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Environmental Chemical Exposuresand Disturbances of Heme SynthesisWilliam E. Daniell,' Henry L. Stockbridge,2 Robert F. Labbe,3James S. Woods,1 Karl E. Anderson,4 D. MontgomeryBissell,5 Joseph R. Bloomer,6 Ralph D. Ellefson,7 Michael R.Moore,8 Claus A. Pierach,9 William E. Schreiber,10Ayalew Tefferi," and Gary M. Franklin2

Porphyrias are relatively uncommon inherited or acquired disorders in which clinicalmanifestations are attributable to a disturbance of heme synthesis (porphyrin metabolism),usually in association with endogenous or exogenous stressors. Porphyrias are characterized byelevations of heme precursors in blood, urine, and/or stool. A number of chemicals, particularlymetals and halogenated hydrocarbons, induce disturbances of heme synthesis in experimentalanimals. Certain chemicals have also been linked to porphyria or porphyrinuria in humans,generally involving chronic industrial exposures or environmental exposures much higher thanthose usually encountered. A noteworthy example is the Turkish epidemic of porphyria cutaneatarda produced by accidental ingestion of wheat treated with the fungicide hexachlorobenzene.Measurements of excreted heme precursors have the potential to serve as biological markers forharmful but preclinical effects of certain chemical exposures; this potential warrants furtherresearch and applied field studies. It has been hypothesized that several otherwise unexplainedchemical-associated illnesses, such as multiple chemical sensitivity syndrome, may representmild chronic cases of porphyria or other acquired abnormalities in heme synthesis. This reviewconcludes that, although it is reasonable to consider such hypotheses, there is currently noconvincing evidence that these illnesses are mediated by a disturbance of heme synthesis; it ispremature or unfounded to base clinical management on such explanations unless laboratorydata are diagnostic for porphyria. This review discusses the limitations of laboratory measures ofheme synthesis, and diagnostic guidelines are provided to assist in evaluating the symptomaticindividual suspected of having a porphyria. Environ Health Perspect 1 05(Suppl 11:37-53 (1997)

Key words: biological markers, diagnosis, environmental exposure, hexachlorobenzene,laboratory, lead, metals, multiple chemical sensitivity, occupational exposure, porphyria,porphyrins

Manuscript received 8 July 1996; manuscript accepted 29 August 1996.The authors wish to extend their sincere gratitude to Ellen K. Silbergeld, University of Maryland, for her

thoughtful, critical review of this article during its preparation. Statements made in this article do not necessarilyrepresent Dr. Silbergeld's opinions. This article was adapted from the Working Document: Evaluation ofIndividuals with Environmental Chemical Exposures and Suspected Abnormalities of Heme Synthesis(1 1 February 1996), which was prepared through the Office of the Medical Director at the Washington StateDepartment of Labor and Industries, Olympia, Washington. A statement of potential conflicts of interest wasincluded with this article at the time it was submitted for consideration by this journal.

Address correspondence to Dr. W.E. Daniell, Department of Environmental Health, University ofWashington, Box 357234, Seattle, WA 98195-7234. Telephone: (206) 685-3160. Fax: (206) 543-8123. E-mail:[email protected]

'Department of Environmental Health, University of Washington, Seattle, Washington; 2Washington StateDepartment of Labor and Industries, Olympia, Washington; 3Department of Laboratory Medicine, University ofWashington, Seattle, Washington; 4Department of Preventive Medicine and Community Health, University ofTexas, Galveston, Texas; 5Department of Medicine, University of California at San Francisco, San Francisco,California; 6Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama;7Department of Laboratory Medicine, Mayo Clinic and Mayo Foundation, Rochester, Minnesota; 8Departmentof Medicine, University of Queensland, Coopers Plains, Queensland, Australia; 9Department of Medicine,University of Minnesota and Abbott Northwestern Hospital, Minneapolis, Minnesota; 10Pathology andLaboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada; "Mayo Clinic andMayo Foundation, Rochester, Minnesota.

Abbreviations used: ADP, ALA-D deficiency porphyria; AIP, acute intermittent porphyria; ALA, &-aminole-vulinic acid; ALA-D, ALA dehydratase; CEP, congenital erythropoietic porphyria; CHP, chronic hepatic porphyria;Copro-O, coproporphyrinogen oxidase; 2,4-D, 2,4-dichlorophenoxyacetic acid; EPP, erythropoietic protopor-phyria; FEP, free erythrocyte protoporphyrin; HCB, hexachlorobenzene; HCP, hereditary coproporphyria; HEP,hepatoerythropoietic porphyria; MCS, multiple chemical sensitivity (syndrome); PBB(s), polybrominatedbiphenyl(s); PBG, porphobilinogen; PCB(s), polychlorinated biphenyl(s); PCT, porphyria cutanea tarda; 2,4,5-T,2,4,5-trichlorophenoxyacetic acid; TCDD(s), tetrachlorodibenzodioxin(s); Uro-D, uroporphyrinogen decarboxy-lase; VP, variegate porphyria; ZPP, zinc protoporphyrin.

IntroductionPorphyrias are inherited or acquiredmetabolic disorders in which the clinicalmanifestations are attributable to char-acteristic patterns of overproduction ofspecific heme precursors and their accumu-lation in certain tissues as a consequence ofdecreased activity of a specific enzyme(s) inthe heme synthesis pathway, usually inassociation with stimulation of the initialstage of the heme-forming system by endo-genous or exogenous stressors. When clini-cally active, and in some cases even whenlatent or in clinical remission, porphyriasinduce high levels of heme precursors inblood, urine, and/or stool.

Porphyrias are relatively uncommonconditions but are probably underrecog-nized. The prevalence of genetic predispo-sition to porphyria probably has beenunderestimated in the general populationbecause of the limited availability of clinicaltests for specific heme-synthesis enzymesand because prevalence studies generallyhave focused more on family members ofaffected individuals and less on the generalpopulation. The symptomatic manifesta-tions of porphyria, particularly the noncuta-neous manifestations, are often nonspecificand may not be accompanied by supportingphysical signs. Most clinicians encounterand recognize few if any cases of porphyriain the course of their careers, and in gen-eral their levels of suspicion for these con-ditions are low; yet diagnosis of porphyriais critically dependent on the clinician firstsuspecting it as a possible cause of a patient'ssymptoms and then ordering the specificessential diagnostic tests.

Some of the inherited porphyrias occurcommonly as toxicogenetic conditions,where the genetically acquired trait is clini-cally latent until clinical manifestations ofporphyria are triggered idiosyncratically byexposure to certain therapeutic drugs, chem-icals, or alcohol. Porphyria also can occuras an acquired toxin-induced condition,where biochemical and clinical manifesta-tions of the porphyria are actually causedby exposure to certain chemicals in indi-viduals with no evident genetic predisposi-tion. In addition, certain chemicals canproduce disturbances of heme synthesisthat are associated with alterations ofsubcellular structure and functions andcharacteristic changes in patterns of heme-precursor excretion. These measurablechanges offer potential biological markersfor detecting harmful effects of specific

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chemical exposures while their biologicaleffects are still preclinical and potentiallyreversible. Most clinicians, however,including specialists in occupational andenvironmental medicine, are relativelyuninformed about these phenomena.

It has recently been proposed that avariety of chemical-associated illnesses forwhich there are no widely accepted specificdiagnostic tests or etiologic explanations-such as multiple chemical sensitivity(MCS) syndrome, Persian Gulf War ill-nesses, conditions associated with siliconebreast implants, and various fatigue syn-dromes-may represent either mild chroniccases of porphyria, or, at least in part, man-ifestations of acquired abnormalities inheme synthesis. Although diagnoses of por-phyria and porphyrialike conditions insuch cases are commonly dismissed byother clinicians and researchers, these diag-noses are increasingly put forth as objectivejustification for recommending majorlifestyle modifications and therapeuticinterventions in individual cases.

This article reviews pertinent medicaland scientific literature in order to providea foundation to address current issuesrelated to environmental chemical expo-sures and possible disturbances ofheme syn-thesis. The first three sections of this reviewprovide necessary background informationabout heme synthesis and the classificationand biochemistry of porphyria; moredetailed information is available in thecited general references (1-10).

Heme SynthesisHeme is a biological compound that, whencombined with certain proteins, plays acentral role in a variety of vital physiologicfunctions, including oxygen binding andtransport (hemoglobin and myoglobin), res-piratory electron transport (cytochromes a,a3, bl, c, and cl), activation and decomposi-tion of hydrogen peroxide (catalase and per-oxidase), and other oxidation-reductionfunctions (cytochromes P450 and b5). Hemesynthesis, which is often also referred to asporphyrin metabolism, occurs in all humancells. It is especially productive in the eryth-ropoietic cells in bone marrow, wherenearly all of the heme is used for hemoglo-bin, and in the liver, where the heme isprimarily needed in cytochrome P450.

Heme, or iron-protoporphyrin IX, isproduced by a metabolic pathway thatinvolves eight enzyme-controlled steps (seebelow and Figure 1).

Succinyl CoA and glycine are com-bined to form 6-aminolevulinic acid

Cytoplasm

4c+Uroporphyrinogen

5iCoproporphyrinogen I

6-Aminolevulinic acid2

Porphobilinogen

3Hydroxymethylbilane

+4Uroporphyrinogen III

5b

Coproporphyrinogen IlIl

Figure 1. Heme Biosynthesis Pathway. Numbers designate individual enzyme-catalyzed steps of heme biosynthe-sis. Enzyme name, step 1, ALA synthase; step 2, ALA dehydratase; step 3, PBG deaminase (uroporphyrinogen syn-thase); step 4, uroporphyrinogen-ill cosynthase; step 5, uroporphyrinogen decarboxylase; step 6,coproporphyrinogen oxidase; step 7, protoporphyrinogen oxidase; step 8, ferrochelatase. "ALA is synthesizedwithin mitochondria in step 1 and then traverses to cytoplasm, where step 2 occurs. Coproporphyrinogen IlIl is syn-thesized in cytoplasm in step 5 and then traverses into mitochondria, where step 6 occurs. bintermediate precur-sors not shown for step 5 (7-, 6-, and 5-carboxyl porphyrinogens) or step 6 (3-carboxyl porphyrinogen). cAlternativepathway at step 4 (hydroxymethylbilane to uroporphyrinogen 1) is not enzymatically catalyzed.

(ALA), an amino acid committedexclusively to heme synthesis.

* Two ALA molecules are condensed toform porphobilinogen (PBG), a mono-pyrrole.

* Four PBG molecules are polymerizedto form the linear tetrapyrrole hydroxy-methylbilane.

* Hydroxymethylbilane is convertedenzymatically to the cyclic tetrapyrroleuroporphyrinogen III (or nonenzymati-cally to an isomer, uroporphyrinogen I,which does not act as an intermediatein heme synthesis).

* Sequential decarboxylations producea series of 7-, 6-, and 5-carboxyl pro-phyrinogens and then coproporphy-rinogen III.

* Further sequential decarboxylationsproduce 3-carboxyl porphyrinogen andthen protoporphyrinogen IX.

* Protoporphyrinogen IX undergoes oxi-dation to protoporphyrin IX.

* Protoporphyrin IX is chelated with fer-rous iron to produce heme.Heme and protoporphyrin IX are the

only directly formed intermediates in theheme synthesis pathway that are actualporphyrins rather than porphyrinogens.However, porphyrinogens are readily oxi-dized to the respective porphyrin forms,particularly when removed from the body.Most laboratory assays measure and report

porphyrinogens in porphyrin form. For theconvenience of a summary term, we use theterm heme precursors loosely to includemeasured porphyrin forms of porphyrino-gens as well as the true intermediates ofheme synthesis.

Classification of PorphyriasMost types of porphyria are known tooccur only as inherited conditions; how-ever, one type of porphyria [porphyriacutanea tarda (PCT)] is known to occur ineither an acquired or an inherited manner.The inherited porphyrias are attributableto an autosomal dominant or autosomalrecessive genetic defect affecting a singleenzyme in the heme synthesis pathway butrarely are attributable to coexistent defectsseparately affecting two different heme-synthesis enzymes.

Porphyrias historically have been classi-fied in various ways (see Table 1), includ-ing: specific enzyme(s) principally involved;nature of underlying inherited defect, if any(autosomal dominant, autosomal recessive,acquired); principal origin(s) of the excessheme precursors (hepatic, erythropoietic,mixed); usual temporal pattern of sympto-matic manifestations (acute or chronic);and principal nature of symptomaticmanifestations (neurologic, neurocuta-neous, cutaneous). These classificationsoverlap and are not mutually exclusive.

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Table 1. Classification of porphyrias and lead intoxication.

Enzyme Principal origin ofIncidence deficiency excess heme precursors Symptom pattem Symptomspattem step no.' Erythropoietic Hepatic Acute Chronic Neurologic Cutaneous

Porphyrias

AIP AD 3 + +I 'C L -'.. --4---~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~., -.~~E"V-14....W.. ...i.S

PCT AD ovrr 5 + + +

HEP AR .+ + .

VP AD7 ± .1 i±SMI !i 1 w i1wl11|1111!!1SB101111~~~~~~~~~~~~~~~~~~~~Mu1H1k 'i~i~~ ~ ,,', ~ IVI%&W.-' --'flu.-[4~f444Wl.k.W~lfllIM*

Lead intoxication Acquired 2,6 8b + + + + +

Abbreviations: AD, autosomal dominant inheritance; AR, autosomal recessive inheritance; +, usually present; ±, variably present. aHeme synthesis enzyme designated by stepnumber (see Figure 1). blhere is apparent interference by lead with these steps. In cases of lead intoxication, there may be mechanisms other than lead inhibition of theenzymes at steps 6 and 8 [Rossi ( 11)].

Porphyrias with NeurologicManifestationsSymptoms in the neurologic porphyrias[ALA dehydratase (ALA-D) deficiency por-phyria (ADP) and acute intermittent por-phyria (AIP)] and noncutaneous symptomsin the neurocutaneous porphyrias [heredi-tary coproporphyria (HCP) and variegateporphyria (VP)] tend to occur in intermit-tent acute attacks, with variable degrees ofsymptom presence between attacks. Clinicalonset of the neurologic and neurocuta-neous porphyrias usually occurs in postpu-bertal adolescents or adults; attacks aremore common in women than men. Onlysix cases of ADP have been reported (7).AIP is one of the more prevalent types ofclinically manifested porphyria, at least inthe United States.

All the neurologic and neurocutaneousporphyrias (except ADP) are classified ashepatic porphyrias because the liver is aprincipal site in which the enzyme defi-ciency manifests itself biochemically. Liverfunction abnormalities are common but aregenerally mild or moderate (12). Severalstudies have indicated that individuals withAIP are at risk for developing hepatocellularcarcinoma (13-15). The principal site ofenzyme expression has not been identifiedclearly for the small number of patientsreported to have ADP.

The neurologic manifestations of acuteattacks are indistinguishable for the variousneurologic and neurocutaneous porphyrias,and these conditions are differentiated bybiochemical or genetic testing. The princi-pal manifestations reflect broad dysfunctionof the nervous system. Frequent symptomsinclude: abdominal pain, constipation, nau-sea, vomiting, tachycardia, hypertension,fever (i.e., autonomic nervous system);

weakness, back and extremity pains,localized or extensive pareses or paralysis,paresthesias, hypoesthesias (i.e., peripheralnervous system); and psychiatric or behav-ioral symptoms (i.e., central nervoussystem). Hyponatremia may reflect inap-propriate secretion of antidiuretic hormone(i.e., hypothalamus). The intensity ofsymptoms, particularly pain, can be severe;associated physical signs, however, arecommonly either absent or disproportion-ately less intense. Symptoms are generallyless severe in HCP (7).

Most individuals who possess thegenetic defect associated with one of theneurologic or neurocutaneous porphyriasshow no recognizable clinical manifesta-tions of porphyria throughout their lives.Only about 10% of individuals geneticallypredisposed to AIP or VP, and probably nomore than one-third of those predisposedto HCP, ever develop the characteristicattacks of acute symptoms (4,7,16-18). Ifand when symptoms develop in a geneti-cally predisposed individual, the initialonset and any subsequent recurrences oftenare linked to precipitating factors such asuse of a therapeutic drug, exposure to aprovocative chemical, alcohol consump-tion, tobacco smoking, an infection, themenstrual cycle, or fasting (1).

The frequency and severity of symptomattacks vary widely both among and withinindividuals. In most cases, symptomattacks have discernible onsets, are charac-terized by moderate or severe pain, last sev-eral days or longer (up to weeks or months),and are followed or separated by periodsof clinical remission. Attacks are usuallyrecurrent but not necessarily (18). Somemanifestations of acute attacks can persistafter the other clinical and biochemical

manifestations subside or resolve; this isparticularly true for peripheral motor neu-ropathy, which can take up to 1 year toresolve or can become permanent (19,20).Permanent neurological deficits are notnecessarily accompanied by heme precursorabnormalities when a patient's porphyria isotherwise in remission.

In contrast to the typical acute presen-tation, symptoms in some cases can occurnondistinctly in time and/or be relativelymild and some symptoms, particularly painor psychiatric manifestations, can becomechronic, often with variable severity overtime (7,9,21). Given such complex presen-tations as well as the frequent paucity ofphysical signs, the diagnosis of porphyriacan be delayed or missed entirely. Accord-ingly, screening surveys of psychiatricinpatients have reported a relatively highprevalence of individuals with previouslyunrecognized porphyria (22-24). Onerecent study, however, found no significantincrease in major psychiatric illness among344 consecutive patients with AIP (25).

Porphyrias with Neurologic and/orCutaneous ManifestationsThe neurocutaneous porphyrias, VP andHCP, can manifest cutaneous lesions aswell as acute noncutaneous symptomattacks. One review of 110 cases of HCPreported that 30% experienced acute pho-tosensitive responses, usually in associationwith acute symptom attacks (16). In con-trast, acute photosensitivity is not commonin VP; the cutaneous lesions tend to bechronic and they occur more often in indi-viduals who do not have acute symptomattacks than in those who do (26,27). Thecutaneous lesions of VP are particularlycommon in hot climates-about 75% in a

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series of South African patients with clini-cally active VP (27). The cutaneous lesionsof VP and chronic cutaneous lesions ofHCP cannot be distinguished clinically orhistologically from PCT (see below).

Porphyrias with Cutaneousoanifestations

The cutaneous porphyrias include all theporphyrias classified as erythropoietic [con-genital erythropoietic porphyria (CEP),hepatoerythropoietic porphyria (HEP), anderythropoietic protoporphyria (EPP)] plusthe nonerythropoietic porphyria PCT. Thehereditary and acquired forms of PCT usu-ally manifest first in adulthood, whereasclinical onset of the erythropoietic porphyr-ias usually occurs in infancy or childhood.There is no gender predilection in PCT,although in the past it has tended to occurmore often in men. The erythropoietic por-phyrias are relatively uncommon: fewerthan 200 cases of CEP and fewer than 20cases ofHEP have been reported (7,28,29).

The cutaneous manifestations of CEP,PCT, and HEP (and VP and HCP) can bedifficult to distinguish on the basis of clini-cal or histologic appearance, although thelesions of the autosomal recessive condi-tions, CEP and HEP, are usually moresevere. Cutaneous manifestations include:skin fragility, vesiculobullous skin erup-tions evolving into crusted ulcers, andresidual hyperpigmentation or scarring,primarily occurring in sun-exposed areas ofthe body (7,30). Facial hypertrichosis isalso common. The pattern of EPP differssubstantially and is generally less severethan in the other cutaneous porphyrias; itis characterized by acute photosensitivitymanifesting as pain, pruritus, and erythemawith exposure as short as minutes, and gen-erally without vesiculobullous lesions orscarring unless sun exposure is prolonged(30,31). Liver injury is usually not foundwith the cutaneous porphyrias other thanin some patients with EPP, in which case itcan be severe and potentially fatal.

Porphyria Cutnea TardaPorphyria cutanea tarda is one of the mostprevalent types of porphyria; it occursmore often as a sporadic condition than asa familial inherited condition. The geneticdefect associated with the familial form isinherited in autosomal-dominant manner.Hepatic uroporphyrinogen decarboxylase(Uro-D) activity is decreased in all forms ofPCT; erythrocyte Uro-D activity, however,is normal in the sporadic form but isdecreased in all but a small subgroup of

familial cases (32). The sporadic and famil-ial forms of PCT usually occur in associa-tion with exogenous factors such as alcohol,oral estrogens, iron, and certain chemicals,or with certain medical conditions, particu-larly liver diseases; both are successfullytreated in the same manner (2,7). FamilialPCT, therefore, can be regarded at leastpartially as an acquired condition in whichthe inherited enzyme deficiency may onlyincrease an individual's susceptibility todisease (33).

PCT is usually associated with somedegree of liver abnormality (2,3,8). Liverfunction tests are nearly always abnormal tosome degree. Moderate siderosis and vari-ous degrees of fibrosis or necrosis are oftenfound upon biopsy, and cirrhosis developsin a small proportion of patients. It is com-mon for PCT to develop in individualswith preexisting liver disease of a variety oftypes, particularly alcoholic liver disease orchronic hepatitis C (34-38). Patients withchronic liver disease also can develop mildor moderate degrees of porphyrinuria(increased urine porphyrins) in biochemicalpatterns that can be consistent with PCTbut without associated cutaneous lesions(see "Chronic Hepatic Porphyria") (2,39).Patients with PCT are reported to be at riskfor developing hepatocellular carcinoma(13,40-42); conversely, some benign ormalignant liver tumors can overproduceuroporphyrin and induce PCT (43-45).

Biochemistry of PorphyriasDeficient activity of a heme-synthesisenzyme results in accumulation of theheme precursors proximal to the defi-ciency, although overall production ofheme is generally adequate (1-10,46). Thewater solubility of the heme precursorsdecreases progressively with successive stepsof heme synthesis. Excess protoporphyrinis excreted exclusively in stool; copropor-phyrinogen, uroporphyrinogen, and the 7-,6-, and 5-carboxyl porphyrinogens areexcreted in both urine and stool, and ALAand PBG are excreted predominantly inurine. Porphyrins in stool undergo varyingdegrees of transformation by normalenteric bacteria (47,48). When a heme-synthesis enzyme deficiency is expressedmore in one tissue than another, heme pre-cursors that accumulate in one tissue canbe transported in blood to other tissues,with subsequent conversion to later inter-mediates in the heme synthesis pathway;experimental ALA loading, for example,leads to prompt coproporphyrinuria inhumans (49-51).

In general, the neurologic (and neuro-cutaneous) porphyrias are characterized byexcessive excretion of the porphyrinogenprecursors, ALA and PBG, in urine; thecutaneous (and neurocutaneous) porphyr-ias are characterized by the accumulationof porphyrins in blood and excessive excre-tion of porphyrins in urine and/or stool(46,52-54). Individual types of porphyriacan be differentiated by the pattern ofheme precursors (i.e., the absolute valuesand the ratios to each other) in a patient'surine, blood, and stool, if collected eitherduring or soon after attacks of neurologicsymptoms or while porphyric skin lesionsare actively manifested. Excretion of ALAand PBG in urine usually normalizeswithin several weeks following an acuteattack in VP or HCP (16,26,27); in AIP,urine excretion commonly subsides butmight not completely normalize duringremissions (19,20). Compared to an acuteattack, urine porphyrinogen precursors(i.e., ALA and PBG) may be lower in VPwhen solely cutaneous lesions are present,but urine porphyrins are still increased todiagnostic levels (27). Porphyrins arealways increased in the plasma of patientswith active cutaneous lesions (10).

When an individual has symptomaticmanifestations of a porphyria (i.e., whendinically active and not latent or in remis-sion), the level of the most excessivelyexcreted heme precursor is typically at leastseveral-fold greater than the values reportedfor the upper limit of normal. In reportsdescribing the neurologic porphyrias, forexample, urine ALA and PBG in AIPincreased acutely to 8 to 150 and 30 to 200mg/day, respectively (normal upper limits, 3to 7.5 mg/day, depending on the labora-tory) (7,20,26,55-57); and in ADP, urineALA was markedly elevated and urine anderythrocyte porphyrins were elevated up to100 times (7,10). In reports of the neurocu-taneous porphyrias, urine coproporphyrinin HCP increased acutely to 4 to 190 timesnormal (16,58). In VP, one small case seriesreported the lowest values of urine ALA,PBG, uroporphyrin, and coproporphyrin inacute attacks to be 9, 12, 190, and 27 timesthe mean control values, respectively (26),which was consistent with the higher aver-age values reported in a larger series (27). Inreports of the cutaneous porphyrias, urineuroporphyrin in PCT increased to about10 to 375 times normal in over 300 reportedcases (except in one case (59), where asingle value was within normal range)(60-70), and in EPP, reported erythrocyteprotoporphyrin values ranged from 2.4 to



90 times normal (31,65,71), with theexception of a single patient in whom itwas elevated by only a factor of 1.4 (31).Finally, in reports of the less common(autosomal-recessive) cutaneous porphy-rias, urine porphyrins in CEP were 20 to60 times normal (7) and in HEP, urineuroporphyrin increased by at least 20-foldand erythrocyte porphyrins increased by5- to 10-fold (28,29,72). Plasma porphy-rin concentrations are markedly increasedwhen there are active skin lesions due toany cutaneous porphyria (10).

Secondary PorphyrinuriaSome of the tests used to diagnose theporphyrias are nonspecific and are abnor-mal in a variety of circumstances other thanthe porphyrias. Porphyrinuria can becaused by porphyrias, by a number of othermedical conditions, especially those affect-ing the liver or bone marrow, and by a vari-ety of exogenous factors such as alcohol andcertain drugs and chemicals that disturbheme synthesis or stress heme-dependentmetabolism (1,73-76). The term sec-ond4ry porphyrinuria is commonly appliedto the porphyrinuria occurring with condi-tions and factors lacking a primary enzymedefect in heme synthesis. It usually involvesmild or moderate coproporphyrinuria, withno or little excess uroporphyrin in urine,and is also often called coproporphyrinuriaor secondary coproporphyrinuria.

The detection of porphyrinuria haspotential utility as a biological indicator ofexposure to chemicals with porphyrinogenicproperties and as a staging measure for sub-clinical development and progression ofchronic hepatic porphyria (see "ChronicHepatic Porphyria and EnvironmentalChemicals and Effects on Measures ofHeme Synthesis"). However, with thenoteworthy exception of lead poisoning(see "Environmental Chemicals and Effectson Measures ofHeme Synthesis"), the por-phyrin excess in secondary porphyrinuriahas no recognized clinically detectableconsequences of its own. Most reviewersattribute symptoms associated with sec-ondary porphyrinuria (other than lead poi-soning) to the condition or agent causingthe porphyrinuria or to an unrelated cause,not to a disturbance in heme synthesis(2,3,10,75,77-81). Still, although theporphyrinuria itself may be benign, anassociated medical condition may be farfrom benign.

Accumulation or excessive excretion ofheme precursors does not necessarily meanthere is a deficiency of a heme pathway

enzyme(s); other possible mechanismsexist. Increased erythropoiesis can produceincreases in urine coproporphyrin, asdemonstrated by induced anemia in experi-mental animals (82). Coproporphyrin nor-mally is excreted in bile and in urine, andimpaired biliary excretion in hepatobiliarydisease leads to increased coproporphyrinexcretion in urine (83). Stimulation ofhepatic heme synthesis (e.g., by certaindrugs) in the absence of deficiency of anyheme-synthesis enzymes can lead toincreased coproporphyrin excretion. Thekidney (primarily the epithelium of theproximal tubules) has been identified as amajor source of porphyrins (primarilycoproporphyrin) in the urine of normaland porphyric individuals as well as lead-and mercury-poisoned individuals (84-87).It is conceivable that toxic effects on renaltubular function could lead to increasedurinary loss of porphyrins along with othersubstances handled by renal tubules withoutinhibition of heme synthesis.

Chronic Hepatic PorphyriaBased on clinical experience (77,88), andwith supporting evidence from animalexperiments (89), Doss (39,75,90) hasdescribed the potential for chronic nonspe-cific disturbances in hepatic heme synthesisto make a transition from secondary copro-porphyrinuria and progress through severalclinically latent stages of chronic hepaticporphyria [(CHP) Types A, B, and C] toPCT (CHP Type D). Each stage in theDoss model (75,88) is differentiable byurine porphyrin quantities and patterns(i.e., initial accumulation of uroporphyrinand later heptacarboxyl porphyrin in theliver), progressively greater degrees of sub-clinical liver injury, and ultimately theoccurrence of cutaneous lesions. The tran-sition from secondary coproporphyrinuriato CHP requires either a genetic defect ortoxic inhibition of Uro-D activity in theliver, generally in combination with liverdisease and precipitating factors such asalcohol or estrogens.

The Doss model, however, is not uni-formly accepted as valid. The evidence inhumans for the Doss model comes fromcross-sectional studies of patient populations,not longitudinal studies (77,88). There areno well-documented cases of individualpatients actually progressing through CHPTypes A, B, and C to PCT, and it has notbeen demonstrated that mild degrees ofporphyrinuria, particularly coproporphy-rinuria, predict the potential for an indi-vidual to develop PCT. The early stages in

the CHP model are characterized primarilyby coproporphyrinuria, which can occurnonspecifically with liver disease and mayreflect a separate phenomenon. There is noevidence or reason to predict that hepaticUro-D deficiency is manifested initially bycoproporphyrinuria before uroporphyrin-uria. In addition, there is evidence thatthe kidney may be the primary source ofcoproporphyrin and other porphyrinsexcreted in normal urine and may also be amajor source in certain porphyrias andtoxin-induced porphyrinurias (84,91,92).

Environmental Chemicalsand Effects on Measuresof Heme SynthesisIn individuals who are genetically predis-posed to developing an acute or cutaneousporphyria (e.g., inheriting one allele), thebiochemical and clinical manifestations ofporphyria can be triggered by a variety ofexogenous factors including certain chemi-cals and therapeutic drugs, alcohol con-sumption, tobacco smoking, infections,and dietary factors, as well as by certainmedical conditions and endogenous factorssuch as the menstrual cycle (1). Exposureto the sun can trigger cutaneous manifesta-tions of porphyria if an excess of por-phyrins already exists. Therapeutic drugsare particularly well recognized as possibleprecipitants of acute porphyria and currentlists of drugs classified as unsafe and drugsthought to be safe for use in acute porphyr-ias are maintained (93-95). Exogenous fac-tors can also cause changes in the hemesynthesis pathway, even in the absence ofgenetic predisposition; in some cases, theseacquired changes have been reported tocause PCT.A number of chemicals, including

halogenated hydrocarbons and metals, areknown to be porphyrinogenic (i.e., capableof inducing changes in heme synthesis,with subsequent overproduction andexcessive excretion of heme precursors) inexperimental animals, generally with expo-sure by ingestion and with doses muchgreater than the range ofhuman experience(96-98). In addition to possible inter-species differences, the differences in doseand pattern of exposure limit the ability toextrapolate from experimental animalstudies in order to assess the potential risksof specific chemicals for humans. Unfortu-nately, there has been only limited system-atic study of the subject in humans. Inhumans, with the noteworthy exceptions ofporphyria caused by hexachlorobenzene(HCB) ingestion and the porphyrinuria

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caused by lead, reports of porphyria orporphyrinuria attributable to chemicalexposures have been infrequent. Thereported findings have generally beenlinked to chronic industrial exposures,industrial accidents, or environmentalexposures that were much higher thanusually encountered.

HechlorobenzeneThe most noteworthy incident of chemical-induced porphyria in humans occurredin Turkey during the late 1950s, whenapproximately 3000 cases of PCT devel-oped from the ingestion of seed wheattreated with fungicides containing about10% HCB (99-106). The wheat wastreated in anticipation of use for plantingbut was instead used for human consump-tion. The syndrome commonly consistedof weight loss, muscle wasting, weakness,hepatomegaly, thyromegaly, arthriticchanges, gross porphyrinuria, hypertri-chosis, and photosensitive dermopathy(99,102). The dermopathy was usuallyhyperpigmented and had vesiculobullouslesions that eventually ulcerated, were com-monly complicated by infections, and leftdepigmented scars and often disfiguringcontractures. Active skin lesions tended toabate within a month after HCB exposureended but often recurred subsequently insummer months, even in the absence ofany new exposure to HCB. Children wereaffected disproportionately more thanadults. Some reports noted the absence ofabdominal crises, mental disturbances, andother neurologic complications (99,100);others noted the presence of abdominal colicand muscle weakness during the acute phase(102). Follow-up studies up to 30 yearslater reported that a majority of the patientsstudied had cutaneous residua, arthriticdeformities, thyromegaly (particularly inwomen), and neurologic manifestationsincluding weakness, paresthesias, and sensoryshading; smaller proportions of patientshad myotonia or cogwheel rigidity withoutother extrapyramidal signs (103-106).

Administration of HCB to experimen-tal animals causes a deficiency of hepaticUro-D and a pattern of porphyrin accumu-lation closely resembling that in humanPCT (89,107-111).

DioxinLaboratory experiments have demonstratedthat tetrachlorodibenzodioxins (dioxin(s)or TCDDs) in high ingested doses are por-phyrinogenic in rodents (112,113), but theevidence in humans is mixed. The strongest

human evidence is from two workplacestudies. In 1964, a study at a New Jerseychemical plant that produced the herbi-cides 2,4-dichlorophenoxyacetic acid(2,4-D) and 2,4,5-trichlorophenoxyaceticacid (2,4,5-T) described two workers withPCT and another with possible PCT, andalso reported that urine uroporphyrinswere positive in 8 of 26 other workers(114). Most workers studied had chlo-racne, prototypically caused by dioxin,which was then a frequent contaminantof 2,4,5-T production. Another study of55 workers arriving at a hospital from aCzechoslovakian herbicide manufacturingplant found a high prevalence of chloracne,11 workers with PCT, and slightlyincreased urine uroporphyrin (< 100 pg/24 hr; reference range not stated) in 21%of those tested (115). One worker in aTCDD-contaminated workplace developedPCT, sarcoma, and probable chloracne; thecombined occurrence of these rare condi-tions suggested an etiological relation tothe TCDD exposure (116,117). Otherworkplace studies, however, have been lessdemonstrative. A later study at the sameNew Jersey plant found no active cases ofPCT and no workers with abnormal urineexcretion of heme precursors except oneworker who formerly had severe PCT(118). Maintenance workers, who had thehighest chemical exposures, were found tohave higher average urine coproporphyrinlevels than other workers, but their individ-ual levels were all within normal limits.Two separate later studies of herbicide pro-duction plants (one of which included theNew Jersey plant mentioned above) (119)were both negative (120).

Limited evidence is also available fromcommunity studies. An explosion at achemical plant in Seveso, Italy, in 1976contaminated the surrounding environ-ment with TCDDs. Two related individ-uals who were found to have a preexistingfamilial defect in Uro-D were reported tomanifest PCT after the explosion withoutanother evident precipitating factor (121).Among 60 Seveso residents from otherfamilies, 8 were reported to have secondarycoproporphyrinuria and 5 had a transitionconstellation to CHP Type A (121) (see"Chronic Hepatic Porphyria"). Anotherstudy, of 115 Seveso residents, found that84% had urine porphyrin patterns consis-tent with coproporphyrinuria or CHPType A, with the greatest prevalence anddegree of abnormality in residents fromareas of highest contamination (122). Alater study, however, reported chloracne

but no PCT attributable to TCDDsamong Seveso children and adolescents(123,124). In two studies of Missouricommunities where TCDD-contaminatedwaste oil had been sprayed for dust controlover several years, there were no cases ofPCT, but one study found significantlyhigher mean uroporphyrin levels in urineand higher prevalence of elevated urineuroporphyrin values (> 13 pg/g creatinine(cre) in 16%, vs 7% controls) among154 residents of a contaminated mobilehome park compared to matched controlsubjects (125,126).

Other Halogenated HydrocarbonsA number of halogenated hydrocarbonshave been demonstrated experimentally tobe porphyrinogenic in exposed animals orin in vitro tests; these are well summarizedin Strik et al. (96) and Marks (97) andwill not be described in this review. Thereported human experience is less exten-sive. Haberman et al. (127) mentionedthat a case of PCT manifested itself soonafter agricultural application of DDT.Lynch et al. (128) reported a case of PCTin association with hepatitis, which wasattributed to chronic toxic exposure andpresumed to be a polychlorinated phenolunintentionially synthesized on a regularbasis by mixing cleaning agents containingbenzylchlorophenol and sodium hypochlo-rite. A worker with severe acute methylchloride intoxication had marked increasein urine and fecal excretion of copropor-phyrin but not uroporphyrin or protopor-phyrin; the coproporphyrinuria decreasedin association with clinical improvement(129). Evaluation of46 workers exposed tovinyl chloride in a polyvinyl chloride pro-duction process found that 36 had copro-porphyrinuria (<820 nmol/24 hr; normal(nl) < 130), including 4 who also hadmildly increased excretion of uroporphyrinand heptacarboxyl porphyrin in urine (130).A study of Michigan farm families exposedto polybrominated biphenyls (PBBs) byingestion of contaminated meat and dairyproducts found that 11 of 126 persons hadcoproporphyrinuria (93-218 pg/liter;nl <78), including 4 with mild uroporphy-rinuria (29-59 pg/liter; nl <24) (131). Of20 individuals in Taiwan, 2 years after poi-soning with polychlorinated biphenyls(PCBs) in food oil, 3 had elevated uropor-phyrin in urine (50-139 pg/liter; controls,7-22), 1 of whom, plus 5 others, had ele-vated coproporphyrin (61-255 pg/liter;controls, 2-48) (132). Two separatestudies of 87 similarly poisoned individuals

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in Yusho, Japan, about 10 years after theevent, found no differences in urine por-phyrins compared to those of controls andidentified PCT in 1 person whose historywas complicated by alcohol use (133,134).Total urine porphyrins were not signifi-cantly different from those of controls in168 chemical plant workers exposed toeither allylchloride, hexachlorocyclopenta-diene, epichlorohydrin, or endrin (135).

LeadLead absorption, both acute and chronic, iswell documented to affect heme synthesis(11). Lead causes accumulation of zincprotoporphyrin (ZPP) in erythrocytes andlarge increases ofALA and coproporphyrinin urine. Lead reversibly inhibits ALA-Dand also appears to interfere with the func-tion of coproporphyrinogen oxidase andferrochelatase, possibly by mechanismsother than direct enzyme inhibition(11,98,136,137). It is conceivable that thecoproporphyrinuria occurs by the samemechanism as in ALA loading of normalsubjects (Biochemistry of Porphyrias).

There is evidence that ALA-D poly-morphism may influence lead toxicokinet-ics. The ALA-D enzyme is comprised ofeight identical subunits coded by an auto-somal gene, at which there are two com-mon alleles (138). The ALA-DI allele hasabout 80 to 90% prevalence in studiedEuropean and American population sam-ples; the ALA-D2 allele, about 10 to 20%(138-142). The three associated pheno-types (1-1, 1-2, 2-2) have similar enzymeactivities in erythrocytes. A number ofstudies have reported that individuals withthe ALA-D2 allele, particularly thehomozygous ALA-D 2-2 phenotype, havehigher blood lead levels on average thanindividuals with the ALA-D 1-1 phenotypeand comparable lead exposures (142-145).The possible mechanism of this reportedphenomenon is not known, although it hasbeen speculated that ALA-D subunitscoded by the ALA-D2 allele might bindlead more efficiently (146). The possibleclinical relevance also is unclear. Someinvestigators have suggested that ALA-D2might render individuals more susceptibleto lead poisoning (143,145); others havesuggested that ALA-D2 might protectagainst lead effects (147).

Lead intoxication is generally classifiedas a secondary porphyrinuria rather thanan acquired porphyria, although some inves-tigators have classified it as a porphyria,noting clinical and biochemical similaritieswith the acute porphyrias (see Table 1),

particularly ADP (39,148-150). It is notknown to what degree the manifestationsof lead poisoning might be attributable, ifat all, to the associated disturbance in hemesynthesis apart from the directly neurotoxicproperties of lead, but there is evidence thedisturbed heme-forming system may havesome contributing role (151,152).

The reported interaction between leadexposure and genetic deficiency of ALA-Dis noteworthy. ADP is attributable tohomozygous autosomal-recessive defectsaffecting ALA-D, with near total loss ofenzyme activity; heterozygous carriers ofthe defect have only partial loss of ALA-Dactivity and usually have no associatedsymptoms (153). However, the heterozy-gous defect appears to increase the vulnera-bility of the individual to the effects ofexogenous agents such as lead, whichreduce ALA-D activity. In case reports ofindividuals who had a heterozygous defectaffecting ALA-D and were exposed tolead, the biochemical abnormalities andclinical features of illness were consistentwith lead intoxication but were muchmore severe than expected for their mildlyelevated blood lead levels (22-43 pg/dl)(149,154,155). The prevalence of the het-erozygous defect in the general populationhas been estimated variably to be below1% (149) or as high as 2% (156).

Other MetalsMercury, arsenic, and other metal exposuresare also reported to affect heme synthesis inhumans, although the associated levels ofporphyrinuria are lower than those seenwith lead. In a survey of dentists attendingan annual professional meeting, those whowere found to have >20 pg/liter mercury inspot urine samples also had 2- to 3-foldhigher levels of coproporphyrin in urine(42 ± 18 pg/g cre or 74 ± 8 pg/liter;mean ± SD) than those with no urinarymercury (coproporphyrin 28 ± 12 pg/g creand 23 ± 6 pg/liter) (87). Urine uropor-phyrin excretions did not differ signifi-cantly; however, pentacarboxyl porphyrinand an atypical porphyrin tentatively iden-tified as keto-isocoproporphyrin were sig-nificantly elevated in urine. A study ofrandom urine samples from 52 workersexposed to inorganic and organic mercury(median urine mercury about 500 pg/liter)found that 23% had elevated copropor-phyrin levels (>70 pg/liter and up to 159pg/liter) compared to only 3% of controlsubjects (157). Similarly, a study of arsenic-exposed smelter workers (urine arsenic129 ± 109 pg/g cre in the high-exposure

group) found more than twice as muchcoproporphyrin (63±30, range 18-171,pg/g cre) but comparable amounts ofuroporphyrin in morning urine specimens,relative to those of nonexposed controlsubjects (coproporphyrin 27± 14, range2-57, pg/g cre) (158). In another study,individuals exposed to arsenic in drinkingwater (urine arsenic mean 65, range 1-130pg/liter) were reported to have no signifi-cant increase in urine porphyrin levels rela-tive to those of a control group, althoughmost exposed individuals did have inver-sion of the urine coproporphyrin/uropor-phyrin ratio (76%, vs 32% of controls)(159). Downey (160) reported a case ofacute intermittent porphyria (laboratoryresults not presented), with symptomsincluding transient dermatitis, severeabdominal pain, diarrhea and vomiting,dyscoordination, memory loss, and visualhallucinations, all of which developed soonafter placement of dental prostheses (con-taining 76% palladium and 10% copper)and resolved "almost immediately" afterremoval of the prostheses.

Experimental animal and in vitrostudies have demonstrated a number ofother metals to be porphyrinogenic, includ-ing aluminum, cadmium (with arsenic orlead), cobalt, gallium arsenide and others;again, these data are summarized in detailin other reviews [Marks (97), Woods (98),Fowler et al. (161)].

Other ChemicalsBleakley et al. (162) noted, in describingelevated fecal porphyrins in rats with sub-chronic cutaneous exposure to diazinon, "ina number of cases seen [ofPCT in humans]... a history of contact with pesticides, inparticular with diazinon." A study of 17pulp production workers exposed to hydro-gen sulfide and methylmercaptan reportedreduced activities ofALA synthase, ALA-D,and ferrochelatase (relative to referenceranges) in the reticulocytes of 8, 1, and 5workers, respectively, as well as low erythro-cyte protoporphyrin levels in 7 workerscompared to control values (163). In tworeported case series of acute porphyria, paintsand solvents were described as triggers ofacute symptoms in a number of geneticallypredisposed individuals (26,164).

Measures ofHeme Synthesisas Biological MarkersofChemical ExposureMeasurements of the status of hemesynthesis, particularly measurements ofexcreted heme precursors, have potential

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field and clinical utility as biologicalindicators of chemical exposure and chemi-cal effect on body functions (98,165). Thisis well illustrated by the extensive experi-ence with lead, where measurements ofprotoporphyrin in blood [e.g., free erythro-cyte protoporphyrin (FEP) and ZPP] andALA and coproporphyrin in urine havebeen used, and in the case of ZPP are stillused, as sensitive markers of lead intoxica-tion. Animal experiments have demon-strated that a number of metals producecharacteristic patterns of change in the uri-nary excretion of heme precursors, that theinduced disturbances in heme synthesis aretypically associated with ultrastructuralchanges and alterations of other subcellularprocesses, and that these measurable changesoccur before the effects of metal exposurewould be clinically evident (98,161).Therefore, measurements of heme precur-sors may offer a means of detecting harm-ful effects of specific chemical exposuresin humans at stages of biological responseto exposure that are still preclinical andare presumably most reversible. As the pre-ceding sections indicate, however, only asmall number of studies in humans haveattempted to assess the utility and validityof heme precursor measurements or othermeasures of the status of heme synthesis asmarkers of exposure or preclinical disease inchemically exposed humans.

Unexplained Chemical-associated Illnessesand Measures ofHeme Synthesis

It has recently been proposed that a varietyof chemical-associated illnesses for whichthere are no widely accepted specific diag-nostic tests or etiologic explanations-suchas MCS syndrome, Persian Gulf War ill-nesses, conditions associated with siliconebreast implants, and various fatigue syn-dromes-may represent either mild chroniccases of porphyria, or at least in part, mani-festations of acquired abnormalities inheme synthesis (166-177).

The MCS syndrome (178-180) hasbeen defined by Cullen (181) as "anacquired disorder characterized by recur-rent symptoms, referable to multiple organsystems, occurring in response to demon-strable exposure to many chemically unre-lated compounds at doses far below thoseestablished in the general population tocause harmful effects." No physiologic testhas been widely accepted as correlatingwith symptoms in MCS syndrome,

although a variety of immunologic andneurologic tests have been described asabnormal or diagnostic based on clinicalseries data. Of note, however, one blindedprospective controlled study evaluated apanel of immunologic tests offered by alaboratory with recognized interest in theevaluation of individuals with chemicalsensitivities, and found that the tests didnot distinguish between MCS patients andcontrol patients (182). Still, in spite ofcontinued debates about the origins ofMCS syndrome, there is little doubt that itcan be associated with substantial sympto-matic distress, major lifestyle disruption,and severe degrees of inability to performusual activities ofwork and daily life.

To date, the hypotheses of porphyricmechanisms in these otherwise unex-plained syndromes are based only on indi-vidual cases or case series characterized byrelatively mild increases in porphyrinexcretion and/or decreases in activity ofvarious heme-synthesis enzymes. The datareported to date have not identified anycharacteristic unifying pattern(s) amongthe laboratory test results. Donnay andZiem (171) reported in 1995 that twomedical practices in Washington State hadfound excess porphyrins and/or enzymeabnormalities in patterns that did notmatch those of any inherited porphyrias inover 70% of more than 150 MCS patientswith symptoms of porphyrinopathy.Morton (172) described laboratory find-ings for 38 individuals with chemical sensi-tivities and deficient activity of at least 1 of5 tested heme-synthesis enzymes. Althoughhe did not distinguish between the mar-ginal and low laboratory reference rangesin defining deficiency (see "Limitations ofLaboratory Measures of Heme Synthesis"),22 (58%) had deficiency of coproporphy-rinogen oxidase (Copro-O), 13 (34%) haddeficiency of protoporphyrinogen oxidase,and 27 (71%) had at least 1 excess por-phyrin in feces or urine (values reportedonly as yes or no). Of note, 7 of the 22individuals with Copro-O deficiency and 4of the 16 individuals with deficiency ofenzymes other than Copro-O had nodetected excess of porphyrins in feces orurine. Ziem and McTamney presentedsupporting clinical data at a recent scien-tific meeting; those data are not yet avail-able for circulation (177).

Downey (168) described hereditarycoproporphyria in 13 patients (plus 7 familymembers) who arrived at an oral stomatol-ogy clinic with unexplained oral conditionsand multiple systemic complaints: 16 of 19

tested had abnormal Copro-O activity, 3of those 16 had one other abnormal heme-synthesis enzyme, and 14 of 14 tested hadelevated heme precursors in urine or stool.Of the 16 abnormal Copro-O activity val-ues, 14 were in the marginal referencerange (0.06-0.09 relative units) and theother 3 values were just below the marginalrange (0.05 relative units) (182). Ofthe 21elevated heme precursor values (most oftencoproporphyrin), 17 (8 1%) were 1. 1 to 1.8times higher than the respective valuereported for the upper limit of normal, and4 (2 in stool and 2 in urine) were 2.0 to2.3 times higher (182). Downey (167)mentioned that about 90% of 62 patients(including the above-described 19) werefound to have one or more heme-synthesisenzyme abnormalities.

Two types of hypotheses have been putforth in explanation of such laboratoryfindings occurring with otherwise unex-plained chemical-associated illnesses. Thefirst hypothesis is that these illnesses areactually porphyrias, in which illness is pre-cipitated by environmental chemical expo-sures in genetically predisposed individuals(160,167,172). Proponents suggest that thelatent genetic traits for various porphyriasare much more common in the generalpopulation but were not recognized untilthe recent increased availability of labora-tory assays for the activity of heme-synthesisenzymes. It is hypothesized that such genet-ically predisposed individuals can manifestporphyria symptoms in low-grade smolder-ing or chronic and slowly progressive pat-terns (without classically recognizable acuteattacks of severe symptoms) in response toexposures to a variety of substances such as"formaldehyde, other aldehydes, heavyvehicle exhaust fumes, perfumes, other fra-grances, chlorine, chlorinated cleaningagents, possibly any chlorinated hydrocar-bon, substances in poorly ventilated build-ings, and anything that triggers porphyriasymptoms" (172). It is contended that thesechronic symptom patterns are not necessar-ily accompanied by diagnostic abnormali-ties of heme precursors, possibly because a"limited attack especially on a small num-ber of cells or on cells where the heme path-way is not a major functioning system orwhere the affected cells do not have readyaccess to external excretory processes maynot raise porphyrin precursors above nor-mal levels" (167); measurement of heme-synthesis enzyme activities, therefore, isregarded as critical to diagnosis.

The second hypothesis is that geneticpredisposition may not be an essential



component of a porphyric mechanismin MCS syndrome, and that reportedreductions in activity of heme-synthesisenzymes might represent an acquired defi-ciency attributable to exogenous toxicagents or stressors, manifesting as eitheran intoxication porphyria or some otheras-yet undefined disorder of porphyrinmetabolism (169,171). It is hypothesizedthat "the heme pathway may be just oneof many sites in the body adverselyaffected by exposure to toxic chemicals,and that this interaction could account formany of the cutaneous, neurological andpsychological symptoms reported by MCSpatients, including the most controversialsymptom of chemical sensitivity itself"[A Donnay, personal communication;(166)]. Some cases of MCS syndrome arepredicted to be porphyric and the othersnonporphyric (171).

Limitations ofLaboratory Measuresof Heme SynthesisIn evaluating the symptomatic patientwith a suspected disturbance of the hemesynthesis pathway, the interpretation oflaboratory tests must consider the condi-tions of sampling and the limitations ofthose tests.

Methodologic LimitatonsLaboratory test results, in general, can becompromised by a variety of factors,including specimen integrity (reflectingconditions of specimen collection, process-ing, transport, and storage), analyticalquality, limitations of analytical methods,and the applicability and specificity of ref-erence ranges, including the range of nor-mal intraindividual variability. Issues ofspecimen integrity may be particularly rele-vant when specimens are collected andprocessed at one site and then transportedto a geographically distant reference labora-tory, as in nearly all the cases described in"Unexplained Chemical-associated Illnessesand Measures of Heme Synthesis." Speci-men integrity is a particular concern inenzyme activity assays. There is the risk ofobtaining falsely low measurements ofenzyme activity because of the potentiallability of enzymes after removal from thebody. Because of these risks, an abnormaltest result generally should be confirmed byanalysis of a second specimen. The need torepeat a test, of course, must be temperedby the degree of support for a diagnosisfrom other clinical and laboratory data,and by the feasibility of repeating the test

(i.e., the appropriate clinical circumstancesshould still be present).

Reference RangesIn addition to considering methodologiclimitations, the interpretation of any labo-ratory test should consider potential limita-tions of the test reference range. First,reference ranges usually do not include allpossible values for normal people (184).It is common laboratory practice to definea reference range as the mean ± 2 SD,based on the distribution of test results inan ostensibly normal reference sample;therefore, it is expected that 5% of normalindividuals, and possibly more if the distri-bution is skewed, will have low or highoutlying values. Second, the range of possi-ble test values may overlap for normal andaffected individuals, and many tests mightbe more appropriately characterized as hav-ing an inconclusive range or a continuumbetween normal and diagnostic ranges,rather than simply being dichotomized(185-187). Third, the usual ranges of testvalues for normal individuals and for indi-viduals with a given disease may differ sosubstantially that intermediately abnormalvalues might be regarded as nondiagnosticfor that disease.

It is particularly important to recognizethat there can be considerable normalintraindividual variation in physiologictests (183); often for specific tests eitherthis has not been characterized or thedegree of expected variability has not beenmade known to the clinician. Any suchvariation, however, can limit the clinician'sability to interpret a test result confidentlyas abnormal when an individual's singleresult is outside the reference range by anamount that is small relative to expectedintraindividual variability on that test. Onestudy, for example, reported a 3-fold differ-ence between the highest and lowest totalporphyrin measurements in 24-hr urinespecimens (normally comprised mostly ofcoproporphyrin) collected over 7 consecu-tive days from the same person; all valueswere normal except one that exceeded thevalue reported for the upper limit of normalby about 25% (131).

Finally, because a reference range maybe unique to the assay method and the lab-oratory performing the test, test resultsshould be interpreted relative to the labora-tory-specific reference range and/or, if suf-ficient general clinical experience exists,against accepted absolute reference stan-dards. However, a reference range may havelimited representativeness for test subjects,

even when derived from a large sample ofnormal individuals, if test subjects differfrom the reference sample in terms of thecircumstances affecting specimen integrityor in terms of potentially confounding per-sonal factors. For example, the study ofPBB-exposed Michigan farm families (see"Environmental Chemicals and Effects onMeasures") found total urine porphyrinswere significantly higher on average thanthose in the Dutch reference sample, butthey were no different than those in acontemporary control group of Wisconsinfarm families (131).

Enzyme Activity MeasurementsThe diagnostic value of activity measure-ments for heme-synthesis enzymes dependson the presence or absence of symptomsand on other clinical and laboratory data,particularly the level and pattern of anyassociated overproduction of heme precur-sors. Individuals who are genetically predis-posed to a porphyria usually will havedeficient activity of the associated enzyme,yet a substantial proportion will neverdevelop symptoms of a porphyria. There-fore, identification of the genetic trait alonein a symptomatic patient is not by itselfsufficient evidence of symptom causality.In addition, reductions in enzyme activitydo not necessarily reflect a genetic trait;certain exogenous agents such as alcohol orlead can inhibit or interfere with the activ-ity or alter the synthesis of specific enzymesin the heme synthesis pathway (11,74).Conversely, normal enzyme activity mea-surements can reduce the likelihood,although they do not completely eliminatethe possibility, that a person has porphyria.There is typically a substantial degree ofoverlap in enzyme activity values for nor-mal individuals and for individuals with aporphyria characterized by deficiency inthat enzyme (73,188). Also, some porphy-rias do not necessarily manifest the associ-ated enzyme deficiency in erythroid cells,even though it may be present in other tis-sues (e.g., variant AIP, when a mutation isin or near exon 1 of the PBG deaminasegene and affects only the nonerythroidenzyme) (10).

The measurement of specific heme-synthesis enzyme activities is commonlyconsidered a second-line test in the evalua-tion of porphyrias (189). Enzyme activitymeasurements are most often used for iden-tifying a genetic trait for porphyria in fam-ily members of a person with diagnosed orsuspected porphyria to identify individualswho should be counseled on the need to

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minimize or avoid exposure to factorsknown to precipitate porphyria, eventhough they may have been asymptomaticto date (7,188,190,191). Enzyme activitymeasurements also can be useful when asymptomatic person is diagnosed as havinga porphyria of some type-based on symp-tom pattern and a substantial increase inexcretion of heme precursor(s)-but thepattern of heme-precursor excretion doesnot allow differentiation of the specifictype of porphyria; proof of deficient activ-ity for a specific heme-synthesis enzymecan facilitate determination of the specificporphyria. Measurements of enzymeactivity otherwise have limited utility inthe evaluation of suspected porphyria insymptomatic individuals.

CoprovorDhvrinozen Oxidase ActivityDecreased Copro-O activity is the mostfrequently identified enzyme finding, andin many cases is the sole basis for diagnos-ing a disturbance of heme synthesis in theabove-mentioned cases of unexplainedchemical-associated illnesses with reportedlyabnormal measures of heme synthesis (see"Unexplained Chemical-associated Illnessesand Measures ofHeme Synthesis").

The assay for Copro-O activity is thesubject of some controversy. At present,only one laboratory offers a Copro-O activ-ity assay on a commercial basis. That assayis performed by the incubation of ALAsubstrate with a lysed peripheral blood cellspecimen, followed by analysis of the por-phyrins formed; a low yield of protopor-phyrin and a normal or increased yield ofcoproporphyrins indicate a deficiency ofCopro-O (183). The commercial assay,therefore, uses coproporphyrinogen synthe-sized enzymatically in situ from ALA addedto the incubate. In contrast, research labo-ratories that perform Copro-O activityassays in peripheral blood cells do so in iso-lated leukocyte or lymphocyte fractions,using externally synthesized coproporphy-rinogen as specific substrate (16,192-194).

It is widely believed that heme synthesisin humans is confined to nucleated cellsand that Copro-O is a mitochondrialenzyme. However, there is little publishedinformation that addresses the relativedegrees of Copro-O activity in reticulo-cytes, mature erythrocytes, and leukocytes(16,195,196). Heme synthesis in periph-eral blood cells is expected to occur only inleukocytes and perhaps in reticulocytes,and not in mature erythrocytes, which lacknuclei and mitochondria and which consti-tute about 99% of peripheral blood cells.

Mitochondria are present in nucleatedblood cells but are not normally present inmature erythrocytes. Developers of thelysed-blood assay maintain, however, thatcirculating erythrocytes can transform 10 to40% of available coproporphyrinogen III toprotoporphyrinogen during 3-hr incuba-tions, with reticulocytes having at leasttwice as much Copro-O activity as oldererythrocytes, and with overall erythrocyteactivity far exceeding that seen in lympho-cytes (RD Ellefson, personal communica-tion). The subcellular distribution of thereported Copro-O activity in erythrocytes isundear; remnants of mitochondria are onespeculated possibility.

It is generally agreed that because thepercentage of reticulocytes varies amongnormal persons and can change dramati-cally in a number of disease states, interpre-tation of test values from the lysed-bloodassay should at least be adjusted for thereticulocyte count (183). It is also agreedthat regardless of the technique used toassay activity, Copro-O is a particularlylabile enzyme after removal from the body.Vigorous specimen processing or failure toproperly maintain the specimen duringtransport or storage can cause enzymedamage and produce falsely low (i.e., falsepositive) test results (183).

Evaluating the SymptomaticPatient in Whom PorphyriaIs SuspectedThe most important first step towarddiagnosing or ruling out porphyria in asymptomatic patient is for the clinician tomaintain a high index of suspicion for apossible diagnosis of porphyria, whethersymptoms are classic for a porphyria or arevague or unexplained. The conclusive diag-nosis of a porphyria should be based on asystematic approach incorporating medicalhistory, physical examination, and biochem-ical data, and including genetic evaluation ifnecessary. Certain symptom patterns, physi-cal findings, and elements of the exposurehistory may raise the degree of suspicion forporphyria; however, the lack of supportinginformation from these sources cannotexclude a diagnosis of porphyria. Therefore,the systematic approach to evaluating asymptomatic patient with suspected por-phyria must include laboratory evaluation(see also "Biochemistry of Porphyrias").

Laboratory Evaluation-Diagnosis ofPorphyriaThe nature and pattern of a patient'ssymptoms and physical signs may provide

some guidance in the selection of tests forevaluating the symptomatic patient withsuspected porphyria. However, the neuro-logic and cutaneous manifestations of por-phyrias can be nonspecific or atypical andcaution is necessary to avoid being overlyfocused on the basis of clinical appearancein initial test selection. Reviewers makeslightly different recommendations regard-ing the appropriate panel of first-line testsfor the evaluation of suspected porphyria.The most common recommendation-when symptoms suggest possible neurologicmanifestation(s) of an acute porphyria-isfor the measurement of PBG with or with-out ALA in urine (46,52,54,189). Mostreviewers also recommend quantification oftotal or individual porphyrins in urine and,routinely or supplementally, in stool-par-ticularly when symptoms or signs suggestpossible cutaneous manifestations of por-phyria. Measurement of protoporphyrin inblood is often recommended, dependingon the degree of suspicion for erythropoieticprotoporphyria. Anderson (10) alterna-tively recommends measurement of totalplasma porphyrins, plus urine PBG andALA, to determine the presence or absenceof porphyria. A blood lead level, with orwithout a ZPP level, should also be consid-ered because of the similarity of symptomsin lead poisoning and porphyrias withneurologic manifestations.

It is generally less difficult to determinewhether a patient has porphyria than it isto differentiate which specific type of por-phyria is present. The presence or absenceof increases in urinary ALA and PBG andthe relative increases in the individual por-phyrins are particularly helpful in diagno-sis. The nature and pattern of reportedsymptoms can assist in differentiation. Thecited general references and review articles(1-10,46,52,54,189) provide informationregarding the patterns of laboratory abnor-malities to consider in attempting to differ-entiate the specific type of porphyria in thepatient who has laboratory and clinical evi-dence consistent with a porphyria. We willnot discuss further this level of differentialdiagnosis; we will focus on preliminaryscreening steps in the diagnostic evaluationof a possible porphyria.

Laboratory Evaluation-Excluding Porphyriaas the Cause o SymptomsAs discussed in "Biochemistry of Porphyrias,"when a porphyria (or another clinicallyimportant disturbance of heme synthesis,e.g., lead intoxication) of any recognized


A"

46


type is symptomatic, it is almost alwaysaccompanied by substantial overproductionand increased excretion of heme precursors,typically at least several-fold greater thanthe value reported for the upper limit ofnormal. One possible but infrequent excep-tion occurs after a patient with acute por-phyria develops a neurologic deficit thatbecomes permanent while the porphyriais otherwise in remission; biochemicalparameters may return to normal ranges.

Conversely, in a patient who is cur-rently or recently symptomatic and who issuspected to have a porphyria, it is notprobable that the patient's symptoms areattributable to a porphyria of any typeunless a measurement on at least one of thefollowing tests is greater than twice thevalue reported for the upper limit of nor-mal: ALA, PBG, uroporphyrin, or copro-porphyrin in urine; blood total porphyrins;or fecal coproporphyrin (see "Biochemistryof Porphyrias") (90,189). A blood lead levelshould be checked to determine the possi-bility of lead intoxication if lead exposureis possible, if excretion of coproporphyrinor ALA is increased, or if blood porphyrins(e.g., ZPP) are increased. If a person is cur-rently or recently symptomatic and hasreduced activity of a specific heme-synthe-sis enzyme, but laboratory testing does notreveal overproduction of heme precursorsin a pattern and levels consistent with theporphyria associated with deficiency of thatenzyme, then the reduction in measuredenzyme activity has no probable causativerelationship to the person's symptoms.

Satisfaction of these 2-fold-thresholdscreening criteria does not necessarily estab-lish a diagnosis of porphyria. Depending onthe degree and pattern of abnormalities onthese tests, additional testing may be neces-sary to establish or exclude a diagnosis ofporphyria. It is possible that an individualcould have an abnormal heme-precursormeasurement with this degree of abnor-mality as a consequence of something otherthan porphyria (or lead intoxication).Other medical conditions can cause sec-ondary porphyrinuria of this magnitude.Blood porphyrins can also be increasedby this magnitude in conditions otherthan porphyria; for example, iron defi-ciency commonly produces an increase inblood ZPP.

Conversely, failure to satisfy these2-fold threshold-screening criteria does notnecessarily exclude a diagnosis of por-phyria. Heme-precursor measurements inthe range of one to two times the valuereported for the upper limit of normal

should not be interpreted as normal butrather as indeterminate or nondiagnostic.The timing of sample collection relative tothe occurrence of symptoms is critical.When a patient with suspected porphyria isnot currently or recently symptomatic, thelevels of heme-precursor excretion are gen-erally lower and can even normalize withtime (i.e., within days to weeks). If apatient's last symptoms occurred remotelyin the time relative to specimen collection,it may be necessary to repeat the tests dur-ing or as soon as possible after future symp-toms. In view of expected interindividualvariations and the potential processing andanalytic limitations of laboratory tests, par-ticularly at the low range of abnormality,mildly abnormal levels of heme-precursorexcretion generally should be repeatedbefore utilizing them as justification forfurther diagnostic assessment.

Secondary PorphyrinuriasIndeterminate or nondiagnostic levels ofporphyrin excretion might represent a sec-ondary porphyrinuria. With the note-worthy exception of lead poisoning, theporphyrin excess in secondary porphyrin-uria has no recognized clinically detectableconsequences of its own. Medical condi-tions that appear to have only secondaryeffects on the heme synthesis pathwayare appropriately evaluated, with atten-tion focused on the primary condition.Similarly, when chemical exposures are sus-pected as the cause of a patient's symptomsor medical condition, the exposure rela-tionship can be characterized more speci-fically by exposure assessment or byquantification of the suspected chemical(or its metabolite) in blood or urine thanby measurement of heme precursors. Leadpoisoning, for example, is probably themost commonly recognized condition thatcan result from a chemical exposure andthat is accompanied by abnormal measuresof heme synthesis. However, even thoughthese measures provide sensitive indicatorsof lead exposure and effect, the blood leadlevel is a more sensitive and more specifictest for the diagnosis and management oflead poisoning.

Exposure RelationshipsIf it is ultimately determined that asymptomatic patient has a specific por-phyria or another clinically important dis-turbance of heme synthesis (e.g., leadintoxication) and if the possibility of expo-sure relationship or work relationship is atissue, then the exposure history and any

independently available exposure datashould be reviewed to assess the likelihoodthat an exogenous chemical(s) might havetriggered or caused the diagnosed condi-tion. The potential complexity of the expo-sure assessment process is beyond the scopeof this review. Exogenous agents that areknown or suspected to cause or to triggerporphyria are discussed above. Given thatthere has been only limited systematic studyof potentially porphyrinogenic chemicals inhumans, however, consideration of possibleexposure relationships should not be con-fined to chemicals known or suspected toaffect heme synthesis.

ConclusionsThere is little question that individualswho are genetically predisposed to a por-phyria can have clinical manifestations ofporphyria triggered by exogenous chemicals.Most of the experience with such chemicaltriggering of porphyria has involved alco-hol consumption and pharmacologicaldoses of drugs. However, other chemicalexposures have also been reported to trig-ger porphyrias with neurologic manifesta-tions as well as those with only cutaneousmanifestation-PCT in particular.

It is also clear that certain exogenouschemical exposures can actually causeporphyria in the absence of genetic pre-disposition. This is demonstrated most con-vincingly by the Turkish epidemic of PCTcaused by HCB intoxication. Other chemi-cals have also been linked to cases of porphy-ria in humans; the reported cases, however,have been infrequent, have all involvedPCT, and have generally been linked tochronic industrial exposures, industrial acci-dents, or environmental exposures thatwere much higher than normally encoun-tered. Chemical exposures have not beenestablished as a cause (i.e., not just a triggerin the presence of genetic predisposition) ofporphyrias with primarily neurologic mani-festations, although lead intoxication isnoteworthy as a possible exception.

There is no doubt that lead absorptioncan cause substantial disturbance of hemesynthesis. Lead intoxication has prominentmanifestations as well as biochemical abnor-malities that mimic the manifestations ofthe neurologic porphyrias. It is not knownto what degree the neurologic manifesta-tions of lead poisoning might be attribut-able (if at all) to the associated disturbancein heme synthesis, apart from the directlyneurotoxic properties of lead, but there isevidence the disturbed heme-formingsystem may have some contributing role.

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It must be acknowledged that therehave been only a limited number of system-atic attempts to identify porphyria or evensubclinical porphyrinuria in chemicallyexposed humans, and that a large numberof chemicals have been identified as por-phyrinogenic in experimental animals-albeit with doses much greater than theusual range of human experience. In somecases, these effects result from inhibition ofor interference with specific enzymes in theheme synthesis pathway; however, some ofthese effects can be produced by mecha-nisms that do not involve an abnormalityin the heme-forming system. Regardless ofthe causative mechanism, these chemical-related changes have potential utility asbiological indicators of chemical exposuresthat might have significant pathologiceffects. There is a need for further research,including trial field applications, of suchporphyrin biomarkers.

Clinicians should maintain a reasonabledegree of suspicion for the possibility of adisturbance in heme synthesis underlyingany unexplained syndrome characterizedby nonspecific symptoms if even mildlyreminiscent of porphyria, whether or notenvironmental chemical exposures are sus-pected as causative or contributing agents.However, heme-precursor measurementsobtained when a patient is symptomaticshould be interpreted cautiously and/orrepeated if not substantially elevated (i.e.,at least 2-fold greater) relative to the respec-tive value reported for the upper limitof normal.

With the noteworthy exceptions of leadintoxication and the porphyrias in clini-cally active states, the porphyrin excessassociated with porphyrinurias has noknown clinically detectable consequencesof its own. Symptoms associated with suchsecondary porphyrinurias are attributed bymost reviewers to the condition or agentcausing the porphyrinuria or to an unre-lated cause and not to a disturbance inheme synthesis. Such porphyrinuria maybe relevant as a nonspecific laboratory indi-cator of some other operative pathophysio-logic mechanism, much as an elevatedsedimentation rate can provide contribu-tory but nonspecific evidence of disease;however, it is most appropriate for furtherdiagnostic attention to focus on the pri-mary condition or exposure of concern,not necessarily on a mild disturbance ofheme synthesis.A variety of chemical-associated illnesses

with unknown causative mechanisms,notably MCS syndrome, have been reported

in association with abnormal results ofporphyria-related tests. It is hypothesizedthat these test values reflect pathophysio-logic disturbances of heme synthesis thatare directly involved in the manifestationof those illnesses. Proponents of thesehypotheses point out that many MCSsymptoms resemble the neurologic andcutaneous manifestations of porphyrias.Yet, cutaneous symptoms are usually not amajor feature of MCS syndrome, andwhen present, they differ substantiallyfrom the cutaneous manifestations of theinherited and acquired porphyrias. Thenoncutaneous symptoms of MCS syn-drome are much less intense and lessdiscretely episodic than the common neu-rologic manifestations of porphyrias. Theclinical course of porphyrias with neuro-logic manifestations can, as with MCS syn-drome, occur in a chronic or indolentpattern, and does not occur exclusivelyas acute symptom attacks separated bysymptom-free intervals; however, in theneurologic porphyrias, in contrast to MCSsyndrome, the chronic pattern occursmuch less frequently than the pattern ofacute symptom attacks.

Proponents of porphyric hypotheses forMCS syndrome point out another similar-ity with the porphyrias-the propensity forsymptoms to be caused or triggered byexogenous chemical exposures. However,the very low levels of exposure to whichMCS patients are characteristically sensitiveare much lower than the pharmacologicaldoses of drugs that are known to triggerneurologic symptoms in genetically predis-posed individuals. These levels are alsomuch lower than the chemical exposuresreported to date in association with PCT orsubclinical porphyrinurias in humans,where situations generally have involvedchronic industrial exposures, industrial acci-dents, or environmental exposures thatwere much higher than normally encoun-tered, and with the exception of lead intoxi-cation, have not been reported to involvelow-level environmental exposures.

It is conceptually difficult to reconcilewhy MCS syndrome, if mediated substan-tially through a disturbance in heme syn-thesis, would have a markedly greaterdegree of chemical intolerance and greaterfrequency of chronic functional limitationsthan recognized forms of prophyria butwould never manifest symptoms or signs assevere as can occur with porphyrias, andwould have markedly lower or even absentelevations of heme-precursor excretionwhen symptomatic compared to porphyria

patients when symptomatic. One contentionis that circumscribed disturbances of hemesynthesis (i.e., limited to a small number ofcells) can cause symptoms without produc-ing measurable or substantial increases inheme precursors and that heme-synthesisenzyme measurements, therefore, are themore critical, if not the only necessary,diagnostic measure. Although it is well rec-ognized that heme synthesis can be dispro-portionately affected in different organsystems with any one type of porphyria, itis contrary to other clinical evidence todate (with rare exceptions) that a distur-bance of heme synthesis is so profound asto produce symptoms without also produc-ing substantial elevations of heme precur-sors. Similarly, there is no other evidenceto date that a heme-synthesis enzymewith low activity, particularly when mar-ginal activity values are overinterpreted aslow, can be linked to symptoms withoutan accompanying substantial increase inheme precursors.

It is sometimes contended in individualcases that the timing of specimen collectionwas not sufficiently dose in time to a symp-tom episode to depict the full degree ofheme-precursor abnormalities associatedwith that individual's symptoms. Althoughthe biochemical abnormalities of some por-phyrias may return to normal ranges duringperiods of clinical remission, such normal-ization generally requires at least severaldays or weeks, if it occurs at all. Given thetypical frequency of symptom episodes inindividuals with MCS syndrome, and if adisturbance of heme synthesis is an opera-tive pathophysiologic mechanism, the tim-ing of specimen collection should not bean issue.

To date, the efforts to relate MCSsyndrome and other unexplained chemical-associated illnesses to disturbances of hemesynthesis have all involved selected caseseries with no consistent or stated case defi-nitions, using only laboratory referenceranges for comparisons, and with little orno attention to the methodologic andinterpretive limitations of measures ofheme synthesis. Based on the limited exist-ing data, it is reasonable at least to considerthe possibility that there might be a higherthan previously recognized prevalence ofabnormalities in measures of heme synthe-sis among patients with MCS syndrome.However, to date there is no convincingevidence that there is-or is not-anysuch increased prevalence of abnormalmeasures of heme synthesis associated withMCS syndrome. Given the current paucity



of evidence in favor of disturbances ofheme synthesis being operative in MCSsyndrome and other unexplained chemical-associated illnesses, and unless supportiveevidence can be provided by well designedand controlled studies, the proposedrelationships with disturbances of heme

synthesis should, at most, be consideredspeculative and unestablished. In casesinvolving these otherwise unexplained ill-nesses, and when there is no demonstrablesubstantial accumulation or increasedexcretion ofheme precursors, it is prematureor unfounded either to apply a diagnosis of

porphyria (or any less defined pathologicdisturbance of heme synthesis) or to usesuch diagnoses as specific justification forlifestyle alterations or therapeutic interven-tions; porphyria-specific interventions,such as intravenous hematin, should not berecommended in such instances.

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