288

Dermatologic Therapy, Vol. 18, 2005, 288–291 Printed in the United States · All rights reserved

Copyright © Blackwell Publishing, Inc., 2005

DERMATOLOGIC THERAPY

ISSN 1396-0296

Blackwell Publishing, Ltd.

Itch and pruritus: what are they, and how should itches be

classified?

J

EFFREY

D. B

ERNHARD

University of Massachusetts Medical School, Worcester, Massachusetts

ABSTRACT:

Itch and pruritus are two terms for the same thing. In this essay I will argue that castingabout for a distinction between them creates only confusion. Once that matter is settled, it is stillnecessary to come up with a clinical classification for itches of different types. No system yet pro-posed, including the one that will be suggested here, is perfect.

KEYWORDS:

classification, itch, pruritus

“Faced with two terms for the same thing, one tendsto cast about for a distinction.”

†

W. V. Quine (1)

Introduction

Some writers used pruritus to signify itchingwithout visible skin lesions. Others use theterms,

essential pruritus

,

pruritus sine materia

,or

generalized pruritus

to describe itching with-out a rash as well. But such terms can be mislead-ing when a patient with itching caused by renalfailure or another underlying systemic diseasehas excoriations or a nonspecific rash created byrubbing and scratching. And does not a patientwith extensive, severe atopic dermatitis also havegeneralized pruritus? Although distinctions can be

drawn, and many have been, itch and pruritus aresynonymous. I prefer (and suspect that it is safer),to use a few extra words to describe different clinicalsituations whenever pruritus or itching is present.Casting distinctions away, would recommend thatitch and pruritus be used interchangeably (2). Iwould feel this way even if pruritus were not oneof the most frequently misspelled words in all ofmedicine (3,4). Pruritus is also practically impos-sible to pronounce.

†

Prurigo

presents some additional and contro-versial semantic problems of its own. On his tourof the western islands of Scotland, Samuel Johnsonnoted that the local inhabitants would speakauthoritatively whether they knew what they weretalking about or not, stating, “such is the laxity ofHighland conversation, that the inquirer is kept incontinual suspense, and by a kind of intellectualretrogradation, knows less as he hears more” (5).That is how I feel when reading about the variousforms of prurigo. I acknowledge that there is a verydistinctive disorder that all trained dermatologistswould recognize as

prurigo nodularis

, but I am notsure they would agree about what causes it. Theymight agree that there is something called prurigosimplex, something called prurigo simplex subacuta,and something called “itchy red bump disease,” –and that some cases of these may or may not bethe same as each other, or as papular urticaria.Curious readers with strong minds who are unafraid

Author’s Note: This paper is modified very slightly from the author’s handout at the Itch Forum at the Annual Meeting of the American Academy of Dermatology, New Orleans, LA, February 18, 2005, and as further modified from the abstract for a presentation at the EADV Spring Symposium, Sofia, Bulgaria, May 21, 2005.Address correspondence and reprint requests to: Jeffrey D. Bernhard, MD, FRCP, Edin, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, or email: bernharj@ummhc.org.

†

The introductory quotation and the following paragraph in this essay are taken nearly verbatim from the introduction to reference 2.

Itch: clinical classification

289

of intellectual retrogradation are referred to thestandard textbooks of dermatology to learn moreabout these terms. By the way, I do not accept theterm “itchy red bump disease,” or at best regard itas a perpetually provisional diagnosis, which letsthe doctor off the hook until the real diagnosis ismade (6).

The definition of itch

Savin suggested that itch be defined as “a sensa-tion that, if sufficiently strong, will provokescratching or the desire to scratch” (7). It may bepleasurable or not. As the present author pointedout at the time, “defining things is a pleasurableactivity that provokes quibbles or the desire toquibble.” So the only quibble I might have withSavin’s definition – which I think is otherwise justright – would be the need to acknowledge thatscratching may be either conscious or reflexive:

Itching is a sensation that, if sufficiently strong,will provoke either conscious or reflex scratchingor the desire to scratch.

Not all itches arise in the skin

Not all itches arise in the skin. Itches that arise asa consequence of pathology in the central orperipheral nervous system can be said to have a“neurogenic” or “neuropathic” origin; the termsare most often used interchangeably. Indeed,neuropathic is sometimes used to describe theparticular character of itch or pain of neurogenicorigin (for example: coexistent stinging, burning,tingling, pain, formication, paresthesia, hypesthe-sia, hyperesthesia, etc.). Notalgia paresthetica andbrachioradial pruritus are examples of two neuro-genic itches that often, but not always, have aneuropathic character. Meralgia paresthetica, anentrapment neuropathy of the lateral femoralcutaneous nerve, sometimes itches, but numb-ness or other paresthetic sensations are usuallymore prominent.

Although the final pathway of certain meta-bolic itches, such as cholestatic pruritus, mayinvolve chemicals, such as opioid peptides, thataffect the brain rather than the skin, I would arguethat such itches should not be considered prima-rily neurogenic, but rather simply “metabolic” or“systemic.” As I argued for simple synonymy inthe case of itch and pruritus, I would also arguethat attempting to make practical distinctionsbetween neurogenic and neuropathic is too taxing

to the brain and fraught with hazards, such as intel-lectual retrogradation, of its own.

Itch classification

An itch classification scheme has been proposedby Twycross and colleagues (8). It has four cate-gories, whose definitions the present author hastaken the liberty of paraphrasing:•

Pruritoceptive itch

: itch originating in the skin,as in scabies or urticaria

•

Neuropathic itch

: itch arising as a consequenceof neuroanatomic pathology, such as from abrain tumor

•

Neurogenic itch

: itch arising as a consequenceof neurochemical activity, such as the action ofopioid peptides in cholestatic liver disease

•

Psychogenic itch

: “as in the delusional state ofparasitophobia.”This classification is imperfect in several ways.

First, although itch may occur as a consequenceof neuroanatomic and/or neurochemical pathol-ogy, the terms neuropathic and neurogenic arenot customarily employed to make such a distinc-tion, not even by neurologists. Neuropathic isoften used to describe the character of pain andmay also be used to describe the character of itch-ing of neurogenic origin (which may burn, tingle,prickle, or display other paresthetic aspects). Ibelieve, as I have just argued, that neuropathicand neurogenic should be considered synonymousfor practical clinical purposes. If by a Procrusteaneffort the terms were forced to take on the burdenof the distinction suggested in the Twycrossscheme, it would still be impossible to rememberwhich is which. I do admire the attempt toaddress the pathophysiological nature of itch aris-ing in the skin by naming it pruritoceptive (byanalogy with the term nociceptive, which is usedto describe pain). But although this term could beadopted as a scientific one that addresses thephysiology of itch, it is not useful clinically. Theterm has other problems as well. Like pruritus, itis not easy to pronounce. In the end it is alsorather too broad, and does not encourage clinicalor pathophysiologic subcategorization of itches aris-ing in the skin. I am not aware that “nociceptive” iswidely used to describe or classify pain in theclinic: in the end it is too broad despite its utilityin the study of sensory physiology. While

nocicep-tor

is used by scientists to describe pain receptorsin the skin, I like “itch receptor” more than theterm “pruritoceptor,” which, by analogy withnociception, would presumably follow from

Bernhard

290

accepting “pruritoceptive.” By the way, I suspectthat “pruritoceptive” as a term for describing itchoriginating in the skin is, if not tautological, stilllikely to fall subject to Quine’s razor.

Another problem, as the authors point out, isthat “there is no reason why one type of itch can-not coexist with another – for example, in the itchof atopic eczema, neurogenic as well as prurito-ceptive itches seem to arise in the same patients.”I would point out that atopic eczema might alsoinvolve neuropathic aspects, as they define theterm, when lichenification with neural prolifera-tion is present. And the itch of atopic dermatitiscan certainly assume psychogenic features aswell. In other words, atopic dermatitis can fall, toone degree or another, in each of their four cate-gories. Although overlapping or mixed categoriesare going to occur in any clinical categorization ofitch (including the one I am about to propose),there just are not enough categories in the Twy-cross scheme to be helpful in the clinical approachto the patient.

In a clinical classification scheme, the term“dermatologic” would be substantially more use-ful than pruritoceptive, although not withoutsome problems of its own. For these and otherreasons, I cannot agree with the claim that theTwycross classification is “clinically relevant,”although parts of it go at least part way in formu-lating a useful approach to categorizing the“pathomechanisms of pruritus.” Perhaps twoclassification schemes will be needed, one todescribe the basic scientific, pathophysiologicalaspects, and one that could help doctors in theclinic figure out what is wrong with their patients.

Erasmus said that thanks to folly, there are asmany grammars as there are grammarians. With anod to Erasmus, I offer here a slightly modified

version of a clinical itch classification scheme hepresented at the American Academy of DermatologyAnnual Meeting Itch Symposium on March 21,2003 (Table 1). I have found it helpful in the clinic.It meets several particular requirements for clini-cal practice: (a) it is relatively unambiguous (e.g.,it does not attempt to make a fine distinctionbetween neurogenic and neuropathic); (b) it isrelatively easy to remember; and (c) it may behelpful in defining an approach to the evaluationof a patient with itching, whether there is a rashor not. I think it could still bear some discussionand improvement.

How would such a scheme work in practice?Let us take several simple and then several morecomplex and challenging examples. Dermatitisherpetiformis is a dermatologic disease of immu-nologic origin: it would fall under “dermatologic”and “systemic.” Psoriasis and atopic dermatitiswould be considered primarily dermatologic,although it is obvious that they have “systemic”origins as well. Systemic mastocytosis would fallunder systemic; cutaneous mastocytosis wouldfall under dermatologic. Although itch is notordinarily a feature of the lesions of cutaneouslupus, in this categorization a patient with itchycutaneous lupus (which does occur), would fallunder the dermatologic, systemic, and mixed cate-gories. The patient who presents with generalizeditching and no readily detectable cutaneous abnor-malities might fall into the systemic category as aconsequence of something like Hodgkin’s disease orrenal failure, but could also have a strictly derma-tologic condition such as very subtle dryness andwinter itch. Cholestatic itch would fall under thesystemic category (underlying systemic disease)as well as the neurochemical subtype of neuro-genic itch (as the mechanism involves opioid

Table 1. Clinical categories of itch

Dermatologic: arising from diseases of the skin such as psoriasis, eczema, winter itch, scabies, and urticaria

Systemic: arising from diseases of organs other than the skin, such as primary cholestatic liver disease, renal failure, Hodgkin’s disease, polycythemia vera, and certain multifactorial (e.g., metabolic) situations

Neurogenic/neuropathic: arising from diseases or disorders of the central or peripheral nervous system, such as brain tumor, multiple sclerosis, neuropathy, and nerve compression or irritation (e.g., notalgia paresthetica and brachioradial pruritus). Such itches often have a neuropathic character. This category also includes several systemic disorders in which itch arises as a consequence of chemical or metabolic factors.Subtype: neuroanatomic (as in entrapment neuropathies)Subtype: neurochemical (as in liver disease)

Psychogenic: “purely psychological” or “purely psychiatric”

Other: because all classification schemes need an other

Mixed: because categories may overlap and/or coexist

Itch: clinical classification

291

peptides acting on the nervous system).In the evaluation of a patient with generalized

pruritus and heretofore undetected cholestasis,the proposed classification scheme is helpfulbecause it directs (or shall we say enables), thephysician to consider systemic as well as derma-tologic and other causes of itching without a rash.The presence, absence, or nature of skin lesions isnot an explicit feature of the proposed classifica-tion scheme because the distinction breaks downtoo easily. For example, scabies incognito is a der-matologic disease that can present with virtuallyundetectable skin changes, whereas a patient withHodgkin’s disease can have notable (e.g., eczema-tous) cutaneous changes created by rubbing andscratching, although it is not a skin disease per se.In any event, a classification scheme is only onepart of an overall approach to the patient withpruritus of undetermined origin. Several ways ofthinking may be employed together; we haverecently presented an algorithmic approach basedon whether the itch is localized or generalized,and whether detectable cutaneous abnormalitiesare present or not (9).

When it comes to experimental research andto categorizing diagnosed itches according tounderlying pathophysiology, something closer to

the Twycross scheme will probably be useful inaddition to the clinical scheme I have proposedhere. A first go at such a pathophysiologicalscheme, eliminating the distinction betweenneuropathic and neurogenic, is provided inTable 2. I do recognize that in arguing that neuro-genic and neuropathic should be used inter-changeably, I am open to an argument made byJoseph Williams, namely, “When you use twowords for one concept, you risk making yourreader think you mean two concepts.” (10) I don’t.Perhaps such paradoxical concerns and inconsis-tencies are inevitable in thinking about the mostprominent and zen-like of dermatological symp-toms – itch, an invisible sensation in the mostvisual of medical specialties.

References

1. Quine WV. Quiddities. An intermittently philosophicaldictionary. Cambridge, Massachusetts: Harvard UniversityPress, 1987: 24.

2. Bernhard JD, ed. Itch: mechanisms and management ofpruritus. New York: McGraw-Hill, 1994.

3. Bernhard JD. On the dysspelling of pruritus. Fitz J ClDermatol 1994:

2

: 11.4. Bernhard JD. Pruritus. Lancet 1995:

345

: 1584.5. Johnson S. Johnson’s Journey to the western islands of

Scotland and Boswell’s journal of a Tour to the Hebridgeswith Samuel Johnson, LL.D. RW Chapman, ed. London:Oxford University Press, 1970: 45.

6. Bernhard JD. Itchy papular eruptions. In: Bernhard JD, ed.Itch: mechanisms and management of pruritus. New York:McGraw-Hill, 1994: 77.

7. Savin JA. How should we define itching? J Am Acad Derma-tol 1998:

38

: 268–269.8. Yosipovitch G, Greaves MW, Schmelz M. Itch. Lancet 2003:

361

: 690–694.9. Ward J, Bernhard JD. Willan’s itch and other causes of

pruritus in the elderly. Int J Dermatol 2005:

44

: 267–273.

10. Williams JM. Style. Toward clarity and grace. Chicago: TheUniversity of Chicago Press, 1990: 87.

Table 2. A first stab at a provisionalpathophysiologic classification of itch

CutaneousSubtype: MechanicalSubtype: Inflammatory or immunologicSubtype: NeoplasticSubtype: Other

Neurogenic/neuropathicSubtype: NeuroanatomicSubtype: neurochemical

Psychogenic/psychologic