LETTERS TO THE EDITOR

DIAGNOSTIC YIELD OF DIFFERENT ELECTROPHYSIOLOGICAL METHODS IN CARPAL TUNNEL SYNDROME In a recent issue of Muscle &f Nerve, Uncini et a15 claim that stimulation of finger 4 (D4) is the most sensitive method for diagnosting carpal tunnel syndrome (CTS), due to a characteristic double peak potential recorded over median and ulnar nerves by means of surface elec- trodes.

It is true that the topography of the different branches of the median nerve could predispose D4 fi- bers to mechanical compression in the carpal tunnel,' but in our experience the examination of D4 does not always show the earliest diagnosis or the most severe changes in conduction velocity.

In a series of 40 consecutive patients with mild symptoms consistent with CTS, including one infantile case, the following techniques were performed: Distal motor latency (DML) from the wrist to abductor pollicis brevis muscle; median sensory nerve conduction veloc- ity (SCV) digit-wrist on stimulating digits 1 , 3, and 4 (Dl , D3, D4); sensory conduction velocity digit-to-palm and palm-to-wrist on stimulating D3; sensory conduc- tion velocity palm-to-wrist by palm stimulation, and me- dian motor conduction velocity (MCV) across the carpal tunnel on stimulating the nerve at the wrist and palm. These techniques were always performed by means of surface stimulation and recording in accordance with previously reported methods.374

The diagnostic yield of each electrophysiological test and the different combined techniques made at the same time are summarized in Table 1. The highest di- agnostic yield was obtained when all tests were per- formed. Double peak potential on stimulating D1 or D4 had high but not the highest diagnostic significance. In 2 patients, only the symptomatic digit showed this ab- normal shape, whereas the asymptomatic one had a normal shape. It is well known that the clinical manifes- tations of CTS are variable. Paresthesia was the most frequent manifestation of nerve entrapment in a series of 200 consecutive cases of mild CTS recently reported.' Paresthesia varied and could be referred to

Table 1. Diagnostic yield of different electrophysiological tests in 40 patients with mild CTS.

Abnormal results ~ ~~

N Yo Comments

DML

SCV digit-palm and SCV D3-W

palm-W (D3) SCV D1-W

SCV D4-W

SCV D1-W + SCV D4-W

MCV W-Dalm

SCV Dalm-W

MCV W-palm + SCV palm-W

SCV digit-W + MCV W-palm + SCV palm-W

26 33 33

34

34

35

37

37

38

40

65.0 82.5 82.5

85.0

85.0

87.5

92.5

92.5

95.0

100.0

One patient had abnormal SCV Dl-W but normal SCV D4-W.

One patient had abnormal SCV D4-W but normal SCV D1 -W.

One patient had abnormal SCV palm-W but normal MCV W-palm.

One patient had abnormal MCV W-palm but normal SCV palm-W.

DML = Distal motor latency. W = Wrist, D I , 03, 04, = digits 1 , 3, and 4. SCV = Sensory conduction velocity. MCV = Motor conduction velocity.

two, three, four, or all fingers, but rarely to only one in the earliest stages of the entrapment.' At this time the stimulation of the symptomatic digit can provide the earliest information for the diagnosis of the CTS (Fig- ure 1).

MUSCLE & NERVE February 1991 183

R L -

FIGURE 1. Slightly abnormal shape with double but not separate peak on stimulating D4, in a patient in the early stage of CTS, in who the only symptom was bilateral paresthesiae in the fourth digit. Examination of SCV on stimulating digits 1 and 3 and mo- tor and sensory conduction velocities across the wrist were within the normal range. This is one example of the importance of examining the symptomatic digit for early diagnosis of CTS.

Our results are in accordance with the ones of Un- cini et al' and suggest that the stimulation of D4 is a sensitive method for the diagnosis of CTS. However, in our series, the stimulation of D4 does not necessarily provide the highest diagnostic yield of the syndrome.

A careful examination of the testing of 40 cases of mild CTS results in the following conclusions: 1. DML has the lowest diagnostic yield. 2. The performance of more tests when the ones

first performed (DML, SCV digit-wrist, abnor- mal shape of evoked potential on stimulating D1 and D4) are not conclusive improves the di- agnosis (Table 1).

3. The stimulation of the symptomatic digit was the first and most sensitive method for the di- agnosis of CTS in the earliest stage of the dis- ease.

4. The impairment of motor fibers with reduced conduction velocity wrist- palm and/or SCV on stimulating the palm can have a higher diag- nostic yield than other electrophysiological techniques.'*6

A. Cruz Martinez, MD, PhD Seccion de Electromiografia Hospital La Paz 28046 Madrid, Spain

1. Buchthal F, Rosenfalck A, Trojaborg W: Electrophysiologi- cal findings in entrapment of the median nerve at the wrist and at the elbow. J Neurol Neurosurg Psychiatry 1974;37:340- 360.

2 . Cruz Martinez A: Diagn6stico y tratamiento del sindrome del tunel carpiano con especial referencia a 10s resultados del estudio electrofisiol6gico. Neurologi'a (Barcelona). 1989; 4:26-46.

3. Cruz Martinez A, Barrio M, Perez Conde MC, Gutierrez

AM: Electrophysiological aspects of sensory conduction ve- locity in healthy adults. 1 -Conduction velocity from digit to palm, from palm to wrist, and across the elbow as a func- tion of the age. J Neurol Neurosurg Psychiatry 1978;41:1092- 1096.

4. Cruz Martinez A, Barrio M, Perez Conde MC, Ferrer T: Electrophysiological aspects of sensory conduction velocity in healthy adults. 2-Ratio between the amplitude of sen- sory evoked potentials at the wrist on stimulating different fingers in both hands. J Neurol Neurosurg Psychiatry

5. Uncini A, Lange DL, Solomon M, Soliven €3, Meer J, Lovelace RE: Ring finger testing in carpal tunnel syndrome: A comparative study of diagnostic utility. Mwcle Nerve 1989;12:735-741.

6. White JC, Hansen SK, Johnson RK: A comparison of EMG procedures in the carpal tunnel syndrome with clinical EMG correlations. Muscle Nerve 1988;11:177- 182.

1978;41:1097- 1101.

DIAGNOSTIC YIELD OF DIFFERENT ELECTROPHYSIOLOGICAL METHODS IN CARPAL TUNNEL SYNDROME: A REPLY We are pleased that Dr. Cruz Martinez confirmed our results' on the high sensitivity of fourth digit testing in carpal tunnel syndrome (CTS). Regarding the compar- ison with the diagnostic yield obtained with other tech- niques, we think that the population we studied (all our CTS patients had normal distal motor latency: i.e. <4.2 ms) is different from the one reported by Dr. Cruz Martinez (65% of his patients had abnormal distal mo- tor latencies). The severity of the entrapment may af- fect the success of the various techniques. In fact, in se- vere CTS cases the fourth digit potential is usually absent.

In general, however, we agree with his conclusion that there are many different ways to diagnose CTS, and when all are used, the diagnosis should be rarely missed.

A. Uncini D.J. Lange Clinical Neurophysiology Laboratories Columbia Presbyterian Medical Center Department of Neurology College of Physicians and Surgeons of Columbia University New York. NY 10032

1. Uncini A, Lange DJ, Solomon M, Soliven B, Meer J, Lovelace RE: Ring finger testing in carpal tunnel syn- drome: A comparative study of diagnostic utility. Mwcle Newt. 1989; 12: 735- 74 1.

184 MUSCLE 8, NERVE February 1991

CONDUCTION BLOCK IN NEUROPATHIES WITH NECROTIZING VASCULITIS We question the conclusions reached by Ropert and Metral in their recent article.6 It has become increas- ingly clear that a drop in the recorded compound mus- cle action potential (CMAP) amplitude from proximal to distal stimulation is insufficient evidence upon which to conclude the presence of conduction block. There are several alternative interpretations which must be considered.

First, and most obvious, is the technical artifact from submaximal proximal stimulation. We know the authors to be experienced electromyographers and doubt that this is the explanation in the present article, but it is a frequent cause for the spurious diagnosis of conduction block with proximal sites of stimulation where the nerve trunk is deeply located.

Second, in cases where the distal evoked amplitude is reduced and likely to be composed of a few remain- ing motor units, the stimulation at a more proximal site may be associated with interphase shift and cancella- tion, resulting in a further reduction of amplitude of the CMAP. This can occur without abnormal temporal d i~pe r s ion .~ We think this is the likely explanation for the results reported in neuropathies with necrotizing vasculitis.

Third, the findings presented could be the result of an acute ischemic lesion occurring in the segment of nerve between the proximal and distal stimulation sites within a few days preceding the electrophysiological study. This parallels the situation present after nerve section, in which distal stimulation evokes a response but proximal stimulation fails to evoke a response. Se- rial studies over the next several days would show a progressive drop in amplitude of the distal CMAP.2 Thus, it is important to perform sequential recordings when studying acute vascular lesions of nerve.

If the described findings are due to true partial con- duction block in the nerve segment between the stiniu- lating electrodes, this should be determined by apply- ing the criteria of Brown and Feasby.’ It is important, at this time when clinicians are increasingly aware of the therapeutic implications of conduction that this conclusion be reached using strict electrophysiolog- ical criteria and awareness of the possible pitfalls in in- terpretation.

David R. Cornblath, MD Department of Neurology Johns Hopkins University School of Medicine Baltimore, MD 21 205 Austin J. Sumner, MD Department of Neurology LSU Medical Center New Orleans, LA 701 22-2822

1. Brown WF, Feasby TE: Conduction block and denervation in Guillain-Barre polyneuropathy. Rrairi 1984; 107:2 19- 239.

2. Gilliatt RW, Taylor JC: Electrical changes following section of the facial nerve. Proc Royal Soc Med 1959;52: 1080- 1083.

Pestronk A, Cornblath DR, Ilyas AA, et al: A treatable mul- tifocal motor neuropathy with antibodies to GM l ganglio- side. Ann Neuiol 1988;24:73-78. Pestronk A, Chaudhry V, Feldman EL et al: Lower motor neuron syndromes defined by patterns of weakness, nerve conduction abnormalities and high titers of antiglycolipid antibodies. Ann Neurol 1990;27:3 16-326. Rhee EK, England JD, Sumner AJ: A computer simulation of conduction block: Effects produced by actual block ver- sus interphase cancellation. Ann Neurol (in press). Ropert A, Metral S: Conduction block in neuropathies with necrotizing vasculitis. Muscle Nerve 1990; 13: 102- 105.

CONDUCTION BLOCK IN NEUROPATHIES WITH NECROTIZING VASCULITIS: A REPLY The didactic comments of Drs. Cornblath and Sumner’ regarding our findings4 are appreciated. First, we thank the authors for their confidence in our experi- ence. We agree with the need for strict criteria regard- ing analysis of the compound muscle action potential amplitude when making a determination of conduction block. An interesting method for identification of mo- tor conduction block despite desynchronization has re- cently been p ~ b l i s h e d . ~ In our article, we used and agree with the criteria of Brown and Feasby (see our ref. 8). The electrophysiologic finding of conduction block is of great interest for both theoretical and practi- cal reasons.

Second, while we know of the single abstract on this subject by these authors,” certainly we could not discuss those arguments about phase cancellation without tem- poral dispersion, which they have proposed in their un- published reference. Phase cancellation is a well-known phenomenon; however, as Kimura et a12 said, this is much more pronounced for sensory responses than compound muscle action potentials. This would be par- ticularly true when only a few axons remain conducting without evidence of temporal dispersion. This was the situation in our cases, as evidenced by the very low am- plitude of the distal CMAP. We see no reason to sus- pect that phase cancellation without temporal disper- sion should be particularly likely in cases of necrotizing vasculitis.

Finally, the main point of our observation is that re- versible conduction block, mimicking that generally as- sociated with demyelinative processes, may be observed in axonal dysfunction of ischemic origin. We feel that the disappearance of conduction block associated with an improvement of distal CMAP amplitude, as seen in two of our cases, is a strong argument for what we con- sider transitory anoxic conduction block. This is very different from the conduction block seen with acute traumatic axonal neuropathy, which is invariably fol- lowed by a progressive drop in the distal CMAP ampli- tude.

Stephane Metral, MD Hopital de Bicetre 94275 Kremlin Bicetre, France

MUSCLE & NERVE February 1991 185

1. Cornblath DR, Sumner A: Conduction block in neuropa- thies with necrotizing vasculitis. Muscle Nerve 1990; 13.

2 . Kimura J, Machida M, Ishida T, et al: Relation between size of compound sensory or muscle action potentials, and length of nerve segment. Neurology 1986;36:647-652.

3 . Rhee EK, England JD, Sumner A: A computer simulation of “conduction block’ produced by phase cancellation. (Ab- stract) Muscle Nerve 1987;10:645-646.

4. Ropert A, Metral S: Conduction block in neuropathies with necrotizing vascuhtis. Mutcle Nerve, 1990; 13: 102- 105.

5 . Roth G, Magistris MR: Identification of motor conduction block despite desynchronisation: A method. Electromyogr Clan Neurophysiol 1989;29:305-3 13.

186 MUSCLE & NERVE February 1991