Electrodiagnostic testing in hand surgery

948 Lettersto the Editor We believe the problem of repetitive stress injury (RSI)/(CTD) is a multifactorial problem involving physical, individual, and psychosocial factors. Specific reference to CTS represents only a small population of patients within the large context of RSIs. Identification of the multiple factors that are associated with these problems is necessary for successful m a n a g e m e n t of this complex issue. Only by dealing with this problem in a fair, open-minded way can we hope to solve the potential C T D "crisis."

Susan E. Mackinnon, MD Christine B. Novak, PT, MS Washington University School of Medicine Suite 17424 East Pavilion One Barnes Jewish Hospital Plaza St. Louis, MO 63110

References 1. Mackinnon SE, Novak CB. Clinical perspective: repetitive strain in the workplace. J Hand Surg 1997;22A:2-18. 2. Nathan PA, Keniston RC, Myers LD, Meadows KD. Obesity as a risk factor for slowing of sensory conduction of the median nerve in industry: a cross-sectional and longitudinal study involving 429 workers. J Occup Med 1992;34: 379-383. 3. Nathan PA, Meadows KD, Doyle LS. Occupation as a risk factor for impaired sensory conduction of the median nerve at the carpal tunnel. J Hand Surg 1988;13B: 167-170. 4. Hales TR. Letter to the editor. J Hand Surg 1991;16B: 230-231.

5. Nathan PA. Response to letter to the editor. J Hand Surg 1991;16B:231-232. 6. Hales TR, Bernard BP. Epidemiology of work-related musculoskeletal disorders. Orthop Clin North Am 1996;27: 679-709.

Electrodiagnostic Testing in Hand Surgery To the Editor: We found the Clinical Perspective article, "Electrodiagnostic Testing in Hand Surgery," by Dr. Campion to be interesting and helpful.l In the section on carpal tunnel syndrome (CTS), however, the ranking of the tests "in sequence of progressive abnormality as CTS becomes more pronounced" is considerably different than has been our experience. The Kimura "inching" technique, with a 0.40-ms critical value, has been found to be more sensitive than any of the other standard electrodiagnostic measurements2, 3 (Table 1). On the basis of our findings, the ranking of the Kimura "inching" technique in Dr. C a m p i o n ' s sequence would be accurate for the 0.50-ms critical value, as used by Ross and Kimura, 4 but not for the 0.40-ms critical value as shown in the article. In our experience, the 8-cm median/ulnar comparison is more sensitive than the 14-cm sensory distal latency or the 0.50-ms critical value for the Kimura technique. Because the efficiency of classification does not differ much (Table 1), we consider it important to use the most sensitive measurements.

Table 1. Sensitivity, Specificity, and Efficiency of Classification of Electrodiagnostic Measurements for Diagnosing Carpal Tunnel Syndrome*

Electrodiagnostic Measurement Any electrodiagnostic abnormality Kimura technique of 1-cm inching: normal value < 0.40 ms/1-cm segment Palmar latency: normal value < 2.2 ms at 8-cm distance Comparison of median sensory latency to ulnar sensory latency using the ring finger, 8-cm distance; normal value < 0.40 ms Sensory distal latency to index: normal value < 3.6 ms at 14-cm distance Kimura technique of l-cm inching: normal value < 0.50 ms/1-cm segment Motor distal latency: normal value < 4.2 ms at 8-cm distance

Sensitivity (%)

Specificity(%)

Efficiency(%)

93.5 84.4

78.3 80.5

83.0 81.8

72.3

87.0

81.9

62.7

90.0

80.6

55.3

92.2

79.5

55.0

92.2

81.1

37.6

93.3

74.6

*Carpal tunnel syndrome diagnosis based on medical history, specific hand/wrist symptoms (numbness, tingling, nocturnal awakening), and response to treatment (n = 7,466 hands of industrial workers and patients).

The Journal of Hand Surgery / Vol. 22A No. 5 September 1997

We agree with Dr. Campion that the diagnosis of CTS ideally should be confirmed by the finding of more than 1 electrodiagnostic abnormality. In our experience, this almost always includes an abnormal maximum latency difference (MLD) with the 0.40ms critical value. As we have, Seror 3 has found the MLD with a 0.40-ms critical value to be the most valuable measurement for assessment of the mildest forms of CTS. As noted by Dr. Campion, the Kimura "inching" technique requires more time to perform than 8-cm or 14-cm latencies, but it also localizes the nerve lesion. 3-5 Precise localization of the nerve lesion is useful in helping to understand the etiology of CTS. 2,5,6 We obtain the standard wrist-palm (8-cm) and wrist-finger sensory latencies (14-cm) as part of the "inching" technique; this amounts to less than 10 minutes per side, which is more than offset by the value of the additional diagnostic information. We have found M L D patterns to be highly reproducible over time. Based on 5-year2,6 and 11-year follow-up studies, the MLD with a 0.40-ms critical value detects abnormalities sooner than does any other electrodiagnostic measurement and correlates better with duration of neuropathy and progression of symptoms. In our experience, the 0.40-ms critical value is greater than 2 SDs from the mean value for healthy adult subjects, 2 and 66% of subjects (workers and patients) with an MLD of exactly 0.40 ms report specific hand/wrist symptoms consistent with CTS. We recommend the MLD (0.40 ms) to anyone who is investigating CTS complaints, particularly for patients for whom there are "normal" electrodiagnostic findings according to other measurements. 2,3

Peter A. Nathan, MD Richard C. Keniston, MD Kenneth D. Meadows, PT Richard S. Lockwood, BSc Portland Hand Surgery and Rehabilitation Center 2455 N. W. Marshall, Suite 1 Portland, OR 97210-2997

References 1. Campion D. Electrodiagnostic testing in hand surgery. J Hand Surg 1996;21A:947-956. 2. Nathan PA, Keniston RC, Meadows KD, Lockwood RS. Predictive value of nerve conduction measurements at the carpal tunnel. Muscle Nerve 1993;16:1377-1382.

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3. Seror P. Sensitivity of the various tests for the diagnosis of carpal tunnel syndrome. J Hand Surg 1994;19B:725-728. 4. Ross MA, Kimura J. AAEM case report #2: the carpal tunnel syndrome. Muscle Nerve 1995;18:567-573. 5. Nathan PA, Srinivasan H, Doyle LS, Meadows KD. Location of impaired sensory conduction of the median nerve in carpal tunnel syndrome. J Hand Surg 1990;15B:89-92. 6. Nathan PA, Keniston RC. Carpal tunnel syndrome and its relation to general physical condition. Hand Clin 1993; 9:253-261. In Reply: I thank Dr. Nathan and his colleagues for their comments. The merit of the order of carpal tunnel syndrome (CTS) tests I listed is that it is based on sound physiologic principles, but experience, technical expertise, and a dialogue between hand surgeon and electrodiagnostic physician may, as Dr. Nathan indicates, justify more reliance on 1 of the tests than on others. Their preference for maximum latency differential (MLD) with a 0.4-ms critical value is well founded in their very extensive experience with industrial workers, but I have found that in a more general EDX practice, in which many of the patients referred for CTS studies are over 65 years of age, the M L D technique, with its multiple sites of stimulation, is less well tolerated than the 8.0-cm palmar latency or the median/ulnar latency comparison at the ring finger.

David S. Campion, MD 436 North Bedford Drive, Suite 311 Beverly Hills, CA 90210

Carpal Tunnel Pressure To the Editor: The article by Seradge et al., "In Vivo Measurement of Carpal Tunnel Pressures in the Functioning Hand" (J Hand Surg 1995;20A:855-859), claims that the wick catheter provides spurious high measurements of carpal tunnel pressure (CTP) because of enfolding of wick fibers into the catheter. We have never experienced this problem except when wick fibers are packed too tightly into a catheter. Therefore, we believe the reported problems were due to improper preparation of wick catheters. The use of their 2.3-mm outer diameter metal catheter presents problems of its own. First, this needle has twice the diameter as that of commonly used