Diagnosis of carpal tunnel syndrome

Abstract

SIR, I read with interest the paper by O'Gradaigh and Merry on the use of Bayes's theorem to produce a diagnostic algorithm for carpal tunnel syndrome [1]. Bayes's theorem may have a useful place in clinical practice, as also indicated by our own previous study [2] with regard to isotope scanning of sacroiliac joints in suspected inflammatory spondylitis as well as with HLA-1327 typing of these patients. As pointed out by the authors, our own recent study did reveal long waiting times for the simple nerve conduction tests that are so useful in evaluating patients with paraesthesia, including suspected carpal tunnel syndrome [3]. I would, however, like to voice some concerns on diagnosing the majority of patients using the suggested algorithm and without the use of nerve conduction tests, as advocated by the authors for a number of reasons. First, I would like to question the use of a single criterion for diagnosing carpal tunnel syndrome in this study (the definitive test being sensory amplitude < 10 mV or motor latency > 3.7 ms). In my opinion a better validated study is necessary (before advocating less use of nerve conduction tests) with more stringent criteria for carpal tunnel syndrome that are generally acceptable not only for scientific studies but also in routine practice. The authors do not indicate which nerves they tested to indicate the sensory amplitude and motor latency, but one would presume that they mean the median nerves. For more diagnostic accuracy for carpal tunnel syndrome other criteria need to be included, such as median nerve sensory velocity, median nerve motor latency compared with the ulnar nerve motor latency in the same hand, and indeed compared with the other hand (especially if asymptomatic). In most centres, to the best of my knowledge, median nerve motor latency > 3.7 ms may still be normal, as on average the accepted maximum normal value for this test is 4.0 ms [4], but even this is arbitrary as people's hand sizes vary. One can easily appreciate the magnitude of the possibility of false-positive tests, as many patients who may be symptomatic in the hands for various other conditions, for example cervical disc disease, may well have median nerve motor latencies falling between 3.7 and 4.0 ms, in which case a false-positive diagnosis of carpal tunnel syndrome would have been made by the single criterion used by the authors. Furthermore, I would like to point out that even an obviously positive Phalen's test, with or without the presence of Tinel's sign (which in my experience has been quite a useless test because of many false positives), can be demonstrated in many patients whose nerve conduction tests have proved absolutely normal. Such patients may include those with cervical disc disease, cervical myelopathy, anxiety and depression, and non-specific complaints in patients with conditions such as fibromyalgia or in early peripheral neuropathy. One further query that I have with the authors is with regard to the time taken to designate a positive Phalen's test (1 min); the standard description of the test suggests reproduction of symptoms within 20 s. It has been my experience that some patients do not understand the significance of the reproduction of symptoms; in particular, patients who do not actually have median nerve compression may inappropriately respond positively, indicating a positive test when they do not experience the type of symptoms experienced by actual carpal tunnel syndrome patients. Hence confusion often arises in clinical practice, especially in busy clinics. The above comments are based on my long experience in dealing with this group of patients and basic nerve conduction tests. In addition, my colleagues and I have performed [3-5] and are currently undertaking a number of studies which raise concerns about the type of conclusion O'Gradaigh and Merry have drawn from their study. For example, in patients with paraesthesia (but normal neurophysiological tests) in the hands (with or without positive Phalen's and/or Tinel's test), when assessed by the Hospital Anxiety and Depression scale, a significant proportion show high scores (paper in preparation). I agree with the authors, however, that in milder and early cases of carpal tunnel syndrome (with three positive clinical tests as they described), apart from splinting of the hand and wrist, a steroid injection may be appropriate whilst waiting for electrophysiological tests; in my experience about half the patients do respond. Such patients do not require decompression surgery straight away, as often advocated, particularly by orthopaedic and plastic surgeons. Finally, I would like to draw your readers' attention to the proposed guidelines for carpal tunnel syndrome management that we have published in the paper quoted by the authors [3].