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With steady improvement and standardisation of methods,1 nerve conduction studies have become a reliable means of testing peripheral nerve function. They supplement clinical observation by precisely localising the lesion and characterising the conduction abnormalities.2 Delineating the extent and distribution of the neural lesion by this means also helps to quantify the degree of involvement. Optimal results can be expected only with proper choice of techniques, which in turn depends on the type of lesions under consideration. Thus, the studies must be conducted as an extension of the clinical examination, rather than a laboratory test.
To document sequential changes for clinical follow up in general and for drug trials in particular, conduction studies must yield high sensitivity and reproducibility. A question often posed, but rarely tested in regard to these criteria relates to the length of the nerve segment under study. Other factors being equal, should shorter or longer segments be studied for better results?
Segmental stimulation in short increments
In the evaluation of a focal lesion, studies of a longer segment tend to lower the sensitivity of the test because the inclusion of the unaffected segments in calculation dilutes the effect of slowing at the site of lesion. By contrast, studying a shorter segment helps to isolate a localised abnormality and provides better resolution of restricted lesions that may otherwise escape detection.
For example, patients with carpal tunnel syndrome show a sharply localised latency increase, averaging 0.8 ms across a 1 cm segment. This, compared to a normal value ranging from 0.16 to 0.21 ms, clearly indicates a focal abnormality.3 An abrupt change in waveform of the recorded response provides an additional, and perhaps more convincing, finding that nearly always accompanies an increase in latency across the site of compression. In fact, waveform analysis often localises a focal lesion unequivocally even in …