[The sensory potential of the ring finger. The value of electromyographic diagnosis of carpal tunnel syndrome].

Accurate diagnosis of carpal tunnel syndrome relies on electromyography. The usual criteria (sensory conduction velocity and motor latency) fail to identify 10% of cases of carpal tunnel syndrome. The sensory innervation of the ring finger (digit IV) is shared by the median and ulnar nerves. This anatomic feature is the basis of a novel electromyographic test: measurement of the sensory potential of digit IV by an orthodromic method with stimulation of the finger via a ring electrode and recording at the wrist. Analysis of the sensory potential of digit IV in 200 subjects with no median nerve compression afforded an invariably synchronised trace with no error of interpretation since it required no additional measurements. A study of 179 cases in 124 patients suspected of carpal tunnel syndrome showed pathological traces that could be graded into five levels of severity. Comparison with sensory conduction velocity and distal motor latency confirmed its validity and especially its usefulness in mild forms. The sensitivity of the test is excellent since false negatives are theoretically due to anomalies in the sensory innervation of digit IV, the reported occurrence of which varies widely (about 15%). The were no false positives. Its reproducibility, painlessness and sensitivity have led us to make this test compulsory in all patients suspected of CTS in our practice.