Relation between needle electromyography and nerve conduction studies in patients with carpal tunnel syndrome

Four hundred eighty cases of electrodiagnostically confirmed carpal tunnel syndrome were reviewed to determine if the findings on nerve conduction studies could predict the presence or absence of fibrillation potentials or motor unit changes on the needle examination of the abductor pollicis brevis (APB). The needle examination is more uncomfortable and the ability to predict the findings in this setting from standard nerve conduction studies (NCS) would make the test more acceptable to patients. All patients had median and ulnar nerves (both sensory and motor) tested, as well as the needle evaluation of the APB. Two hundred thirty-one patients had an abnormal needle evaluation as defined by presence of one of the following conditions: abnormal spontaneous activity, increased motor unit action potential (MUAP) amplitude, or increased MUAP polyphasia. One hundred five patients had fibrillation potentials. The mean median motor and sensory amplitudes and latencies, as well as age, did differ in the normal and abnormal needle examination groups, but the sensitivity for predicting an abnormality ranged from 57% to 68%. The ratio of the median to the ulnar amplitudes did not improve the sensitivity of predicting the abnormal needle findings. Motor and sensory evoked potential latencies were the most important predictors of an abnormal needle examination.