DISTRIBUTION OF ELECTROPHYSIOLOGICAL ABNORMALITY IN LAMBERT-EATON-MYASTHENIC-SYNDROME

Objective-To assess the distribution of electrophysiological abnormali ty in Lambert-Eaton myasthenic syndrome (LEMS) to identify the most se nsitive muscle to use in routine examination. Methods-Surface recorded compound muscle action potential (CMAP) amplitudes were made from abd uctor digiti minimi, abductor pollicis brevis, anconeus, biceps brachi i, and trapezius in 10 patients with LEMS. The effect of 3,4-diaminopy ridine (3,4-DAP) was recorded in each muscle in nine patients. CMAP am plitudes were measured at rest and immediately after 10 seconds maxima l voluntary contraction in each muscle. Values were compared with resu lts obtained from 12 healthy controls. Results-Resting CMAP amplitudes were reduced in at least one muscle in all patients compared with con trols, most markedly in abductor digiti minimi and anconeus. The admin istration of 3,4-DAP resulted in significantly improved resting CMAP a mplitudes in trapezius only. After maximal voluntary muscle contractio n, characteristic increments in CMAP amplitude of over 100% above rest ing values were seen in abductor digiti minimi and abductor pollicis b revis in seven patients, anconeus and biceps brachii in five patients. No patient had this level of increment in trapezius. Conclusion-Despi te predominantly proximal limb weakness seen clinically in patients wi th LEMS, the most sensitive muscles for detecting characteristic elect rophysiological abnormalities of low resting CMAP amplitude and increm ent of over 100% after 10 seconds maximal voluntary contraction are ab ductor digiti minimi, abductor pollicis brevis, and anconeus.