Electrophysiological and muscle pathological characteristics of Lambert - Eaton myasthenic syndrome

Objective To summarize the electrophysiological and muscle pathological characteristics of Lambert - Eaton myasthenic syndrome (LEMS). Methods and Results Twelve LEMS patients referred to China - Japan Friendship Hospital between January 2010 and December 2021 were included. Clinically, they all showed proximal muscle weakness and reduced or disappeared tendon reflexes. Electrophysiologically, all 12 patients showed decreased compound muscle action potential (CMAP) amplitudes, normal distal motor latency (DML), and normal motor and sensory conduction velocities. Ten patients underwent repetitive nerve stimulation (RNS) test: all had amplitude increments of more than 100% at high - frequency RNS, and 8 had amplitude decrements of more than 15% at low - frequency RNS. Three patients completed the post-exercise facilitation (PEF) test: the ulnar nerve CMAP amplitude reached a peak immediately after maximum voluntary contraction of the abductor digiti minimi for 5-10 s and then gradually decreased to near baseline level after contraction for 60 s; the ulnar nerve CMAP amplitude reached a peak immediately after maximum voluntary contraction for 10 s, and then rapidly decreased to near baseline level after 15 s. On needle EMG, 8 patients showed normal motor unit action potential (MUAP), 3 showed short - duration MUAP mimicking myopathy, and one showed long - duration MUAP mimicking neuropathy. Two patients with normal MUAP underwent biceps biopsy, which showed selective type Ⅱ fiber atrophy. Conclusions Electrophysiologically, CMAP amplitudes of LEMS patients are usually diffusely low or borderline. LEMS patients have amplitude increments of more than 100% at high-frequency RNS, and some have amplitude decrements of more than 15% at low-frequency RNS. PEF test shows a transient but significant CMAP amplitude increase after 5-10 s maximum voluntary contraction. Needle EMG is usually normal, but some show myogenic lesions, and some show neurogenic lesions. Muscle biopsy shows selective type Ⅱ fiber atrophy. Needle EMG myogenic lesion may not be associated with pathologically selective type Ⅱ fiber atrophy.