Repetitive nerve stimulation studies in the Lambert-Eaton myasthenic syndrome

We compared changes in amplitude and area of surface recorded compound motor action potentials (CMAPs) during 20-Hz repetitive nerve stimulation and after maximum voluntary contraction in patients with the Lambert-Eaton myasthenic syndrome (LEMS), myasthenia gravis (MG), and normal controls. There was greater potentiation of CMAP amplitude after voluntary contraction than during 20-Hz stimulation in 10 of 14 LEMS patients; CMAP area increased more after exercise than during 20-Hz stimulation in all LEMS patients. Although abnormal potentiation of CMAP area and amplitude was seen in equal numbers of LEMS patients, more LEMS patients demonstrated a greater than 100% potentiation of CMAP area than of CMAP amplitude. We conclude that maximum voluntary contraction is preferable to brief 20-Hz RNS to demonstrate potentiation in LEMS because it is at least as sensitive and is less painful. Measurement of CMAP area in LEMS patients is not better than measuring the change in CMAP amplitude in demonstrating abnormal potentiation. Testing of a single hand muscle for potentiation in LEMS does not demonstrate abnormal potentiation in all LEMS patients.