RESUMEN
Isaacs' syndrome consists of spontaneously occurring muscle activity of peripheral nerve origins. This syndrome arises in association with/without polyneuropathy and rarely with malignancy. A 63-year-old man was admitted to our hospital due to generalized painful muscle stiffness. He complained of difficulty with standing and with finger exten-sion after grasping. Chvostek's and Trousseau's signs were noticed. Electrolytes, calcium, CK, and LDH were in the normal range. Small cell lung cancer was diagnosed by a needle biopsy. Electrophysiological testing revealed normal nerve conduction studies with the exception of a grossly abnormal EMG. Continuous neuromyotonic discharges with firing rates of 120-200 Hz were seen at rest. The amplitude of the response typically waned with 0.5-1.5 seconds of duration. The discharges persisted throughout sleep, after diazepam injection, and with brachial plexus blockage.Muscle stiffness improved with the administration of oral phenytoin. Under chemotherapy and radiotherapy, tumor remission was partially achieved and neurological symptoms markedly improved.