A Case of Lambert-Eaton Myasthenic Syndrome with Small-Cell Lung Cancer and Transient Increase in Anti-Acetylcholine-Receptor-Binding Antibody Titer
A 57-year-old male was admitted to the hospital due to dry mouth and eyes and progressive proximal limb weakness without diurnal fluctuation of 2 months duration. He also complained of mild transient positional dizziness, dysarthria, and dysphagia. He had smoked one pack of cigarettes a day for 30 years. His family history was unremarkable. A neurologic examination showed mild dysarthria, proximal muscle weakness, and absent deep tendon reflexes. His cranial nerve, cerebellar, and sensory functions were all normal. A repetitive nerve stimulation test (RNST) was performed on the right abductor digiti quinti (ADQ), flexor carpi ulnaris (FCU), orbicularis oculi, nasalis, and trapezius muscles following Oh's method using the Toennies two-channel NeuroScreen system (Jaeger-Toennies, Hochberg, Germany). Postexercise facilitation (PEF) of compound muscle action potentials (CMAP) immediately after maximal voluntary contraction for 30 seconds and increment responses for 1 second at 50-Hz high-frequency stimulation were also recorded for the ADQ and FCU. The RNST results fulfilled all of the criteria for electrophysiological LEMS patterns: low-amplitude CMAP at rest, a decrement on low-frequency (3 Hz) stimulation, PEF of more than 100%, and an approximately 900% increment on high-frequency (50 Hz) stimulation ().\nFollowing an injection of acetylcholinesterase inhibitor (intramuscular injection of 1.5 mg of neostigmine methylsulfate into the deltoid muscle), arm elevation endurance improved from 30 seconds to 2 minutes, and this improvement lasted for up to 30 minutes postinjection; such an observation is an additional indicator of LEMS. The titer of AChR-abs was 0.587 nmol/L (normal: <0.2 nmol/L). Chest computed tomography (CT) revealed a 1.6-cm mass in the anterior segment of the right lower lobe and enlarged lymph nodes in the subcarina and lower paratracheal areas (). SCLC was confirmed by a transbronchial lung biopsy. The patient was successfully treated with radiation (5.4 Gy for the primary tumor and regional lymph node area) and chemotherapy (irinotecan and cisplatin for the first cycle, followed by etoposide and cisplatin for the fifth cycle) for LEMS and SCLC, as confirmed by a follow-up electrophysiological examination () and chest CT () performed 13 months after the first evaluation. The patient was not treated for MG, but the titer of AChR-abs had decreased to 0.001 nmol/L at a follow-up performed 5 years after successful treatment.