Myopathic EMG findings and type II muscle fiber atrophy in patients with Lambert-Eaton myasthenic syndrome

Abstract

Objective: Lambert-Eaton myasthenic syndrome (LEMS) is a rare condition, which may mimic myopathy. A few reports have described that EMG in LEMS may show changes compatible with myopathy, and muscle biopsies have been described with type II as well as type I atrophy. The EMG results were, however, based on qualitative EMG examination and the histopathological methods were not always clear. The objective of this study was to investigate if the previous EMG findings could be confirmed with quantitative EMG (QEMG) and to describe muscle histology in LEMS.

Methods: QEMG, nerve conduction studies and muscle biopsy were performed in four consecutive LEMS patients.

Results: QEMG showed significantly decreased mean MUP duration and muscle biopsy showed marked type II fiber atrophy.

Conclusion: EMG and biopsy abnormalities mimicking myopathy may often be found in patients with LEMS.

Significance: LEMS is a debilitating, but treatable disease, which often precedes detection of a malignancy and it is therefore of obvious importance to diagnose these patients with speed and certainty. Hence it is important that neurophysiologists and neurologists are aware that EMG and histological abnormalities mimicking myopathy may be found in LEMS patients so that these findings do not prolong or misdirect the diagnostic process in these patients.

Keywords: Lambert-Eaton myasthenic syndrome; Muscle biopsy; Myopathy; Quantitative EMG; Type II muscle fiber atrophy.