CMAP Amplitude of Median Nerve - A Prognostic Marker of Amyotrophic Lateral Sclerosis

Abstract

Background: Amyotrophic lateral sclerosis (ALS) is an incurable relentlessly progressive neurodegenerative disease. A nerve conduction study and electromyogram are routinely used for diagnosis, and the compound muscle action potential (CMAP) amplitude of the median nerve in nerve conduction study is a key prognostic marker at the time of diagnosis enabling carly therapeutic trial. Previous studies have shown that a decreased median nerve amplitude strongly correlates with the functional status of ALS, making it a useful quantitative prognostic marker.
Objectives of the study: The study aims to determine the association of CMAP amplitude of median nerve with prognosis of ALS.
Materials and Methods: The research was conducted at the Department of Neurology, BSMMU, Dhaka, from April 2023 to September 2024, and it involved 38 subjects. Among them, 19 ALS patients had normal CMAP amplitude of median nerve, while 19 ALS patients had decreased CMAP amplitude of median nerve and met the selection criteria. Patients with ALS were diagnosed based on the Revised El Escorial diagnostic criteria, and Severity of their condition was assessed using the ALS functional rating scale-revised (ALSFRS-R). Data were analyzed using SPSS (26 versions) for Windows to observe the association between CMAP amplitude of the median nerve with the functional status of ALS.
Result: ALSFRS-R was significantly reduced at 6 months than at the time of diagnosis in ALS patients with decrease CMAP amplitude of median nerve than normal CMAP amplitude. Multivariate analysis revealed ALSFRS-R at diagnosis and decrease CMAP amplitude of median nerve were significantly associated with poor outcomes. Survival model analysis revealed ALS patients with decrease CMAP amplitude of median nerve had shorter survival time than with ALS with normal CMAP amplitude patients.
Conclusion: Patients with decrease CMAP amplitude of median nerve may experience shorter survival times and poorer functional outcomes. This can serve as a valuable prognostic marker for patients with ALS.