The Compound Muscle Action Potential as Neurophysiological Marker for Amyotrophic Lateral Sclerosis.

Objectives: To definite the peripheral nervous involvement in ALS through the repeated use of the compound motor action potential (CMAP) to test the progression of disease, to determine different change of phrenic CMAP and forced vital capacity (FVC) in spinal and bulbar onset, and to establish clinical and neurophysiological features of patients with poor prognosis. Material & Methods: CMAP from phrenic, ulnar, and medial plantar nerves, Medical Research Council (MRC) score, revised ALS functional rating scale (ALSFRS-R) and FVC were evaluated in 117 ALS patients every three months in one year-period. Results: Bulbar onset patients had lower FVC but similar amplitude of phrenic CMAP at baseline compared to spinal onset patients. The patients with poor prognosis had lower phrenic CMAP and FVC at baseline. CMAP values, when compared to the rate found in the previous visit, reduced significantly in both poor and good prognosis groups during the entire follow-up period, while the FVC reduced significantly only in the first three months. Conclusions: CMAP is a reproducible sensitive marker for motor neurons loss and collateral reinnervation in ALS also in a short period of time. The changes in CMAP, MRC, FVC and ALSFRS-R score resulted correlated, but CMAP is the only parameter with the advantage to demonstrate objectively the progression of disease in both patients with poor and good prognosis for the entire period of follow-up. It should be used as clinical outcome of ALS in clinical trials, taking advantage of its objectivity and selectivity for peripheral nervous system study.