CORTICOMOTOR THRESHOLD IS REDUCED IN EARLY SPORADIC AMYOTROPHIC-LATERAL-SCLEROSIS

The pathogenesis of idiopathic amyotrophic lateral sclerosis (ALS) rem ains unknown, but accumulating evidence suggests a neuroexcitotoxic me chanism may have some credence. Glutamate-induced hyperexcitability of cortical or spinal motoneurons may be expected to manifest itself as a reduced threshold for activation of these structures. We have measur ed corticomotor threshold to the first dorsal interosseous (FDI) muscl es of 48 patients with sporadic ALS using magnetic brain stimulation a nd have correlated the findings with physical signs of upper and/or lo wer motor neuron degeneration. We find that if FDI in patients with AL S shows no weakness, wasting, or signs of an upper motor neuron lesion , mean corticomotor threshold is significantly lower than in 102 healt hy control FDI muscles (P = 0.02). In contrast, FDI muscles showing si gns of lower motor neuron degeneration only or mixed upper and lower m otor neuron signs are associated with a raised corticomotor threshold (P = 0.008, P < 0.001, respectively). We conclude that early in ALS, a t a time when hand muscle function is normal, corticomotor threshold i s reduced and suggest that this may be a manifestation of abnormal exc itability of cortical or spinal motoneurons to neurotransmitters, whos e action will ultimately lead to cell death. (C) 1997 John Wiley & Son s, Inc.