Responses to transcran1al magnetic stimulation (TMS) in the masseter muscles of patients with amyotrophic lateral sclerosis (ALS)

TMS of the motor cortex can document abnormalities of cortico-spinal system conductivity and excitability in ALS. The motor potentials evoked (MEPs) by TMS in muscles supplied by cranial nerves have not been extensively investigated in ALS, despite the frequent impairment of the bulbar region. We recorded MEPs from the masseter muscles of 30 ALS patients (16 M - 14 F; aged 42-86 ys.) as compared to 22 age-matched normal controls and 9 patients with spondylothic myelopathy. R-MEPs to direct stimulation of the trigeminal root were normal in ALS. Responses evoked by cortical stimulation in the preinnervated jaw-closing muscles (C-MEPs) were absent or delayed in 63.3% of ALS. Abnormalities of masseter CMEPs were observed in patients both with (77.8%) and without (41.7%) clinical bulbar signs. C-MEPs were normal in patients with spondylotic myelopathy. The investigation of corticobulbar projections may disclose frequent abnormalities in ALS, useful in the differential diagnosis between ALS and other spinal cord disorders.