Responses of masseter muscles to transcranial magnetic stimulation in patients with amyotrophic lateral sclerosis

We recorded motor responses evoked by transcranial magnetic stimulation (TMS) in the masseter muscles of 30 patients with amyotrophic lateral sclerosis (ALS), 10 patients with cervical spondylotic myelopathy (CSM) and 22 age-matched normal controls. Responses to direct activation of the trigeminal motor root (R-MEPs) were normal both in ALS and CSM patients. Responses to activation of cortico-bulbar descending fibers (C-MEPs) were absent or delayed in 19 ALS patients (63.3%). Abnormalities of masseter C-MEPs were more frequent than abnormalities of limb MEPs and could be observed both in patients with (77.8%) and without (41.7%) clinical bulbar signs. Masseter C-MEPs were normal in all CSM patients. Recording masseter responses to TMS can reveal the frequent impairment of cortico-bulbar projections in ALS and can be useful in the differential diagnosis of spinal cord compression disorders mimicking ALS because of combination of upper and lower motor neuron signs.