Corticobulbar tract involvement in amyotrophic lateral sclerosis. A transcranial magnetic stimulation study

We investigated corticobulbar tract function by recording from the tongue and orofacial muscles and using transcranial magnetic stimulation (TMS) in 30 patients with amyotrophic lateral sclerosis (ALS) in order to improve the diagnostic yield in the detection of subclinical upper motor neuron (UMN) dysfunction. A UMN lesion was assumed when the peripheral conduction time and amplitude of the M-wave were within normal range and either the response to cortical stimulation was absent, or the central conduction time was delayed (> mean + 2.5 SD). Only two patients showed clinical evidence of UMN involvement in the cranial nerves, while TMS demonstrated corticobulbar tract dysfunction in the orofacial muscles in 17 patients (57%) and to the tongue in 15 patients (50%). Following recording at both sites, corticobulbar tract dysfunction was confirmed in 21 patients (70%). Twelve out of 13 patients with bulbar symptoms had evidence of additional subclinical corticobulbar tract involvement using TMS. In seven out of 15 patients with isolated limb involvement, subclinical corticobulbar tract dysfunction was demonstrated. Our results indicate the presence of early and, in most cases, subclinical corticobulbar tract involvement in the pathways to the orofacial muscles and tongue in ALS.