Sensitivity of transcranial magnetic stimulation of cortico-bulbar vs. cortico-spinal tract involvement in Amyotrophic Lateral Sclerosis (ALS)

Background An upper motor neuron (UMN) lesion in amyotrophic lateral sclero sis (ALS) is often difficult to identify because clinical signs maybe discr ete or masked by severe simultaneous LMN lesions. We compared the diagnosti c sensitivity of transcranial magnetic stimulation (TMS) to cranial muscles and limb muscles in the detection of UMN lesions. Design We investigated c orticobulbar and corticospinal tract function to the tongue/orofacial muscl es and abductor digiti minimi/tibial anterior muscles with TMS in 51 patien ts with ALS to compare the diagnostic yield in the detection of UMN dysfunc tion. An UMN lesion was assumed when the following were found: the peripher al conduction time and amplitude of the M-wave were within the normal range , the response to cortical stimulation was absent, the TMS evoked/M-wave am plitude ratio was reduced, and the central motor conduction time or the int erside difference was delayed. (> mean+2.5 SD). Results On the basis of the se criteria a UMN lesion to the orofacial muscles was identified in 24 pati ents (47 %), to the tongue in 27 (53 %), and to the upper and lower limbs i n 13 (25 %) and 22 patients (43 %), respectively. Combined abnormalities fr om all sites increased the diagnostic yield to 39 patients (76 %). TMS of t he limb muscles confirmed a UMN lesion in only 15 (54 %) of the 28 patients with clinically confirmed UMN involvement. This number increased to 23 pat ients (82 %) if tongue and orofacial muscles were taken into acount. Conclu sion Our results indicate the early and in most cases subclinical corticobu lbar tract involvement of the central motor pathways to the orofacial muscl es and tongue in ALS. TMS of the tongue and orofacial muscles had a higher sensitivity in identifying UMN lesions than that of the upper and lower lim bs.