Optimising the detection of upper motor neuron function dysfunction in amyotrophic lateral sclerosis--a transcranial magnetic stimulation study.

Evidence of upper motor neuron (UMN) dysfunction is essential in making the diagnosis of amyotrophic lateral sclerosis (ALS). Central motor conduction (CMC) abnormalities detected using transcranial magnetic stimulation (TMS) are presumed to reflect UMN dysfunction. CMC is, however, often normal in patients with classical sporadic ALS. The aim of the study was to determine whether the utility of the CMC measure in ALS could be enhanced. We measured CMC to four pairs of muscles (abductor digiti minimi (ADM), biceps, vastus medialis (VM) and abductor hallucis (AH) in 20 controls and 25 ALS patients. The commonest abnormality detected in the ALS patients was an absent MEP, found in 11 patients (44 %) and in 25 of 200 muscles examined. Studying a minimum of three muscles increased the probability of detecting UMN dysfunction. Weakness in the muscle as well as selecting a distal rather than a proximal muscle was significantly associated with an abnormal CMC. Interside differences in CMC were significantly more pronounced in the patient group. In 30% of patients a significant interside difference in AH CMC time was the sole abnormality, suggesting mild UMN dysfunction on the side with the longer CMC.