Quantification of upper motor neuron loss in amyotrophic lateral sclerosis

Objective: To quantitatively estimate upper motor neuron (UMN) loss in ALS. Methods: We used the recently developed triple stimulation technique (TST) to study corticospinal conduction to 86 abductor digiti minimi muscles of 4 8 ALS patients. This method employs a collision technique to estimate the p roportion of motor units activated by a transcranial magnetic stimulus. At the same time, it yields an estimate of lower motor neuron (LMN) integrity. Results: The TST disclosed and quantified central conduction failures attri butable to UMN loss in 38 sides of 24 patients (subclinical in 15 sides), w hereas conventional motor evoked potentials detected abnormalities in only 18 sides of 12 patients (subclinical in two sides). The increased sensitivi ty of the TST to detect UMN dysfunction was particularly observed in early cases. Increased central motor conduction times (CMCT) occurred exclusively in sides with conduction failure. In sides with clinical UMN syndromes, th e TST response size (but not the CMCT) correlated with the muscle weakness. In sides with clinical LMN syndromes, the size of the peripherally evoked compound muscle action potentials correlated with the muscle weakness. Conclusion: The TST is a sensitive method to detect UMN dysfunction in ALS. It allows a quantitative estimate of the UMN loss, which is related to the functional deficit. Therefore, the TST has a considerable impact on diagno stic certainty in many patients. It will be suited to follow the disease pr ogression and therapeutic trials. (C) 2000 Elsevier Science Ireland Ltd. Al l rights reserved.