The present study was designed to ascertain the sensitivity of the transcortical magnetic stimulation (TMS) and triple stimulation technique (TST) to demonstrate upper motor neuron involvement in ALS.

The diagnosis of ALS requires the presence of lower (LMN) and upper (UMN) motor neuron involvement. LMN involvement is demonstrated by clinical and EMG findings whereas UMN neuron signs are dependent on the clinical examination. Though such signs often are ambiguous, no electrophysiological methods are currently included in the diagnostic criteria for UMN involvement.

TMS including TST to detect UMN was examined in 144 consecutive ALS patients at the time of diagnosis. EMG was used to confirm LMN affection. TST is a collision technique to demonstrate conduction failure in corticospinal fibers and was as far as possible performed in both arms and conventional motor evoked potentials (MEP) were carried out in the legs to assess central motor conduction time (CMCT).

Clinical UMN signs occurred in 81 of 144 patients whereas 63 patients had only LMN abnormalities. The TST in the arms showed a central conduction failure in 63% of 142 patients (2 patients had no cortical responses), while only 15% had prolonged CMCT. In the legs, prolonged CMCT was found in 50% of patients. TMS showed UMN dysfunction in 76% of 144 patients. Compared to prolonged CMCT in the arms in pure LMN involvement, TST increased the detection of UMN involvement by a factor 4.7. The combined TST and conventional MEP disclosed a central abnormality in 62% of pure LMN patients.

TMS with TST is a sensitive method to detect corticospinal dysfunction in ALS. This TMS protocol is applicable in the clinical routine supporting the proposal that MEP abnormalities are a reliable marker of UMN damage and could be incorporated into diagnostic criteria for ALS.