Transcranial magnetic stimulation identifies upper motor neuron involvement in motor neuron disease

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Abstract
Objective: To evaluate the sensitivity of transcranial magnetic stimulation (TMS) to identify upper motor neuron involvement in patients with motor neuron disease. Background: Diagnosis of ALS depends on upper and lower motor neuron involvement. Lower motor neuron involvement may be documented with electromyography, whereas definite evidence of upper motor neuron involvement may be elusive. A sensitive, noninvasive test of upper motor neuron function would be useful. Methods: TMS and clinical assessment in 121 patients with motor neuron disease. Results: TMS revealed evidence of upper motor neuron dysfunction in 84 of 121 (69%) patients, including 30 of 40 (75%) patients with only probable upper motor neuron signs and unsuspected upper motor neuron involvement in 6 of 22 (27%) patients who had purely lower motor neuron syndromes clinically. In selected cases, upper motor neuron involvement identified with TMS was verified in postmortem examination. Increased motor evoked potential threshold was the abnormality observed most frequently and was only weakly related to peripheral compound muscle action potential amplitude. In a subset of 12 patients reexamined after 11 ± 6 months, TMS showed progression of abnormalities, including progressive inexcitability of central motor pathways and loss of the normal inhibitory cortical stimulation silent period. Conclusions: TMS provides a sensitive means for the assessment and monitoring of excitatory and inhibitory upper motor neuron function in motor neuron disease.

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