A double determination of central motor conduction time in the assessment of Hirayama disease

To investigate central motor conduction time (CMCT) in patients with Hirayama disease (HD) and to analyse the role of motor nerve root lesions in the pathogenesis of HD.

CMCT measured by F-wave (CMCT-F) and by paravertebral magnetic stimulation (CMCT-M) was performed on both abductor pollicis brevis (APB) and abductor digiti minimi (ADM) in 41 HD patients and 22 controls. All patients underwent neck-flexion magnetic resonance imaging evaluation.

Prolonged CMCT (CMCT-F and/or CMCT-M) was recorded in at least one tested muscle from 7/41 (17.1%) HD patients, and 4 cases presented significant prolonged CMCT-M with normal CMCT-F on the side with significant cervical cord forward-shifting. This asymmetric forward-shifting was identified in 13 HD patients, and forward-shifting on the symptomatic side was more obvious. Compared to the controls (ADM: 0.9±0.3ms; APB: 0.8±0.3ms) and the other 28 HD patients (symptomatic side: ADM: 0.8±0.2ms, APB: 0.8±0.3ms), increased nerve root conduction times were demonstrated in these symptomatic sides (ADM: 1.5±0.7ms; APB: 1.2±0.6ms) (P<0.05).

Motor nerve root may be main lesion site in some HD patients, especially on the symptomatic side of patients with asymmetric neck-flexion cervical cord forward-shifting.

Compared to spinal motor neuron lesions, damage to motor nerve root (intra- and/or extra-medullary motor roots) may play an equally important role in the pathogenesis of HD. Abnormally increased forward traction in shorter nerve roots may be the cause for the main damage in motor nerve root.