Physiologic decrease of single thenar motor units in the F-response in stroke patients

Objective: To investigate the left-right difference and the reproducibility by the F-wave motor unit number estimation and to compare the motor unit number between the hemiplegic and unaffected side in stroke patients. Setting: A referral center and institutional practice providing outpatient care. Subjects: Seven healthy volunteers and, 15 consecutive stroke patients. Design: Diagnostic statistical test and correlational study. Method: Submaximal stimuli were used to evoke a sample of surface motor unit action potentials (S-MUAPs) in the P-waves that are entirely representative of the relative numbers of detected S-MUAPs of different sizes. The average S-MUAP amplitude was calculated from a selected population of F-wave responses for each abductor pollicis brevis (APB) muscle, The motor unit number was calculated by dividing the maximum M-potential negative peak amplitude by the average S-MUAP negative peak amplitude. Result: There was no statistical difference between motor unit numbers on either side and between test and retest in this motor unit number estimation method among normal subjects. The motor unit number on the hemiplegic side was significantly lower than on the unaffected side (p < .05, Mann-Whitney test) among stroke patients. Conclusion: The motor unit could decrease in the hemiplegic side after a moderate-to-severe hemiplegic stroke and this decrement might be due to the transsynaptic degeneration secondary to an upper motor neuron lesion.