F/M ratios in polyneuropathy and spastic hyperreflexia.

The ratios of F to M amplitudes were determined by dividing the mean amplitude of 15 F-waves (mF AMP) by the maximum evoked motor response amplitude recording from the abductor pollicis brevis and calf muscles. Data were compared from age-matched control subjects and patients with polyneuropathies or spastic hyperreflexia. In comparison to normals, mF AMP/M--but not mF AMP--values were increased at statistically significant levels in all patients groups. M amplitudes were decreased (versus normal, P less than 0.005) in the patients with either peripheral or central lesions. In these patients, the normal significant correlation between mF AMP and M amplitudes may be disrupted, whereas the data suggest that increased mF AMP ratios in neuropathies are related to their duration. These results indicate that mF AMP/M ratios can be increased not only with increased central excitability but also in peripheral neuropathies. Although mechanisms may differ, the data are consistent with a similar physiological response to peripheral and central injury and emphasize that M amplitudes may be decreased with central lesions.