F/M ratios in polyneuropathy and spastic hyperreflexia

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Abstract
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 patient groups. M amplitudes were decreased (versus normal, P < 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.