F-waves and facilitated late responses of the mentalis muscle in patients with a cerebrovascular accident

F-waves in the extremities result from the backfiring of antidromically act ivated anterior horn cells and F-waves of the mentalis muscle can be also e licited after stimulation of the marginal mandibular branch of the facial n erve. In order to investigate the influence of the descending pathway of th e excitability of the facial motonucleus, the F-wave of the mentalis muscle and the facilitated late response, which follows F-waves and which seems t o be the snout reflex due to their similar latency and habituation, were st udied in 11 conscious patients with a hemispheric cerebrovascular accident (CVA) presenting with hemiparesis, and in 10 unconscious patients with CVA or head injury. The duration and the persistence of the F-waves increased s ignificantly statistically on the normal side in the CVA patients compared with those of the palsy side and the normal subjects. In comatose patients the F-waves and the facilitated late response were not elicited. The latenc y (46.1 +/- 13.3 msec) of the facilitated late responses in the unconscious patients tended to increase compared with the latency (36.6 +/- 4.3 msec) in the conscious patients. These findings suggest that the hyperexcitabilit y of the facial motoneuron is ipsilateral to any hemispheric lesion; the he mispheric lesion exerts a bilateral excitatory influence on the interneuron of the facilitated late response: and that the reticular formation may inf luence the facial motoneuron and any interneurons concerned in the facilita ted late response. F-waves and facilitated late responses should be further examined as neurophysiologically useful diagnostic methods.