Brainstem excitability is increased in subjects with palmomental reflex.
OBJECTIVE
The palmomental reflex (PMR) is a brief contraction of the mentalis muscles caused by a scratch over the thenar eminence, i.e. a brainstem reflex to afferents of upper limb. Using electrophysiologic methods, we studied the characteristics of brainstem excitability in PMR subjects.


METHODS
Ten healthy PMR subjects were included in the study. Brainstem excitability was assessed with electrical stimulation at the trigeminal nerve, median nerve, ulnar nerve, and sural nerve with recordings at the mentalis muscles. A comparison was made by the probability between the mechanical scratch and the electrical stimulation to evoke the visible muscle contraction of mentalis.


RESULTS
An electrical stimulus was able to elicit mentalis muscle responses (MMR(electrical)) in all the subjects if the stimulus was of sufficient strength. Using electrical stimulation, the median nerve at the wrist was the best site to evoke MMR(electrical). However, in PMR subjects, the probability of MMR(electrical) to median nerve stimulation was less than that of MMR(scratch), i.e. the clinical findings of PMR. Significantly lower thresholds and higher amplitudes were noted in PMR subjects only when the median nerve was stimulated. The onset latency did not show any difference between the two groups despite the stimulation sites.


CONCLUSIONS
The facial motor neurons to median nerve stimulation are more sensitive in PMR subjects. In healthy PMR subjects, this indicates that the excitability increases only in the specific neuronal circuits between the lower cervical spinal cord and the facial motor nucleus in the rostral medulla. MMR(electrical) is a physiologic phenomenon, and PMR is a sign of increased brainstem excitability.
Liao
Kwong-Kum
KK
Department of Neurology, Taipei Veterans General Hospital, National Yang-Ming University School of Medicine, Taipei, Taiwan.
Chen
Jen-Tse
JT
Lai
Kuan-Lin
KL
Lin
Chia-Yi
CY
Liu
Chih-Yang
CY
Kao
Chuen-Der
CD
Lin
Yung-Yang
YY
Wu
Zin-An
ZA
D000368:Aged; D000369:Aged, 80 and over; D001933:Brain Stem / Q000502:physiology*; D004558:Electric Stimulation; D005152:Facial Muscles / Q000502:physiology*; D005154:Facial Nerve / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology; D012018:Reflex / Q000502:physiology*