[Importance of intraoperative monitoring of ABR and compound action potential of the eighth cranial nerve during microvascular decompression surgery].
An intraoperative recording of a direct compound action potential of the 8th cranial nerve with pre, intra and postoperative monitorings of auditory brainstem evoked response (ABR) was carried out for the purpose of clarifying mechanism and prevention of postoperative hearing dysfunction following microvascular decompression surgery as a treatment for hyperdysfunction syndrome of cranial nerves. In 221 patients with hemifacial spasm and tic douloureux out of 510 patients operated on by microvascular decompression surgery, ABR was monitored before, during and after surgery and furthermore, in the recent 94 patients among them, an intraoperative direct recording of a compound action potential of the acoustic nerve was performed simultaneously. Among these 94 patients, postoperative mild hearing dysfunction was encountered in 11 patients and severe reduction of hearing acuity in 2. As a result of our studies of monitoring action potential of the 8th cranial nerve and ABR, the following conclusions were drawn. When a latency of component V of ABR was delayed by more than 1.5-2.0 msec during surgery, and delay of latency of main negative peak (N1) of an action potential of the acoustic nerve exceeded by more than 1.0 msec, the occurrence rate of postoperative hearing dysfunction became positively high. There were two different types of changes in both ABR and action potentials; one type was a delay of an action potential in combination with that of ABR latencies and the other one was merely a delay in ABR components without following a remarkable change in action potentials. The former change might due to be an insult of whole neural pathways of the 8th cranial nerve including a cochlear nucleus and the latter change may due to be functional changes of the structures proximal to the cochlear nucleus. The delay of latencies in ABR and action potentials could be imposed either by a traction of the 8th cranial nerve or by a compression of the region of cochlear nucleus by a blain spatula as well as a vascular insufficiency of the tiny vessels which supplied these neural structures. To avoid the surgical insult to the acoustic nerve itself and more proximal neural pathways, the traction of any neural structures by a brain spatula should be lasted less than 5 minutes with more than 2 minutes intervals when judged from the results of our monitoring studies and the pressure of the compression on the cerebellar cortex is much preferable to be less than 20 mmHg for the safety throughout whole surgical procedures.
Nishihara
K
K
Hanakita
J
J
Kinuta
Y
Y
Kondo
A
A
Yamamoto
Y
Y
Nakatani
H
H
D000328:Adult; D000367:Age Factors; D000368:Aged; D001933:Brain Stem / Q000502:physiology*; D005072:Evoked Potentials, Auditory; D005152:Facial Muscles; D005260:Female; D006801:Humans; D007431:Intraoperative Complications / Q000517:prevention & control; D008875:Middle Aged; D008991:Monitoring, Physiologic; D011930:Reaction Time / Q000502:physiology; D013035:Spasm / Q000601:surgery*; D014277:Trigeminal Neuralgia / Q000601:surgery*; D000159:Vestibulocochlear Nerve / Q000502:physiology; D061285:Vestibulocochlear Nerve Injuries