|
(Continued from page 125)
located on both sides of the brain. There is considerable evidence that the inferior colliculus does not produce any noticeable farfield potentials. A prolongation of the latency of peak V with the latency of peak III unchanged may thus be assumed to indicate that structures of the ascending auditory pathway located between the cochlear nucleus and the inferior colliculus are affected, i.e. the superior olivary complex, trapezoidal body, or lateral lemniscus.
It is presently believed that it is the shift of the peaks in the BSAER (prolongation of their latency) that is the most important indication that neural conduct{on has been affected, although the amplitudes of the potentials are probably also important in detecting injuries to the vestibulocochlear nerve or ascending auditory nervous system. If the changes are caused by manipulations of the vestibulocochlear nerve, all peaks except peak I will shift and many neurophysiologists then prefer to monitor the latency of peak V because that peak is usually easier to identify than peak III. We do not know exactly how much the latency or amplitude of the BSAER can change before there is a significant risk of permanent postoperative hearing loss.
logic deficit varies widely. Thus, some surgeons do not regard changes less than total obliteration of the BSAER to be indicative of a significant risk of postoperative hearing loss, whereas others assume that changes of 1.5-2.5 ms indicate a significant risk it is provided good results. The latencies of the various peaks in the BSAER show very little spontaneous variation, and a change of 0.25-0.5 ms represents a clear indication that the system under test has been affected. One major advantage of reporting even small changes is that if the surgeon is aware of when the changes began, he/she will be able to determine what caused the change. This is might occur, this author is convinced that a shift of 1-1.5 ms indicates that action needs to be taken, and a shift in latency of 2-2.5 ms indicates that the risk of postoperative hearing loss is high.
ANESTHESIA AND MONITORING
It is well known that the type of anesthesia, the patient's blood pressure, cerebral blood flow, body temperature, hematocrit, and blood gas tensions all affect the functioning of the patient's central nervous system and thus the intraoperatively observed neurophysiological measures.
The cranial base tumor cases are typically performed using isoflurane or a modified balanced narcotic procedure. However, muscle relaxants are rarely used, as the monitoring of the EMGs related to cranial nerve function is a major factor in the successful outcome of these cases.
The anesthesiologist typically uses constant infusion techniques to minimize the use of inhalation agents and to maintain as constant as possible a baseline level of functioning. The neurophysiologist notes whenever a medication bolus is given in anticipation of decrements in response quality.
Throughout the surgical procedures, close communication is (Continued on page 127)
|
|
