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(Continued from page 120)
Frequency Following Response (FFR). A continuing AC response generated in several nuclei of the brain stem.
Sustained Cortical Potential (SCP). A continuing DC response generated in the cerebral (auditory) cortex.
Contingent Neganve Variant (CNV). A very late DC shift developing in certain situations between a warning signal and an expected imperative stimulus.
On-effects: The on-effects are subdivided on the basis of their latencies as to fast, middle, slow, or late. A particular wave is designated by P if it is vertex positive and by N if it is vertex negative. A subscript is used to give the on-effects', most characteristic latency in a normal hearing adult at a sensation level of 60 decibels (dB). For example, a late cortex wave would be P 3(6) potentials from electrodes placed on the vertex and mastoid. Complementary techniques have been described to enhance the information obtained by a BSAER. These techniques include direct recording of the eighth nerve potential, electrocochleography, and BSAER data processing. Direct-nerve action-potential recording has the advantage of providing the information in a very short time, thus, giving the surgeon rapid feedback. Electrocochleography parallels the information provided by direct action potential of the auditory nerve. Although it is slower, electrocochleography does provide information recorded directly from the cochlea. Data processing may capture gradual changes in amplitude and latency of several BSAER peaks.
A stable V-wave on a BSAER predicts good postoperative hearing. Loss of hearing is usually associated with loss of the V-wave. Combining BSAER monitoring with other techniques seems most useful in predicting hearing outcome. Changes in BSAER may also predict brain-stem impairment, which has led to its use in skull-base surgery and for vascular lesions of the vertebrobasilar system. It has predicted moderately well patients who will have postoperative neurological problems. Brain-stem impairment seems more severe when the BSAER from the contralateral side is lost. Permanent intraoperative loss of waves II through V usually predicts severe neurological deficits or brain death. Nevertheless, a BSAER fails to monitor some brain-stem pathways anatomically distant from the acoustic pathways. This creates a false sense of intraoperative safety. For this reason some authors recommend the addition of an SSEP to the BSAER when the brain-stem is at risk.
Monitoring the function of cranial nerve VIII is used to aid in preserving hearing, locating CN VIII, and determining if the overall function of the brain stem is altered. The classic BSAER consists of a minimum of five and a maximum of seven peaks. The first five peaks, Jewett Waves I through V, are the principal peaks used in clinical practice. All occur with 10 msec of a brief click or tone presentation. Wave I is generated in the auditory portion of CN VIII. Its latency is approximately 1.5-2.1 msec in a normal adult. Wave I is present on the ipsilateral side to the stimulus but is not (Continued on page 122)
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