Hearing preservation following suboccipital removal of acoustic neuromas

Advances in the diagnosis and intraoperative management of acoustic neuromas have greatly reduced the incidence of neurologic deficits following their removal. Ninety-three patients underwent acoustic tumor removal during a 41/2-year period, and hearing preservation was attempted in 20 cases. Hearing was preserved in 65% of the entire series, and excellent results were obtained in tumors less than 1.5 cm. No patient with a tumor greater than 1.5 cm had serviceable hearing preserved when total tumor removal was performed. Two patients, one with neurofibromatosis and one with an acoustic neuroma in an only-hearing ear, had planned partial tumor removal with preservation of hearing. Preoperative auditory brainstem response results were not predictive of postoperative hearing preservation. Intraoperative auditory brainstem response monitoring demonstrated that loss of wave V consistently correlated with loss of hearing postoperatively, whereas persistence of wave V (with a latency prolongation not exceeding 3.00 ms) was predictive of successful hearing preservation regardless of latency increases.

Effect of anesthesia on acoustically evoked middle latency response in guinea pigs

Increased recent interest in the middle latency response (MLR) has necessitated a clarification of the possible effects of anesthesia on the response. Our study was designed to examine the changes, in the guinea pig MLR, which occurred during anesthesia with ketamine, xylazine or both ketamine and xylazine. Under anesthesia the response remained present and the threshold remained stable. After anesthesia, significant changes in amplitude, latency, and general morphology of the waveform took place, however these were consistent and predictable. For studies requiring the MLR, it is best to avoid anesthetic agents. However, with care the MLR can be used as a reliable measure of auditory system sensitivity under anesthesia.

Usefulness of 1000 Hz tone-burst-evoked responses in the diagnosis of acoustic neuroma

The auditory brain stem response (ABR) has become widely recognized as a sensitive and cost-effective screening modality in neuro-otologic diagnosis. However, the audiometric characteristics of the test ear may obscure the interpretation of the click-evoked ABR, particularly in the face of high-frequency hearing loss. It is often unclear whether latency delays or absent responses are attributable to retrocochlear disease or simply to the magnitude of the patient's hearing loss. The acoustic click stimulus commonly used in ABR testing activates predominantly the basilar membrane in the 2000 to 4000 Hz range. Because many cochlear and retrocochlear processes are associated mainly with hearing loss in this range, we have found it helpful in selected cases to use 1000 Hz tone-burst stimuli to circumvent the effects of elevated hearing thresholds on the ABR. In this article, our experience with the use of 1000 Hz nonlinearly gated tonebursts in 17 patients with acoustic neuroma is presented.

Middle-latency responses. II. Variation among stimulation sites

We investigated the relationship between thresholds of the electrically evoked auditory brain-stem response (EABR) and the electrically evoked middle-latency response (EMLR), and the variation in EMLR thresholds and dynamic ranges with site of stimulation. The EABRs and EMLRs were recorded in albino guinea pigs in response to electrical stimulation at the round window, promontory, scala tympani, and modiolus. The EABR and EMLR thresholds were similar. There was no significant difference between thresholds for round-window and scala tympani stimulation. Amplitude/intensity functions for the EMLR differed with site of stimulation. The EMLR seems to be comparable with the EABR for assessing the electrical excitability of the auditory pathway with less electrical artifact contamination. In this respect, round-window and scala tympani stimulation sites are equally efficacious.

Neurophysiologic intraoperative monitoring: II. Facial nerve function

Intraoperative facial nerve monitoring provides a potentially useful adjunct to recent surgical advances in neurotology and neurosurgery. These measures further aid the surgeon in preserving facial nerve function by enhancing visual identification with electrical monitoring of mechanically evoked facial muscle activation. Facial nerve monitoring in neurotologic surgery may achieve the following goals: (1) early recognition of surgical trauma to the facial nerve, with immediate feedback made available to the surgeon through monitoring of mechanical activation; (2) assistance in distinguishing the facial nerve from regional cranial nerves and from adjacent soft tissue and tumor with selective electrical stimulation; (3) facilitation of tumor excision by electrical mapping of portions of tumor that are remote from the facial nerve; (4) confirmation of nerve stimulability at the completion of surgery; and (5) identification of the site and degree of neural dysfunction in patients undergoing nerve exploration for suspected facial nerve neoplasm or undergoing decompression in acute facial palsy. This paper provides an overview of intraoperative facial nerve monitoring principles and methodology and reports a recent clinical investigation that demonstrates the utility of facial nerve monitoring in translabyrinthine acoustic neuroma surgery.

Middle-latency responses. I. Electrical and acoustic excitation

The electrically evoked auditory brain-stem response has been used in the past to assess auditory system function with regard to cochlear prosthesis application. The brief latency of the response makes it susceptible to electrical artifact contamination, and waveform identification is often difficult. As a possible alternative for a noninvasive measure of system excitability, the middle-latency response (MLR), elicited by electrical stimulation, was investigated. Middle-latency responses were recorded in response to acoustic and round-window electrical stimulation in albino guinea pigs. Acoustic and electrically evoked MLR waveforms were similar, as were their respective latency/intensity functions. Amplitude/intensity functions for the electric MLR showed greater variability than acoustically evoked MLR functions. The electric MLR is readily evoked and relatively insensitive to electrical artifact in the guinea pig. It is potentially a useful tool in assessing the integrity of auditory pathways and consequently in the development of diagnostic tests for cochlear implant candidates.

Normal auditory brainstem response in patients with acoustic neuroma

Auditory brainstem response testing has been a major breakthrough in audiologic screening for acoustic neuroma because of its high degree of sensitivity. Although it is not uncommon for other cerebellopontine angle masses to present with normal ABR findings, reports of eighth nerve tumors with false-negative auditory brainstem response tests are quite rare. A series of 120 acoustic neuromas resected at the University of Michigan was reviewed and revealed two such patients. These two patients presented with asymmetric sensorineural hearing loss and unilateral tinnitus and were found to have completely normal auditory brainstem response. The diagnosis of acoustic neuroma would have been delayed if a comprehensive evaluation had not been pursued.

An intrasubject comparison of electric and acoustic middle latency responses

The middle components of the auditory evoked response (middle latency response, MLR) were evoked by acoustic clicks from the normal-hearing ear and by charge-balanced biphasic current pulses from the severe-to-profoundly hearing-impaired ears of patients undergoing labyrinthectomy for the management of intractable vertigo. A vertex-positive peak with a latency ranging from 27 msec to 38 msec (Pa) was characteristic of both the electric and the acoustic MLR. In subjects, the electric Pa always preceded the acoustic Pa in latency. In addition, the electric Pa had a sharper appearance than did the acoustic Pa. The electric MLRs were elicited by a range of stimulus intensities and persisted after the completion of labyrinthectomy.

Neurophysiologic intraoperative monitoring: I. Auditory function

As a result of advances in neuro-otology and neurosurgery, surgeons often operate in the vicinity of sensitive and delicate neural structures in an attempt to restore their function, remove tumor, or alleviate distressing and disabling symptoms. Concerns about preservation of neural function during these surgical procedures have provided the need and motivation for the development and adaptation of neurophysiologic techniques designed to provide feedback about impending trauma. The most common functions that are at risk in neuro-otologic surgery are related to the auditory nerve and centers and the facial nerve. This paper deals with principles and practices of intraoperative monitoring of auditory function. The following applications are illustrated and discussed: 1) intraoperative monitoring of auditory function during posterior fossa tumor resection, 2) auditory brain stem response and VIIIth nerve monitoring during retrolabyrinthine vestibular nerve section, 3) monitoring of auditory function during microvascular decompression of cranial nerve VII, VIII, or IX, and 4) special applications related to cochlear implant surgery.

Pitfalls in neurotologic diagnosis

Several cases illustrating potential pitfalls in neurotologic diagnosis from the viewpoint of the otologist are presented. The role of auditory brain stem response testing is specifically emphasized in cases with discordant audiologic and radiographic findings. Included are one case of a "false negative" ABR in a patient with an intracanalicular acoustic neuroma, a case of a "false positive" CT scan in a patient with Meniere's disease, and a case of a patient with normal hearing in whom an acoustic neuroma was discovered serendipitously.

Electrically evoked middle-latency auditory potentials in cochlear implant candidates

Electric middle-latency auditory evoked responses (EMLRs) to transtympanic promontory stimulation were obtained from 19 of 22 ears of profoundly hearing-impaired patients evaluated for cochlear implant candidacy. The EMLRs were characterized by positive polarity peaks with latencies ranging from 20 to 30 ms, with the majority of responses exhibiting peaks in the range of 26 to 30 ms. Generally, the configuration of the EMLRs closely resembled the configuration of acoustic MLRs. While in most cases, behavioral thresholds to identical promontory stimulation were slightly lower, EMLR thresholds closely approximated behavioral electrical promontory thresholds. The EMLR thresholds correlated positively with implanted thresholds and exhibited a negative correlation with implanted dynamic current ranges.

Predictive value of ABR in infants and children with moderate to profound hearing impairment

Click and 500 Hz tone pip ABR thresholds obtained in infancy were compared to pure-tone thresholds in 25 preschool-aged patients with moderate to profound hearing impairments. The electrophysiologic thresholds obtained in infancy were generally lower than the recent audiometric thresholds. Clicks best predicted the lowest pure-tone threshold within the 1 to 8 kHz range. The 500 Hz tone-pip thresholds were closest to the lowest of 250 Hz and 500 Hz pure-tone thresholds. Regression analyses involving audiometric and electrophysiologic thresholds and 95% confidence intervals of the differences between their means provide a basis for behavioral threshold prediction in this patient group.

Test-retest variability of auditory event-related potentials

Auditory event-related potentials (ERP, P3 components) were recorded from normal young adult subjects (17 to 37 yr) during two sessions scheduled 1 to 2 weeks apart. The test-retest intrasubject variability and the intersubject variability of the latency and amplitude values of components N1, P2, and P3 were investigated and analyzed. Between subjects, there was no evidence of systematic age-related changes of component latencies and amplitudes: the latency of P2 came closest to exhibit such an effect. Within subjects, there were no statistically significant changes of component latencies and amplitudes between test and retest, although ERP component latencies tended to be reduced on retest, especially the latency of P3.