7.2.1 Pure-Tone Air and Bone Conduction

The audiogram is a graphic representation of a person’s hearing sensitivity across a range of frequencies important for speech understanding, and provides information about the integrity of the entire auditory system from the peripheral end organ to the level of the auditory cortex in the brain. Behavioral threshold search is performed utilizing earphones (insert or circumaural) and a bone conduction oscillator. Patients with VS will often demonstrate asymmetrical hearing thresholds that are poorer in the affected ear, though hearing may be completely normal with no significant asymmetry noted. The type of hearing loss typically associated with a VS is sensorineural, with a potential sensory component due to changes in the cochlea itself or a strictly neural hearing loss from isolated auditory nerve impact. The pure-tone audiogram itself does not provide site-of-lesion information but can be used in conjunction with other testing measures to determine the status of the cochlea versus the auditory nerve, as described later. In 1977, Johnson classified a large cohort of patients with VS according to the patterns of hearing loss configuration: high frequency (66%), flat (13%), trough shape or mid frequency (12%), and low frequency (9%).s. Literatur At least one subsequent large study confirmed similar percentages.s. Literatur

7.2.2 Word Recognition Testing

Speech reception thresholds (SRT) are determined by having the patient repeat a familiar list of two-syllable words called spondees. The SRT is identified as the presentation level at which the subject correctly identifies 50% of the spondees presented. Word recognition testing is then performed at a “comfortable” listening level (usually between 35 and 50 dB above the SRT) using monosyllabic words. This test assesses the patient’s ability to recognize and repeat an unfamiliar list of words utilizing audition alone (without lip reading). Patients with retrocochlear pathology often have poorer word recognition scores (WRS) than might be expected given the pure-tone thresholds and SRT in the affected ear.

7.2.3 Rollover

Patients with VS often demonstrate a positive rollover test, as defined by a decline in WRS as a function of increasingly louder presentation levels. A rollover index of 0.42 to 0.45 has been found to be suspicious for a retrocochlear lesion.s. Literatur ^,​ s. Literatur However, the sensitivity of the rollover test is generally regarded to be poor, considering that normal results can be obtained on a significant number of patients with confirmed VS. Still, a positive rollover test in conjunction with other abnormal findings (asymmetrical hearing loss and/or absent acoustic reflexes) strongly suggests that retrocochlear pathology may be present.

7.2.4 Acoustic Reflex Thresholds/Decay

Acoustic reflex thresholds are tested by recording the lowest level that a change in tympanic membrane (TM) compliance can be observed as the stapedius muscle contracts in response to a loud sound stimulus. A small contribution to the acoustic reflex threshold also comes from the tensor tympani muscle.s. Literatur ^,​ s. Literatur The afferent limb of the ipsilateral acoustic reflex travels via the auditory nerve to the cochlear nucleus and then to the superior olivary complex and the facial nerve nucleus in the pons, where the efferent limb via the facial nerve on the same side activates the stapedius muscle. This contracts, reducing mobility of the stapes and thus the entire ossicular chain, causing a measurable reduction in TM compliance recorded in the same ear. The contralateral acoustic reflex crosses to the superior olivary complex and facial nerve nucleus in the contralateral brainstem before traveling via the facial nerve to activate the stapedius muscle in the opposite ear. When a VS is present along the auditory nerve pathway, normal transmission of the afferent auditory signal to the brainstem is altered, either by compression or vascular disruption, often resulting in an elevated or absent acoustic reflex when the sound stimulus is presented to the affected ear even if there is excellent residual hearing. The ipsilateral reflex in the unaffected ear should not be impacted. Patients with VS may also demonstrate positive reflex decay, which means that the reflex cannot be sustained while a continuous tone is presented to the affected ear (typically 10 dB above the acoustic reflex threshold). When reflex decay is present, the reflex measured in the contralateral ear will rapidly decline to at least half of the initial compliance value. Studies have shown that abnormal acoustic reflexes have a sensitivity of only 75 to 82% for confirmed cases of VS, but a low specificity of only 11 to 30%.s. Literatur ^,​ s. Literatur The low sensitivity rate is due primarily to the very small prevalence of acoustic tumors in the population undergoing audiometric testing for hearing complaints. Thus, the significance of abnormal acoustic reflex testing should be interpreted only in conjunction with other auditory test measures.

7.3.1 Auditory Brainstem Response

Auditory brainstem response (ABR) evaluations can provide information about the integrity of the auditory pathway as afferent signals travel to the brainstem. Studies have shown that the sensitivity of the ABR in diagnosing VS is between 85 and 95%. The sensitivity rate for small tumors remains somewhat controversial, with reported values ranging from 58 to 92% for tumors measuring less than 1 cm.s. Literatur ^,​ s. Literatur ^,​ s. Literatur For tumors less than 1 cm, Don et al advocated use of the stacked ABR, which utilizes a derived-band technique by recording click responses in the presence of high-pass masking filters of various cutoff frequencies and then subtracting each response from subsequent runs. Their study showed a sensitivity of 95% and a specificity of 88% for small VS. Though not widely adopted, additional studies replicating this work may lead to more widespread use of the stacked ABR technique for screening purposes.s. Literatur

During a diagnostic evaluation for asymmetrical auditory symptoms such as hearing loss, tinnitus, perceived distortion of sound, or aural fullness, an ABR may be requested to screen for retrocochlear pathology. The neurodiagnostic ABR is performed while the patient lies quietly with eyes closed, or even more optimally, in a state of natural sleep. It is a far-field recording, with surface electrodes positioned on either the forehead or the vertex and at the level of the ear (either on the mastoid, earlobe, or anterior to the tragus). A loud click stimulus is delivered to the ears monaurally via insert earphones at 80 to 95 dB HL at a moderate rate of 20 to 30 clicks per second. This stimulus represents information mainly from the basilar (high-frequency) portion of the cochlea. Free-running EEG and EMG are monitored throughout the recording, which is ideally paused whenever large muscle artifact is present due to patient movement, swallowing, or tensing of the jaw. These artifactual EMG signals can obliterate the much smaller EEG response from the auditory pathway. Averaged EEG information collected within the first 10 ms after the onset of each stimulus is analyzed for delays in the evoked waves related to the auditory pathway and/or gross deviation from the anticipated response morphology. Five peaks are identified in a normal ABR waveform and are currently believed to originate from the following neural generators:

The morphology of the ABR is qualitatively analyzed for the presence or absence of waveform components, as well as for their reproducibility. Timing characteristics, such as absolute and interpeak wave latencies, are analyzed within and between ears. Significant findings include prolonged Wave I–III, III–V, or I–V interpeak latencies, delayed absolute latency of Wave V, or a significant interaural difference between ears for any of these parameters. The presence of a tumor within the internal auditory canal or cerebellopontine angle can affect the timing relationship of the ABR in the affected ear or cause a complete absence of synchronized neural activity. See Fig. 7‑1 for an example of an ABR recording.

The person’s hearing status as measured via conventional pure-tone audiometry may not correlate with the ABR response in the predicted fashion. For example, a person may have completely normal hearing thresholds but demonstrate an abnormal or even absent ABR. Likewise, a person with moderate hearing loss and a tumor of large size may still have a measurable ABR. When discussing management options for tumors, knowing the ABR status of the patient is sometimes helpful. When surgical intervention for hearing preservation is considered, patients with normal ABRs are likely to have better outcomes.s. Literatur The ABR also provides a baseline for intraoperative neurophysiologic monitoring of auditory nerve function during resection of cerebellopontine angle tumors via a middle cranial fossa or retrosigmoid approach. Currently, given the widespread access and very high sensitivity and specificity of gadolinium-enhanced MRI for detecting VS, ABR is no longer used at most centers in the United States as a diagnostic screening tool for VS. However, many centers still utilize preoperative and/or intraoperative ABR testing for cases where VS hearing preservation surgery is a consideration.