Eighth Nerve Disorders Research Articles

Contributions from the auditory nerve to the brain-stem auditory evoked potentials (BAEPs): results of intracranial recording in man

Intraoperative recordings obtained from electrodes placed on the scalp (vertex and earlobe or ear canal) in response to click stimulation were compared with recordings made directly from the auditory nerve in patients undergoing microvascular decompression (MVD) operations to relieve hemifacial spasm (HFS) and disabling positional vertigo (DPV). The results support earlier findings that show that the auditory nerve is the generator of both peak I and peak II in man, and that it is the intracranial portion of the auditory nerve that generates peak II. The results indicate that the second negative peak in the potentials recorded from the earlobe is generated by the auditory nerve where it passes through the porus acusticus into the skull cavity, and that the proximal portion of the intracranial portion of the auditory nerve generates a positive peak in the potentials that are recorded from the vertex. This peak appears with a latency that is slightly longer than that of the second negative peak in the potentials recorded from the earlobe (or ear canal). The second negative peak in the recording from the ear canal and the positive peak in the vertex recording contribute to peak II in the differentially recorded BAEP. Since our results indicate that the difference in the latency of the second negative peak in the recording from the earlobe and that of the positive peak in the vertex recording represents the neural travel time in the intracranial portion of the auditory nerve, this measure may be valuable in the differential diagnosis of eighth nerve disorders such as vascular compression syndrome.

Patterns of phoneme identification error in cochlear and eighth-nerve disorders.

The purpose of this study was to determine whether patterns of phoneme identification error differ among listeners with cochlear and retrocochlear auditory disorder. Speech intelligibility performance was analyzed in 15 patients with confirmed eighth-nerve disorder and in 15 patients with cochlear disorders, matched to the retrocochlear group for age and audiometric configuration, using confusion matrices derived from responses to a monosyllabic word list. Results indicated that: (1) vowel errors were more prevalent in the retrocochlear group and varied directly with increasing stimulus presentation level; and (2) consonant errors did not differ in type or relative frequency between the two groups, nor was there a level-dependent effect for consonant errors. These results are supported by the results of closed-set vowel identification tests and thus do not appear to be an artifact of open-set testing. Vowel errors may account for a major part of the speech 'rollover' phenomenon typical of retrocochlear auditory dysfunction.

Patterns of phoneme identification errors in cochlear and eighth nerve disorders

The purpose of this study was to determine whether patterns of phoneme identification errors differ as a function of signal presentation level among listeners with cochlear and retrocochlear auditory disorder. An analysis of the speech intelligibility performance of 15 patients with confirmed eighth nerve disorder was conducted, using confusion matrices derived from responses to a monosyllabic word list. The same analysis was conducted on the responses of a group of 15 patients with cochlear disorders, matched to the retrocochlear group for age and audiometric configuration. The results indicated that: (1) vowel errors were more prevalent and varied directly with increasing stimulus presentation level in the retrocochlear group; and (2) consonant errors did not differ in type or relative frequency between the two groups, nor was there a level‐dependent effect. These results are supported by closed‐set vowel identification tests and thus do not appear to be an artifact of open set testing. Vowel errors may account for a major part of the speech “rollover” phenomenon typical of retrocochlear dysfunction.

Latency of the acoustic reflex in eighth-nerve tumor.

We evaluated acoustic reflex morphologic features in four subjects with confirmed, unilateral acoustic neuroma. All four subjects showed marked reduction in absolute reflex amplitude and alteration in the reflex amplitude-intensity function in the ear with the eighth-nerve disorder. The early, fast-rising component of the normal reflex was also typically absent in the ears with tumor. Interaural latency comparisons were made in three ways. At equal reflex sensation levels and equal reflex sound pressure levels, latency ws substantially delayed in the ear with the eighth-nerve disorder. At equivalent reflex amplitudes, however, latency was equivalent in normal ears and ears with eighth-nerve disorder. Results suggest that delayed onset of the acoustic reflex in subjects with eighth-nerve disorder may reflect amplitude and wave-form morphologic effects rather than a latency prolongation per se.

Evaluation of diagnostic audiometric tests.

Five diagnostic audiometric procedures - acoustic reflexes, performance-intensity functions for monosyllabic (phonetically balanced) words, Békésy audiometry, Békésy comfortable loudness audiometry and suprathreshold adaptation test - were critically evaluated by decision matrix analysis and information theory analysis. The subjects were 20 patients with surgically confirmed eighth nerve disorders and 20 patients with presumed cochlear disorders. Each patient completed all test procedures. In terms of decision matrix analysis, sensitivity (correct identification of eighth nerve site) ranged from 45 to 85%. Specificity (correct rejection of patients with cochlear site) ranged from 70 to 100%. Predictive value ranged from 74 to 100% for positive results and from 62 to 82% for negative results. The efficiency of the five tests ranged from 68 to 78%. In terms of information theory, positive test results increased the probability of eighth nerve disorder in any subject from 50% (a priori) to 74-100%. Negative test results decreased the probability of eighth nerve disorder from 50% to 18-38%. Relative differences among the diagnostic effectiveness of the five individual test procedures were not pronounced. However, the suprathreshold adaptation test (results confined to 500, 1 000 and 2 000 Hz) was less efficient than the other four diagnostic techniques.

Diagnostic Value of Crossed vs Uncrossed Acoustic Reflexes: Eighth Nerve and Brain Stem Disorders

Acoustic reflexes to crossed (sound in one ear; probe in opposite ear) and uncrossed (sound and probe in the same ear) stimulation were obtained from 17 patients with eighth nerve or brain stem disorders and 20 normal control subjects. Comparison of crossed and uncrossed reflex thresholds provided an important diagnostic tool for differentiating the eighth nerve from the brain stem site. Egihth nerve disorders were usually characterized by diagonal reflex patterns. In contrast, brain stem disorders were usually characterized by horizontal reflex patterns. Detailed evaluation of the reflex time course in two selected patients with intra-axial brain stem disorder showed unusual abnormalities in the crossed acoustic reflex.

The acoustic reflex in eighth nerve disorders.

Acoustic reflex data were analyzed in 30 patients with surgically confirmed retrocochlear disorder, primarily acoustic neurilemoma. Results confirm the reality of both elevated reflex thresholds and reflex decay as diagnostic signs of retrocochlear disorder. The acoustic reflex was present and normal in 7 patients, present and abnormal in 4 patients, but absent at all test frequencies in 19 of the 30 patients. Results support the diagnostic value of the acoustic reflex test in the evaluation of suspected eighth nerve disorders.

Audiological comparison of cochlear and eighth nerve disorders.

Four new approaches to the diagnostic problem of differentiating cochlear and retrocochlear disorders are described. The four procedures are: 1) the forward-backward discrepancy in the Békésy audiogram; 2) the Békésy Comfortable Loudness (BCL) test; 3) the rollover of the performance-intensity function for PB words; and 4) the acoustic reflex. Findings on these new auditory test procedures were contrasted in three groups of patients having either confirmed cochlear, VIII nerve or brain stem disorders. Results suggest that suprathreshold levels are a more fruitful area in which to explore VIII nerve symptomatology than threshold levels.

Diagnostic significance of PB word functions.

Performance-Intensity (PI) functions for phonetically balanced (PB) words were compared in 41 patients with presumably cochlear disorder and 16 patients with retrocochlear disorder. The PI functions of all of 10 eighth nerve disorders and three of six brain stem disorders showed a pronounced rollover phenomenon. After reaching a maximum the PI function declined substantially with further increase in speech level. Implications of the phenomenon for differential diagnosis of cochlear and retrocochlear disorders are discussed.

The audiological examination as an aid in diagnosis.

THE MOST IMPORTANT contribution of diagnostic audiometry to the total evaluation of the vertiginous patient is the differentiation between cochlear and eighth nerve disorders. There are four auditory tests of particular value in this regard. Used in combination as a unified test battery they have proven effective as valid and reliable indices of site of lesion. 1 The four tests, and their interpretation, are summarized in the Table. The first test in the battery, the short increment sensitivity index (SISI), measures the patient's ability to hear small, short changes in sound intensity. Twenty 1-db increments of 200-msec duration are superimposed at 5-sec intervals on a continuous pure tone signal at a sensation level of 20 db. The patient is instructed to respond whenever there is a change in the loudness of the steady sound. Patients with cochlear hearing loss yield high (positive) scores on this test, usually above 55% correct.

GENICULATE GANGLION FACIAL PALSY.

EVER since Ramsay Hunt's (1907)2description of the syndrome of facial palsy, herpetic rash in and around the ear on the same side often accompanied by a constitutional disturbance and sometimes by deafness and giddiness, there has been a tendency to think of a lesion of the nerve trunk at or near the geniculate ganglion only when the Ramsay Hunt syndrome is present. For some years now, however, I have been aware of a group of cases of peripheral facial palsy in which the onset of the paralysis is unaccompanied by any herpetic vesicles around the pinna or by any constitutional disturbance or eighth nerve disorder, but in which there is loss of lachrymal secretion on the paralyzed side, Cawthorne ( 1963).1Before I included a test of lachrymation as part of the routine examination of eighth nerve function in a case of facial palsy, I used to regard

THERAPY OF NERVE DEAFNESS AND TINNITUS AURIUM: USE OF LARGE DOSES OF THIAMINE HYDROCHLORIDE AND EVALUATION OF RESULTS, WITH A SOURCE OF POSSIBLE ERROR IN INTERPRETATION OF IMPROVEMENT

Among the more recent approaches to the therapy of eighth nerve disorders is that of the administration of vitamins, more specifically the use of the various factors of the vitamin B complex. Of these, vitamin B 1 , or thiamine hydrochloride, by virtue of being considered the anti-neuritic vitamin, has received the greatest attention both in the medical and in the lay press. In ascertaining the efficacy of any treatment, scientific control of the results is essential. A series of cases of deafness comparable without treatment may serve as the control for the series of cases with treatment. If the defect is bilateral and only one ear is treated, as in surgical operation for deafness, the untreated ear furnishes the best control. In a case of long-standing chronic deafness, the previous course may in itself be taken as the control period. In the treatment of deafness, it is necessary to recognize the