[VESTIBULAR EXAMINATIONS. ELECTRONYSTAGMOGRAPHY].
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Head-shaking nystagmus (HSN) is induced by oscillating the head at high frequency in the horizontal plane. This test is used in the clinic to detect the presence of a unilateral loss of vestibular function. HSN has been described as monophasic with fast-phase direction towards either side, or biphasic with the direction of fast phases reversing after a few seconds. Loss of vestibular function amplifies existing non-linearities in the vestibular system, so that imposed sinusoids can induce biases which are the source of HSN. Fifty-one patients suffering from loss of peripheral vestibular function (43 partial, 11 total unilateral tests) were exposed to whole-body sinusoidal stimulation, with increasing head velocities (90-220 degrees/s) at 1/6Hz, to explore the consistency of per-rotatory induced biases. A bias was induced in all cases, but it wandered on either side, healthy or pathologic, unless test head velocities were larger than approximately 180 degrees/s. Given this condition, the slow-phase bias was located towards the pathologic side for all patients with significant bias ( > 5 degrees/s). These observations demonstrate that the sign and amplitude of the bias is variable and is not correlated with the lesioned side, unless high head velocities are imposed. This explains why the direction of the initial phase of HSN in the clinic seems so labile. Subsequent monophasic or biphasic characteristics of HSN are simply the reflection of interactions between two main time constants associated with "velocity storage" and "gaze holding" in the vestibular central processes.
Prenatal and perinatal correlates of abnormal auditory brainstem responses in neonates have been studied extensively. In contrast, vestibular function during the first year of life has received sparse attention. Using a specially modified vestibular test battery, 65 infants (17 low-risk, 48 high-risk) were initially evaluated during their first 6 months of life. Results revealed normal vestibular function in 46 infants (13 low-risk, 33 high-risk) and abnormal findings at either 3 or 6 months in 19 infants (4 low-risk, 15 high-risk). Correlations between vestibular results and variables such as auditory brainstem response results, birth history, and postnatal course in the neonatal intensive care unit were analyzed statistically. While some differences were mildly significant, none were highly significant. The lack of significant correlation between abnormal auditory brainstem response and vestibular results is of particular interest.
The physician must be the captain of the vestibular diagnostic team based on his interest, knowledge, and capability. It is not the battery of vestibular or etiologic function tests used that provides diagnostic answers. It is the intelligent application and use of the indicated vestibular tests that are practically correlated and clinically evaluated that will give the knowledgeable physician significant information. This can best be accomplished when testing is done in physical proximity to the responsible physician and when the tests used are knowledgeably ordered and evaluated. Under such circumstances, vestibular function testing is always cost-effective and therapeutically useful.
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OBJECTIVE: To compare hearing results as a function of vestibular ablation in the treatment of Ménière's Disease, using gentamicin perfusion. STUDY DESIGN: A retrospective review of patients with Ménière's Disease treated by gentamicin perfusion of the inner ear via the MicroWick device. SETTING: A tertiary otologic referral center. PATIENTS AND INTERVENTIONS: The charts of patients treated with gentamicin perfusion via the MicroWick between the years 1998 and 2000 were reviewed. The results for patients with functional hearing in the affected ear were analyzed and were compared with the results in patients without functional hearing. MAIN OUTCOME MEASURES: Audiologic and vestibular test results as well as subjective symptoms. RESULTS: There were 45 patients who met the inclusion criteria. The averages for speech discrimination score and pure tone average before treatment were 92% and 38 dB, and after treatment were 82% and 47 dB. Patients were divided into two groups: Group 1 (20 patients), less than 75% ice air caloric reduced vestibular response (RVR); Group 2 (25 patients), those who reached greater than 75% ice air caloric RVR. There were 8 patients (17.6%) with persistent vertigo; 7 were from Group 1, and 1 was from Group 2, which was statistically significant (p = 0.007)wwww. The pure tone average dropped an average of 3 dB for Group 1 and 15 dB for Group 2. The difference in hearing loss between the two groups was statistically significant (p = 0.01). CONCLUSION: This study suggests that there is a correlation between the degree of vestibular ablation, the control of vertigo, and the risk of hearing loss. Patients with functional hearing seem to have a similar success rate for vertigo control, compared with patients who already had lost functional hearing before treatment. Future investigation may determine if less than 100% RVR, but greater than 75% RVR, is an alternative end point with adequate vertigo control and reduced risk of hearing loss.
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The usefulness of vestibular testing is directly related to the accuracy of the test interpretations. Two factors, subjective analysis of large test data sets and failure to make appropriate age corrections, tend to reduce test accuracy. Correction of these problems can be accomplished by application of physiologically based models of vestibular function and multivariate classification techniques to the test data, thereby creating a more objective test interpretation procedure. Herein we report our results on the use of this strategy for analysis of sinusoidal harmonic acceleration (SHA) test interpretation. For each patient, models reduce the large set of SHA test variables to three key parameters: asymptotic gain, vestibulo-ocular reflex time constant, and bias. In addition, the new technique objectively adjusts these parameters for the patient's age. Finally, each patient's set of parameters are statistically classified as either normal or as unilateral peripheral deficit. Based on learning sets of 57 normals and 30 patients with a full unilateral peripheral deficit, this new technique resulted in a misclassification rate between the categories of normal and full unilateral loss of 3.4%, comparing favorably to the present method's misclassification rate between normal and abnormal of 13.8%. We also analyzed and classified a test group consisting of patients with possible partial unilateral deficits using the same classification function as the normal and full unilateral learning sets. Even though the classifier was not optimized for the partial group, results seemed favorable relative to the human interpreter. These results validate the accuracy and utility of physiological parametric models and multivariate statistical classification in SHA test interpretation.
The recent advances of various diagnostic procedures made possible to detect intracanalicular neuroma in early stage. The operative results have been also greatly improved by transtemporal approach using surgical microscope and dental burr. In addition, the revised suboccipital approach contributed to obtain better operative results. Reviewing our clinical experiences with 15 cases operated from May 1969 to December 1971, appropriate combination of applicable diagnostic procedures and selection of operative method are discussed. Multiple audiologic examinations consisting of pure tone audiometry, speech discrimination, Békésy audiometry, alternate binaural loudness balance test, tone decay and uncomfortable lebel were performed in 11 cases. 9 of these cases manifested retrocochlear type of hearing loss. Spontaneous nystagmus and induced nystagmus, together with balance test, were evaluated for vestibular function. Bithermal caloric test disclosed unilateral canal paresis in all cases. In 13 cases, spontaneous nystagmus was observed. Analysis of optokinetic nystagmus using electronystagmography suggested tumor invasion to the brain stem and cerebellum. Tomography of the internal auditory canal showed abnormal findings in all cases. Cisternography with 2 ml of myodil was performed in 7 cases, all of which were diagnosed definitely by filling defect. Angiography showed abnormal vascular displacement with the tumor of size exceeding 3 cm in diameter. Translabyrinthine approach was performed in 5 cases. Suboccipital approach was applied in 6 cases. Combined approach of these two routes was used in 4 cases. 11 patients returned to their original work postoperatively. To obtain the correct diagnosis of acoustic neuroma in early stage, multiple audiologic examination are indispensable together with detailed vestibular function tests when the unilateral sensory neural hearing loss is suspected. Tomography of the internal auditory canal is also advisable as routine roentonologic study. If the clinical sign is confined to the 8th nerve only, myodil cisternography is useful, however, angiography is reliable in more advanced cases. When the tumor size is below 2 cm and signs of the spread to the brain stem, cerebellum or lower cranial nerves are lacking, translabyrinthine approach is indicated. Suboccipital approach is advisable for more advanced cases or undefinitive cases. Planning of combined approach of both routes is seemed to be not necessary in any case from our experience.
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