Anne Forrest Josey

Clinical electroneurography: Statistical analysis of controlled measurements in twenty-two normal subjects†‡

Electroneurography, an objective electrophysiologic measurement of a muscle compound action potential, is used to assess the integrity of a peripheral motor nerve. This paper describes how to perform electroneurography with particular attention to control of patient and instrumentation variables. Twenty‐two normal adult subjects were tested in a balanced, designed experiment. The resultant average difference between the right and left halves of the face was not significant. The standard deviation of this difference was divided into three components: test‐retest, daily, and intersubject. The data suggest that the most significant source of error in electroneurography is test‐retest variability (p < 0.05) and that repeated and averaged measures on a given patient will increase the precision of the test. As further understanding and experience are gained, electroneurography measurements should be interpreted in light of clinical findings and more traditional tests.

Preservation of Hearing in Acoustic Tumor Surgery: Audiologic Indicators

Preservation of hearing in patients with acoustic nerve tumors can be a goal when tumor size is small and residual hearing is intact. Overall success rates for preservation have been reported to be 20% to 40%. The overall success rate in this series is 30.7%. However, indicators of intact auditory brain stem response (waves I-III-V), good speech discrimination score, and intact acoustic (stapedial) reflex were associated with a 68.2% rate of success. Thus, a comprehensive audiologic evaluation is a guideline for selecting and counselling patients with acoustic tumors before hearing preservation procedures.

Brainstem evoked response audiometry in confirmed eighth nerve tumors.

Brainstem evoked response audiometry has been described as an effective predictor of eighth nerve disease. This study reports audiologic findings in 52 cases of surgically confirmed eighth nerve and cerebellopontine angle tumors. The results of brainstem audiometry were consistent with an eighth nerve lesion in 51 cases. Its use in site of lesion tests is highly recommended.

The acoustic reflex test in cochlear and eighth nerve pathology ears.

The clinical interpretation of acoustic reflex test results in ears with sensorineural impairment should be based on a consideration of three characteristics of the response: the reflex threshold hearing level, the reflex threshold sensation level, and the decay of the reflex. Examination of results in 152 ears with cochlear pathology and in 152 ears with acoustic tumor indicated that in some cases the combination of characteristics may be contradictory as to site of lesion. Regarding a contradictory response pattern as questionable — neither confirming nor denying a specific site of lesion — improves the predictive accuracy of the test through a reduction in the number of false identifications.

Auditory Brain Stem Response—interwave Measurements in Acoustic Neuromas

Auditory brain stem responses (ABR) of 61 patients with tumors of the VIIIth nerve or cerebellopontine angle were analyzed for latency of the I-III, III-V, and I-V interwave intervals. Of the 61 patients, 16 yielded ABR records with repeated waves I, III, and V so that interwave intervals could be measured. Results indicate that not only is the I-III interval often abnormal, but in almost 50% of the cases the III-V interval is also extended. Additional analysis included comparing a group of subjects with cochlear lesions with the 16 patients with VIIIth nerve tumors. These findings indicated a higher degree of specificity than sensitivity for the ABR interwave measurement. These findings, as well as additional analyses, are discussed.

ABR results in patients with posterior fossa tumors and normal pure-tone hearing.

Sixteen patients with confirmed mass lesions of the posterior fossa and normal hearing sensitivity for pure tones were studied. Patients’ main symptoms, auditory brain-stem response (ABR), and lesion size were analyzed. All patients manifested neurologic and/or otoneurologic symptoms or complained of hearing difficulty disproportionate to their pure-tone findings. Interestingly, the patients in this select group were younger (mean = 34 years) than the typical patient with a posterior fossa tumor. ABR results were abnormal in 15 of the patients, although several indices—including absolute and interwave latencies, interaural latency difference, and wave presence/absence—were employed to achieve this sensitivity. Lesion size varied considerably and failed to correlate with ABR or pure-tone results.

Audiologic manifestations of tumors of the VIIIth nerve.

From the first reports of abnormal auditory behavior in the 1890s until the present, the patient with an acoustic nerve tumor has been the subject of considerable discussion. The frequency of reports summarizing test data in this group of patients reflects the need of the medical community to identify these patients as well as the interest among auditory scientists in their changed auditory behavior. The first form of abnormal response in the acoustic tumor ear was observed in 1893 by Gradenigo (l) , who observed adaptation to the tone of a vibrating tuning fork. This abnormal behavior became the basis for a series of tone decay or adaptation tests. Johnson (2) reported that 77% (N = 252) of 329 patients had partial or complete decay on Green’s Modified Tone Decay Test (3). In our own review of 408 surgically confirmed acoustic tumor ears, 64% were found to have retrocochlear signs at one or more frequencies on the Carhart Tone Decay Test or the Modified Tone Decay Test. Bekesy audiometry was also attempted as a means of separating cochlear from retrocochlear ears with the pattern types identified by Jerger (4). While results showed retrocochlear changes with type I11 or IV patterns in some cases, the test was successful in identifying tumor ears only 52% of the time in a 1974 review by Sanders et a1 (5) . A more recent addition to the battery, the physiologic correlate of this behavior, acoustic stapedial reflex decay, has also been noted to occur in the tumor patient, as first reported by Anderson et a1 in 1970 (6). Johnson (2) found abnormal adaptation patterns, either in the decay or the absence of the reflex in 81% of 117 patients. In a review of 1 10 patients, Sanders (5) found 77% of the patterns abnormal by revising the criteria for rating the findings as suggestive of retrocochlear pathology and an additional 13% were questionable. Alternate binaural loudness balancing tests (ABLB) have also revealed interesting results in the acoustic tumor ear. In 1948, Dix et a1 (7) reported the use of the ABLB in separating cochlear from retrocochlear lesions, since the phenomenon of loudness recruitment was noticeably absent in the VIIIth nerve tumor ear. Findings of absence of recruitment occurred in 67% of VIIIth nerve tumor ears according to Sanders et a1 (5 ) , but results in the literature have ranged from 57% to 90% in studies of over 25 acoustic tumor ears (5, 7, 8). The Short Increment Sensitivity Index (SISI) is also a test which shows positive results in cochlear ears but negative results in normal and retrocochlear ears. Jerger (9) reported on 20 patients with Meniere’s disease (all with positive SISI scores) and 11 patients with acoustic nerve tumors (10 with negative scores and one with a questionable score). Cooper and Owen (10) suggested a test modification for assessing DLIs at high intensity (90 dB SL) to heighten the difference between cochlear and retrocochlear ears. However, increasing the test intensity above the 20 dB SL increases the number of tumor patients with positive scores (cochlear results) at low levels to false negative findings of 17% (1 1) and recommended that the test minimum level be 75 dB HL. Speech audiometry has also provided a useful tool with the rollover phenomenon at high stimulus intensities. Based on the concept presented by Jerger and Jerger (12) in 1971, Bess et a1 (13) found that 15 of 18 (83%) acoustic nerve tumors had excessive rollover or other retrocochlear abnormality. The advent of the auditory brain stem response (ABR) in site of lesion testing has markedly enhanced sensitivity in site of lesion testing in patients with hearing who are suspected of having an VIIIth nerve tumor. Our own experience has indicated that ABR testing is effective in identifying 97% of 176 lesions in our series of patients with VIIIth nerve tumors (14). In these patients, changes may occur in latency, morphology, or both in the affected ear. In large tumors, responses from the contralateral side may be affected as well. The changes affect waves I and I1 on the affected side so that I to I11 are prolonged, when they are present. Table 1 indicates the changes which occur and their frequency in a series of 308 surgically confirmed VIIIth lesions (14). The ABR is so sensitive to VIIIth nerve tumors that it may act as monitor for patients who have lesions small enough to escape routine radiologic evaluation. The patient in Figure 1 had serial ABRs and hearing tests over a 3 yr period prior to radiologic confirmation and subsequent surgical removal. Thus, the ABR is helpful in screening relatives of vonRecklinghausen’s syndrome; so much so that after baseline screening with computerized axial tomography (CT scan) or magnetic resonance imaging (MRI) and hearing tests with ABR, annual reevaluation is only audiologic, utilizing hearing tests and ABR. The ABR is also useful in following the patient through the operative procedure if the object is to spare the VIIIth nerve. The patient whose ABR is illustrated in Figure 2