Olsen, Wayne O.

Attempts to specify normal threshold sensitivity by bone conduction have been unsuccessful because of problems in obtaining reliable measurements from commercially available artificial mastoids. Recent design modifications incorporated in the Bruel and Kjaer 4930 artificial mastoids have resulted in greater uniformity among these units. However, the new design has resulted in impedances that are higher than those recommended in current standards. Bone-conduction thresholds referenced to measurements made on B x00026 K 4930 artificial mastoids with the new design were performed on 60 normal listeners by three participating laboratories. The results are reported for consideration in the development of a reference threshold for hearing by bone conduction.

Cochlear and neural auditory function for a series of 30 patients with acoustic neuroma was assessed pre- and postoperatively by transiently evoked otoacoustic emission (TEOAE), auditory brain stem response (ABR), and acoustic reflex measures. Employing a retrosigmoid surgical approach, physical integrity of the acoustic nerve was preserved for 22 patients. TEOAEs were observed preoperatively for 13 patients and postoperatively for 6 of them. Postoperative neural function remained unchanged or improved following surgery for 7 patients, and cochlear function decreased. TEOAE, ABR, and acoustic reflex measurements were considered valuable in providing information about site of lesion and probable mechanisms of injury associated with tumor removal.

Simultaneous four-channel (ipsilateral, contralateral, horizontal, and noncephalic) ABR recordings were reviewed by three audiologists for 187 ears of nontumor patients, and 13 ears of patients having eighth nerve tumors. Waveforms from a subgroup of 63 ears were reexamined by the same audiologists to assess inter- and intra-judge consistency. For nontumor ears, the ipsilateral tracings provided the best identification of Waves I and III; the ipsilateral and noncephalic recordings allowed equal identification of Wave V. Waves I, III, and V generally were more robust from the ipsilateral electrode array than from any of the other electrode montages for the tumor ears. Three-way inter-judge agreement ranged from 82% to 87% for ipsilateral Waves I, III, and V and for noncephalic Wave V. Agreement between pairs of judges ranged from 86% to 100%. Intra-judge agreement ranged from 87% to 94% for ipsilateral Waves I, III, and V, and from 92% to 95% for noncephalic Wave V.

Audiologic test results were reviewed for 30 patients whose hearing was preserved following eighth nerve tumor surgery. Speech reception thresholds, 4,000-Hz thresholds, acoustic reflexes, and ABR wave I absolute latencies were generally unchanged or poorer following surgery; word recognition scores were essentially unchanged. By contrast, ABR waves III and V, and the I–III and I–V interpeak intervals, were generally improved postoperatively, including several patients who had clear waves following surgery despite absent preoperative responses. The improved ABR tracings likely indicate increased neural synchrony due to reduced pressure on the cochlear nerve following surgery, whereas the reduced hearing sensitivity probably reflects cochlear damage coincident to the surgery.

ABR absolute latencies and interpeak intervals were reviewed for 684 non-tumor ears and 75 eighth-nerve tumor ears having various degrees of high frequency hearing loss. For non-tumor ears, the percentage of abnormal absolute latencies for waves I, III, and V increased rather systematically as hearing loss increased, whereas absolute latencies for eighth-nerve tumor ears were largely abnormal regardless of hearing loss. Interpeak intervals were normal for most of the non-tumor ears, but the I–III and I–V intervals were abnormal for most of the tumor ears. The most sensitive index for otoneurologic assessment was a combination of abnormal wave V interaural latency differences or I–V interpeak interval, and the most specific criterion was the I–V interpeak interval. Tumor size influenced sensitivity of most ABR indices.

Multichannel ABR recordings for 30 otoneurologic patients were reviewed independently by three audiologists to assess interjudge consistency in determining absolute latencies and overall interpretation of ABR results. Four months later, the tracings were reviewed a second time to evaluate intrajudge consistency in interpretation of ABR waveforms. Interjudge agreement in marking latencies for waves I, III, and V within 0.2 ms was on the order of 90% or better. Intrajudge consistency was slightly higher. Only rarely did inter- or intrajudge differences in latency measurements exceed 0.3 ms. Agreement in overall interpretation of ABR results as "normal" or "abnormal" was unanimous for 90% of the patients. Across pairs of judges, the agreement for "normal" and "abnormal" classification of the ABR tracings was 97%. Intrajudge consistency for "normal" and "abnormal" categorization of the ABR results was 100% for one judge, 97% for the other two judges.

The exact nature of ipsilateral auditory pathway suppression under dichotic stimulation remains controversial. To help resolve this controversy, the distinctive feature confusions (blend vs nonblend errors) made on a dichotic listening CV test by two hemispherectomees were analyzed. Results support ipsilateral auditory pathway suppression under dichotic CV stimulation. Ipsilateral auditory pathway suppression is discussed in terms of stimulus specificity, subject, and task difference.