Threshold estimation in adult normal and impaired hearing subjects, using Auditory Steady State Responses

Objective: To compare the estimation of hearing threshold values from behavioral and electrophysiological (ASSR) methods, in subjects with normal hearing and sensorineural hearing impairment. In particular we were interested in estimating : (a) the error margin of the ASSR estimated threshold levels with a commercial instrument (Audera) ; and (b) how the ASSR estimated hearing levels depend on the degree of hearing loss. Methods: We have tested 32 subjects (17 male and 15 female) for a total of 61 ears. From those 11 (22 ears) presented normal hearing threshold values ( 0- 19 dB HL) and 21 (39 ears) sensorineural deficits. The latter group was subdivided in three classes namely : (i) 11 subjects (16 ears) with moderate hearing loss (53.7 dB HL ±12.3); (ii) 5 subjects (11 ears) with severe hearing loss (80.6 dB HL ±12.7); and (iii) 6 subjects (12 ears) with profound hearing loss (101 dB HL ± 5.5). Results: The data show that for the normal hearing subjects the ASSR threshold is approximately around 20 dB ( 11 dB SD) for the frequencies 0.25 – 1.0 kHz. For the higher frequencies the ASSR threshold increases up to 40 dB (12.5 dB SD) at 8.0 kHz. In our hearing impaired subjects also, we observed this phenomenon. In fact the ASSR proved to reliably predict the behavioral threshold (+/- 5 dB) especially in the group or most impaired ears. Similar errors were found in the less impaired ears, and for the ASSR at high frequencies. The regression analysis confirmed that the difference between the ASSR-estimated and behavioral threshold values, significantly decreases with the amount of hearing loss. Our data evidenced that for a 10 dB increment of the behavioral threshold, the ASSR threshold increases of 7 dB. The difference between the two methods, of about 27 dB observed in normal subjects tends to cancel in the hearing loss greater then of 95-100 dB HL. Conclusion: The results of our analyses indicate that the threshold estimates are rather discordant with the behavioral thresholds. Particularly, it seems that the correction factor we have applied does not rely on factors adequately modeled (instrumentation-wise) to compensate for the effects of hearing loss on ASSR thresholds. The threshold estimation is adequately modeled for high levels of hearing loss particularly for patients needing a cochlear implant

Advantages of the Retrosigmoid Approach in Auditory Brain Stem Implantation

From April 1997 to December 1999, six patients (five men and one woman), ranging in age from 22 to 37 years with neurofibromatosis type 2 (NF2) were operated on via the classic retrosigmoid-transmental (RS-TM) approach for removal of a vestibular schwannoma (VS) (tumor size from 12 to 40 mm) and for auditory brain stem implantation (ABI). After tumor removal, the floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleos were reached, and the ABI was inserted. More recently, an ABI was implanted via the retrosigmoid approach in a 4-year-old boy with a cochlear malformation (common cavity) associated with cochlear nerve aplasia. Electrically evoked auditory brain stem responses (EABRs) and neural response telemetry (NRT) were performed to verify the correct positioning of the inserted electrodes. No major complications related to ABI were observed. ABI has been activated to date in five of the NF2 patients. Auditory sensations with various numbers of electrodes were evoked in all patients. We consider the RS-TM approach the route of choice for ABI insertion in patients with NF2 and good hearing, offering a chance of hearing preservation, and in patients with complete cochlear ossification, severe head trauma and cochlear fracture, or nerve disruption, or a combination of these. A new indication for ABI implantation via the RS approach is presented by patients with bilateral cochlear nerve aplasia

Intraoperative Monitoring for Hearing Preservation and Restoration in Acoustic Neuroma Surgery

The present article reports on our experience with hearing preservation during 158 acoustic neuroma (AN) operations via the retrosigmoid-transmeatal (RS-TM) approach with the aid of intraoperative auditory monitoring. Several auditory monitoring methods are described. Of these, the bipolar cochlear nerve action potential (CNAP) was found to be the most helpful in preserving hearing. Of 106 patients with useful hearing preoperatively, more than 50% had useful hearing after surgery. Electrical auditory brainstem responses were useful in the placement of an auditory brain stem implant (ABI) in 4 patients with neurofibromatosis type 2 (NF2). All 4 reported speech perception benefit and use their ABIs regularly in their lives. It is our firm belief that intraoperative auditory monitoring has a pivotal role in the preservation and restoration of hearing in AN surgery