Asymmetric hearing loss: toward cost-effective diagnosis.

OBJECTIVE: Unilateral audiovestibular symptoms are commonly seen in clinical practice, and the treating physician frequently considers retrocochlear disease in the differential diagnosis. These considerations often precipitate a lengthy and expensive battery of tests. This study was conducted to evaluate the diagnostic yield of the various tests available in the investigation of these symptoms and to recommend changes to practice patterns, if appropriate. METHODS: We examined a cohort of 310 patients with unilateral audiovestibular symptoms who were referred to our radiology department for investigation. All patients had complete auditory assessments, and 144 underwent auditory evoked potential testing (ABR). There were 258 contrast-enhanced CT (CECT) scans and 86 gadolinium-enhanced MRI (Gd-MRI) scans performed. RESULTS: A total of 12 cerebellopontine angle (CPA) tumours were diagnosed. CONCLUSIONS: MRI is the diagnostic tool of choice in patients with asymmetric sensory neural hearing loss (ASHL), but it is expensive and not always easily available. Analysis of diagnostic accuracy of CECT and ABR, both used as screening tests, demonstrate their limitations and reveals that CECT is no more accurate than ABR, although it is three times more costly. Based on these data, we have changed our diagnostic approach to patients with unilateral audiovestibular symptoms. Patients are either evaluated with MRI or are screened initially with ABR based on clinical suspicion. CECT is no longer routinely included in the evaluation of patients with ASHL. Finally, newer emerging MRI techniques promise faster and less expensive examinations with increased diagnostic accuracy.

The relation between head size and auditory brain-stem response interpeak latency maturation.

In developmental populations, duration of auditory brain-stem response (ABR) I-III, III-V, and I-V vary substantially across individuals, particularly among preterm infants. Adult ABR interpeak latency has a strong correlation with brain-stem size and weaker correlation with head size. To determine if head size might contribute to this increased interpeak latency variability among infants, ABR data were normalized based on head circumference. Normalization by head circumference did not reduce interpeak variability. Further analyses revealed a negative correlation between interpeak latency and head circumference that varied as a function of age. Before 42 weeks conceptional age (CA), a significant relation exists between increased head circumference and decreased duration of the III-V and I-V intervals, but not the I-III interval. For infants older than 42 weeks CA, there was a significant relation between increased head circumference and decreased duration for the I-III intervals but not the III-V and I-V intervals. An age-dependent correlation between decreasing interpeak latency and increasing head circumference suggests that improved neural transmission through the auditory nerve and brain-stem pathway offset or even overcompensate for developmental lengthening of the sensory pathway. Also, developmental time constants obtained from nonlinear curve fit analyses were shorter for normalized than non-normalized data, particularly for the I-V interval. Therefore, correction of ABR data for the length of the sensory pathway may be important to estimate accurately maturation rate for developmental populations.

Frequency-specific maturation of the eighth nerve and brain-stem auditory pathway: evidence from derived auditory brain-stem responses (ABRs).

Previous studies of human auditory development using frequency-specific auditory brain-stem responses (ABRs) have reported that maturation for both peak and interpeak latencies occurs earlier for responses generated by low-frequency stimuli. In two of these studies, low-frequency ABRs presumed to originate from apical locations in the cochlea were likely dominated by activity from higher frequency regions closer to the base. In the present study, the high-pass noise-masking technique was used to generate derived ABRs that represent activity from isolated place specific regions along the basilar membrane. Analysis of auditory brain-stem maturation based on I-V interpeak latency differences with adult means revealed a frequency-specific pattern of development. Developmental changes occurred faster and mature function was attained earlier for ABRs from the mid-center-frequency (CF) derived conditions than from either the highest or lowest CF derived conditions. The differential maturation of mid-CF derived ABRs may reflect the delayed effects of the pattern of development that occurs in the cochlea.

Maturation of the traveling-wave delay in the human cochlea.

The maturation of the traveling-wave delay in the human cochlea was investigated in 227 subjects ranging in age from 29 weeks conceptional age to 49 years by using frequency specific auditory brain-stem responses (ABRs). The derived response technique was applied to ABRs obtained with click stimuli (presented at a fixed level equal to 60-dB sensation level in normal hearing adults) in the presence of high-pass noise masking (slope 96 dB/oct) to obtain frequency specific responses from octave-wide bands. The estimate of traveling-wave delay was obtained by taking the difference between wave I latencies from adjacent derived bands. It was found that the traveling-wave delay between the octave band with center frequency (CF) of 11.3 kHz and that with CF of 5.7 kHz decreased (about 0.4 ms on average) in exponential fashion with age to reach adult values at 3-6 months of age. This decrease was in agreement with reported data in kitten auditory-nerve fibers. The traveling-wave delays between adjacent octave bands with successive lower CF did not change with age.

Frequency dependent maturation of the cochlea and brainstem evoked potentials.

The anatomical development of the human cochlea starts in the middle basal turn and progresses both toward the base and the apex. Behavioral responses, in contrast, appear to emerge for lower frequencies first. Cochlear tuning may continue to change during early development and so effect electrophysiological and behavioral measures of maturation. The maturation of the cochlear traveling wave delay as well as the ABR I-V interval was investigated using the derived response technique which permits a frequency dependent analysis of this maturation. It was found that the traveling wave delay decreased significantly with age for the most basal part of the cochlea; however, it was not affected by age for more apical locations. The I-V delay difference with the adult values was found significantly shorter in the derived octave band with CF = 2.8 kHz than in all other bands for the preterm, term and infant groups. This suggests that the frequency dependent electrophysiological maturation of the brainstem parallels the anatomical development of the cochlea.

Vestibuloauditory impairment in Cogan's syndrome: a case report.

The onset and progression of the auditory and vestibular difficulties which occur with typical Cogan's syndrome are outlined in this case study. These results are documented through a series of auditory and vestibular evaluations highlighting the improvement in hearing sensitivity and changes in vestibular function. The case presented is notable for the rather dramatic and sudden shifts in hearing sensitivity necessitating the use and the discontinuation of amplification.

Audiological tests for evaluation of central auditory disorders.

Three audiological tests (Competing Sentences, Rapidly Alternating Speech, Staggered Spondaic Words) currently being used to evaluate central auditory dysfunction are discussed. Audiological and neurological findings on three patients with central auditory lesions are presented, with special reference to their performance on these central auditory tests. Results suggest the tests are of value in identifying and/or corroborating central lesions, and in providing a more complete explanation of patient communicative difficulties.