To evaluate discrepancies between pure-tone audiometry (PTA) and auditory steady state response (ASSR) tests in non-malingerers and investigate brain lesions that may explain the discrepancies, especially in cases where the PTA threshold was worse than the estimated ASSR threshold.
PTA, speech audiometry, auditory brainstem response, ASSR, and neuroimaging tests were carried out on individuals selected from 995 cases of hearing impairment. Among these, medical records of 25 subjects (19 males, 6 females; mean age = 46.5 ± 16.0 years) with significant discrepancy between PTA and estimated ASSR thresholds were analyzed retrospectively. To define acceptable levels of discrepancy in PTA and ASSR hearing thresholds, 56 patients (27 males, 29 females; mean age = 53.0 ± 13.6 years) were selected for the control group. Magnetic resonance images, magnetic resonance angiograms, and positron emission tomograms were reviewed to identify any neurologic abnormalities.
Pathologic brain lesions were found in 20 cases (80%) in the study group, all of which showed a significant discrepancy in hearing threshold between PTA and ASSR. Temporal lobe lesions were found in 14 cases (70%), frontal lobe lesions in 12 (60%), and thalamic lesions without the frontal or temporal lobe in 2 cases (10%). On repeated PTA and ASSR tests a few months later, the discrepancy between ASSR and behavioral hearing thresholds was reduced or resolved in 6 cases (85.7%). Temporal lobe lesions were found in all 3 cases in which the estimated ASSR threshold worsened with unchanged PTA threshold, and frontal lobe lesions were found in all 3 cases in which the PTA threshold improved but the estimated ASSR threshold was unchanged. No neurological lesions were found in 5 cases (20%) of patients with a discrepancy between ASSR and behavioral hearing thresholds.
Clinicians should not rely exclusively on ASSR, especially in cases of central nervous system including temporal, frontal lobe, or thalamus lesions. If no lesions are found in a neuroimaging study of a patient with a discrepancy between PTA thresholds and estimated ASSR thresholds, further functional studies of the brain may be needed. If clinicians encounter patients with a discrepancy between PTA thresholds and estimated ASSR thresholds, an evaluation of brain lesions and repeat audiologic tests are recommended in lieu of relying solely on ASSR.