[Sound lateralization testing based on interaural intensity difference discrimination and interaural time difference...

Sound lateralization is considered useful in diagnosing retrocochlear lesions. Interaural time difference (ITD) discrimination has been considered more important than interaural intensity difference (IID) discrimination in diagnosing retrocochlear lesions. The purpose of this study was to determine whether testing sound lateralization by IID and ITD discrimination is useful in patients with acoustic neurinomas (AN). Twenty patients (15 to 74 years old) with unilateral AN were studied by testing IID and ITD discrimination using a self-recording device by delivering 500 Hz band noise through head-phones. Auditory brainstem response (ABR) testing and magnetic resonance imaging (MRI) were also performed in all patients. IID discrimination abnormalities were found in 25% of all patients, and ITD discrimination abnormalities in 85%. Five patients (25%) showed both IID and ITD discrimination abnormalities. One of the three patients with both normal IID and ITD discrimination showed waves I and V, and two of them showed waves I, III and V. One of the five patients with both IID and ITD discrimination abnormalities showed no response, two of them showed only wave I, the other two showed waves I, III and V. Two patients with latency prolongation between waves III and V had both IID and ITD discrimination abnormalities, and seven patients with latency prolongation between waves I and III had normal ITD discrimination or slightly abnormal ITD discrimination and normal IID discrimination. Three of the four patients with large tumors (> 3 cm) had both IID and ITD discrimination abnormalities, while two patients with small ear tumors (< 1 cm) had both normal IID discrimination and normal ITD discrimination. Normal ITD discrimination or slightly abnormal ITD discrimination was found in the majority of patients with wave III. Impairment of ITD discrimination was well correlated with wave III abnormalities. Patients with latency prolongation between waves III and V showed a high degree of ITD discrimination abnormality. Impairment of IID discrimination was related to a high degree of cochlear nerve damage or brainstem damage. We emphasize that IID discrimination is important in the neurological diagnosis of AN, in addition to ITD discrimination. Sound lateralization testing was a useful screening test for diagnosing AN, because tumor size influenced lateralization in every patient, and ITD discrimination was closely correlated with the ABR findings.