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Recent advances in audiology have made it possible to determine locus of pathologic lesions in the auditory pathways. However, attention should be paid to examine hearing of head blow patients.Because two or more areas from the middle ear up to the auditory cortex are frequently involved.We have examined 90 cases of head blow with special attention in finding retrocochlear lesions, by combining several audiometric procedures.The following auditory tests were performed in all cases, in addition to routine examinations of otoscopic finding, X-ray photography pure tone audiometry, vestibular tests and EEG.1) Speech AudiometryDiscrimination scores were made by a speech audiometry with and without filters which cut off frequencies higher than 1700 cps or lower than 1200cps.2) Bekesy AudiometryFrequencies of 1000cps and 4000cps were used in this test. Tone decay was examined after an exposure of 80dB for a minute.3) Directional Hearing TestA device was made to measure threshold of the interaural time difference which produced a shift of the tone image, by placing two microphones apart from 0 to 1.2 msec (0 to 42cm). A value over 0.06msec (2cm) was considered to be pathological.Speech audiometry frequently helped us in diagnosis of retrocochlear deafness in cases of poor discrimination scores with an excellent pure tone threshold. Bocca and Calearo devised an apparatus to avoid patients " being accustomed " in conventional speech audiometry. In our series of examinations, we employed 1200cps low pass filter and 1700cps. high pass filter to obtain discrimination scores Control experiment in normal person showed 50 to 70% in each test at 80dB above threshold, whereas in patients of suspected retrocochlear deafness poor scores were obtained. The sum of scores tested with filters did not exceed that without filters.Many authors have reported that the recruitment phenomenon is present in head blow patients.A narrow amplitude in Bekesy audiogram is often observed in patients with positive recruitment.Tone decay is suggestive of retrocochlear lesion.An effective test for demonstrating retrocochlear lesion is to measure threshold of the interaural time difference. Matzker states that a lesion, higher than the crossing-over of the auditory system in the brain stem, showed a high value in the directional hearing test.By combining Bekesy audiometry, filtered speech audiometry, and a directional hearing test, it is possible to determine retrocochlear lesions in the auditory system.

HEARING IMPAIRMENT DUE TO HEAD BLOW DIFFERENTIAL DIAGNOSIS OF RETROCOCHLEAR IMPAIRMENT