- 1) bilateral typical NIHL audiogram and type A tympanogram; 2) hearing threshold above 4k Hz was greater than 25 dB HL; 3) audiogram showed the characteristic 4 kHz or 6 kHz notch (average hearing threshold was 10 dB HL higher than the baseline); 4) up-turn phenomenon appeared above 6 kHz or 8 kHz, and 5) symmetrical hearing loss threshold over bilateral ears and the disparity was less than 10 dB HL. Patients with other otologic diseases were excluded.