[Familial deafness showing hearing pattern of low-tone losses]

Clinical and electrocochleographic studies of low tone familial deafness were carried out in 19 patients of 10 families. These hearing impairments were characterized by an upward-sloping pure tone audiogram with 50dB to 60dB hearing threshold below 2000Hz and with normal or near-normal hearing threshold above this frequency. The type of inheritance in these families showed autosomal dominant mode and no associated malformation. In 7 patients, hearing tests were repeated, it was observed that hearing threshold was not elevated. The short increment sensitivity index (SISI) tests were positive suggesting a hair cell lesion in tested 7 patients. The caloric tests were normal in tested 3 patients. No abnormal internal ear configuration was found in both X-ray and high-resolution computed tomography. Twelve patients with these familial low tone deafness (FLTD), 19 patients with Ménière's disease, and 9 patients with low tone sudden deafness (LTSD) were studied using the transtympanic electrode technique of electrocochleography. All patients of FLTD showed normal negative summating potential (-SP) amplitude, and the cochlear microphonics (CM) showed a nearly same detection threshold as hearing level of the pure tone audiogram at 1 kHz and 0.5 kHz. The evidence suggested that the hearing impairment resulted from a localized abnormality of the cochlear apex, and without endolymphatic hydrops. On the other hand, patients with Ménière's disease and LTSD showed a high -SP amplitude and a satisfactory detection threshold of CM compared with hearing level of the pure tone audiogram. These findings suggest that both Ménière's disease and LTSD relate to endolymphatic hydrops.