Inherited sensorineural low-frequency hearing impairment: some aspects of phenotype and epidemiology.

This contribution forms part of the HEAR project. It describes some phenotypes of inherited low-frequency sensorineural hearing impairment (LFSHI) and estimates the prevalence of this inherited hearing impairment (HI) based on a clinical series. During a 10-year period (1987-1996), 418 subjects (134 males and 284 females), with a median age of 68 years (range 4-98), had been examined with LFSHI, defined as hearing loss most pronounced in the low frequencies (i.e., 250 and 500 Hz > 20 dB HL with better hearing, i.e., > or =15-dB difference at 1 and/or 2 and/or 4 kHz with an air-bone gap <15 dB for the average of 0.5, 1, and 2 kHz). The 418 subjects comprising 0.6 per cent of the total number of subjects examined (N=69,309) were subdivided into four categories: category I positive genetic subjects (N=69); category II, probably genetic (N=339); category III, uncertain genetic (N=6); and category IV, subjects with contradictory audiological findings (N=4). The phenotype in category I demonstrated a symmetrical LFSHI, with a pattern of progression showing a slow deterioration in the high frequencies (i.e., 2, 4, and 8 kHz as a function of age)--the progression comprising 40-45 dB. In the low frequencies (i.e., 250, 500, and 1,000 Hz), a deterioration of 15-25 dB could be demonstrated from the youngest to the oldest age group. In category II, a symmetrical LFSHI was found in 179 subjects, showing the same pattern of progression as in category I. However, in the age group 20-39 years, a significantly poorer hearing was found in the low frequencies compared to category I, implying that several phenotypes may be present in LFSHI. A subgroup (A) in category II exhibited normal hearing in one ear with LFSHI in the opposite ear with the same pattern of progression as in category I. Three other subgroups with LFSHI and flat/sloping audiogram in the opposite ear and asymmetrical LFSHI also showed the same type of progression in the ear with LFSHI as in category I. A prevalence of 0.18/1,000 (95 per cent CI 0.13-0.22) of LFSHI was estimated based on the background population with a fairly constant prevalence throughout life. It is concluded that inherited nonsyndromal LFSHI is a rare disease and that the many different phenotypes of LFSHI probably are associated with pronounced genetic heterogeneity.