Editor—Hearing impairment is extremely heterogeneous, both phenotypically and genetically. It is the most frequent form of sensory impairment in the western world, affecting approximately 1/1000 newborns and approximately half of the people above the age of 80.1 ,2 In all these cases hereditary factors are a prominent cause. So far, more than 60 loci for monogenic non-syndromal hearing impairment have been described and 14 responsible genes have been identified (Van Camp and Smith, Hereditary Hearing Loss Homepage http://dnalab-www.uia.ac.be/dnalab/hhh).

Balance problems are also are relatively frequent, but considerably less is known about the causes. Purely genetic forms of vestibular impairment are extremely rare and no genes have been identified yet. However, it is commonly known that many hearing impaired people also suffer from balance problems. Moreover, it is now recognised that many syndromes with genetic hearing impairment also show a dysfunction of the vestibular system.3 The prevalence of vestibular dysfunction may be severely underestimated, as it often remains unnoticed until specialised vestibular tests are performed. Owing to the intimate relationship between the auditory and the vestibular systems, there are probably many genes with a function in both systems.

DFNA9 is the only form of hereditary non-syndromal hearing impairment where strongly marked vestibular involvement has been described. This locus has been mapped to chromosome 14q12-q13.4Progressive sensorineural hearing impairment is present, usually starting between the ages of 35 and 50 in the high frequencies.5-7 This is paralleled by a gradually deteriorating sense of balance, leading to instability (most notably in the dark). Some, but not all patients periodically suffer from vertigo attacks associated with nausea, tinnitus, or aural fullness, which is reminiscent of the symptoms of Menière's disease.5Eventually, after a disease course of approximately 20 years, patients become severely to profoundly deaf …