Nonsyndromic hereditary hearing loss.

The etiology of hereditary hearing loss is extraordinarily complex. More than 400 genetic syndromes are associated with hearing loss and more than 140 genetic loci associated with nonsyndromic hearing loss have been mapped, with more than 60 genes identified to date. Hereditary hearing loss can be inherited as an autosomal dominant, autosomal recessive, X-linked or mitochondrial (maternally inherited) condition. The overlapping audiologic phenotypes associated with many genes and the variability and/or reduced, sometimes age-related, penetrance of some phenotypic features of syndromic hearing loss can complicate the distinction between various genetic causes of nonsyndromic hearing loss and between nonsyndromic and syndromic hearing loss, especially in childhood. Testing for individual genes associated with nonsyndromic hearing loss, beyond GJB2 which encodes Connexin 26, can become expensive and, without specific phenotypic features to guide selection of genes for testing (such as enlarged vestibular aqueducts, low frequency hearing loss or auditory neuropathy), it is not likely to yield an etiology. Advances in DNA sequencing and the rapid decline in the cost of sequencing presage the availability of testing that can identify the etiology in the majority of cases of genetic hearing loss. However, until comprehensive genetic testing of hearing loss is clinically available and cost-effective, thorough phenotypic and audiologic evaluation and careful documentation of risk factors, infectious exposures and patient and family medical history will continue to be important to efforts directed toward etiologic diagnosis. The complexities associated with interpretation of genetic test results, genetic counseling and genetic risk assessment make consultation with medical geneticists important for many patients.