Hearing loss (irrespective of its degree commonly termed “deafness”) is the most common sensory deficit in humans, affecting approximately 1 in 500 newborns. Deafness has a genetic basis in two-thirds of patients. Mutations in around 100 genes have been identified as causes, and many more are likely to be found in future research. Genetic deafness does not always manifest in infancy, onset may also be in adulthood.
Medical care for childhood deafness should include, besides early support by hearing aids or cochlear implants, genetic counselling of the patients’ parents. It is essential to determine the cause of deafness (environmental versus genetic) – if genetic, the differentiation between isolated (non-syndromic) and syndromic deafness (that involves other organs) is decisive for the medical management. In children who failed in newborn hearing screening, molecular genetic testing should therefore be offered to the parents.
30% of children with an apparently isolated hearing deficit have a syndrome whose additional manifestations will manifest later in life. The most frequent deafness syndrome is Usher syndrome, affecting about 10% of children with congenital or early-onset hearing loss. In Usher syndrome, additional retinal degeneration begins in childhood or adolescence and may progress to severe visual impairment and even blindness. Clinically, the retinal affection cannot yet be detected in infancy. Today, the molecular genetic analysis may differentiate between isolated and syndromic deafness and predict extra-cochlear manifestations soon after the clinical diagnosis of hearing impairment. The most important syndromes affect the kidneys (Alport syndrome), the thyroid gland (Pendred syndrome) and the heart (Jervell and Lange-Nielsen syndrome, SANDD syndrome).
The genetic diagnosis may hence guide the individual medical care, with regular consultation of the respective specialists (e.g. ophthalmologists or cardiologists) to detect additional symptoms early.
Moreover, the genetic diagnosis enables the precise determination of the recurrence risk, e.g. allowing to prepare for birth of another affected child. Finally, the determination of the underlying mutation largely excludes exogenic/environmental factors (such as infections), also in hearing impairment of later onset.
Familial clustering of cancers may indicate that hereditary factors play an important role for cancer development in such families. Genetic counselling and molecular genetic diagnostics may enable targeted preventive screening measures and might be important for individual treatment in some tumour entities.