Congenital sensorineural hearing loss

Congenital hearing loss (CHL) is defined as the hearing loss present at birth and, consequently, before speech development. It is one of the prevalent chronic conditions in children and the main sensor neural disorder in developed countries. The estimated prevalence of permanent bilateral CHL is 1-3 per 1000 live births in developed countries. CHL is caused by genetic factors in more than 50% of the cases. Genetic hearing loss may be the only clinical feature (non-syndromic or isolated forms) or may be associated with other symptoms (syndromic forms). Non-syndromic hearing loss is extremely heterogeneous. About 80% of the cases are autosomal recessive, 15-24% are autosomal dominant and 1-2% are X-linked. Furthermore, less than 1% of CHL resulting from mitochondrial mutations and it presents with a characteristic matrilineal pattern of transmission. Typically, autosomal recessive hearing loss is congenital whereas autosomal dominant is often progressive. The most frequent isolated form of genetic hearing loss in white population of Europe and United States is the gap junction protein beta 2 gene (GJB2) mutation that is the gene encoding connexin-26. Syndromic form represents about the 30% of the cases of CHL and literature reports more than 400 syndromes where hearing loss is accompanied with physical or laboratory findings. Responsible genes are known for many of these scenarios. A genetic diagnosis is required for different reasons, in particular for choosing appropriate therapeutic options, for treating associated medical problem (syndromic forms) and for predicting the progression of the degree. New treatments and screening strategies are available for identifying the responsive gene, e.g., Next Generation DNA sequencing that allows the simultaneous analysis of a large number of genes causing CHL with a higher probability of gene identification. This paragraph will describe the different genes and clinical features involved in CHL both in isolated and in syndromic form.