Course : MOC Part IV: Hearing Loss
One in every 300-500 children born in the United States has hearing loss. The prevalence of hearing loss increases with age; 40-50% of individuals experience hearing loss by age 75. Hearing loss is etiologically heterogeneous and can be caused by both genetic and environmental factors. Sixty percent of congenital and early childhood hearing loss is genetic. Forty percent of congenital and early childhood hearing loss is caused by environmental factors; congenital cytomegalovirus (CMV) infection remains an important cause. The contribution of genes to later onset forms of hearing loss can be difficult to discern, but genetic factors are clearly involved in many cases. Early hearing intervention in deaf and hard of hearing children is effective in facilitating speech and language development. Genetic hearing loss can be inherited as an autosomal recessive, autosomal dominant, X-linked, or matrilineal trait. Thirty percent of genetic hearing loss is syndromic. More than 400 genetic syndromes have been described that include hearing loss as a feature. For some syndromes, the non-auditory features may be subtle or even absent at the time the hearing loss is noted, especially in early childhood. Many forms of syndromic hearing loss demonstrate marked variability. Seventy percent of genetic hearing loss is nonsyndromic. Nonsyndromic hearing loss has been associated with more than 100 genes. Eighty percent of nonsyndromic hearing loss is autosomal recessive. The predominance of autosomal recessive forms of congenital or early childhood hearing loss leads to more than 95% of newborns with hearing loss being born to hearing parents. Mutations in the GJB2 gene encoding the gap junction protein Connexin 26 account for the largest percentage of autosomal recessive early childhood hearing loss in many populations. Determining the etiologic cause of an individual’s hearing loss may impact clinical management, especially if an unsuspected syndromic form of hearing loss is identified. Etiologic diagnosis also improves the precision of genetic counseling and estimations of recurrence in families. However, the extreme etiologic heterogeneity of hearing loss challenges traditional approaches to establishing an etiologic diagnosis, necessitating careful and thorough clinical and diagnostic evaluation including comprehensive medical evaluation and genetic and other medical testing. Evaluation of patients and families with hearing loss also requires attention to the linguistic and cultural distinctiveness that sometimes accompanies hearing loss and deafness.
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