- In most cases, sensorineural hearing loss of unknown origin
- Positive family history often present
- Vestibular symptoms possible, but rare
- In syndromic cases, associated with other clinical abnormalities
Hearing loss is the most common sensory deficit in humans. The prevalence of congenital hearing loss in newborns is approximately 1–3 cases per 1000. More than 60% of these prelingual cases (ie, hearing loss before the acquisition of speech) are attributed to genetic causes. A further 1 in 1000 children becomes deaf before adulthood. In patients over 60 years of age, approximately half show a hearing loss >25 dB HL. A large percentage of these populations is estimated to be likely affected by genetic influences, although age-related epidemiologic studies of the genetic contribution to hearing loss are not available. Finally, more than 100 deafness genes are believed to exist. These figures illustrate the impact of hearing loss on the public health system and the importance of genetic factors.
The most common and useful distinction in hereditary hearing impairment is syndromic versus nonsyndromic hearing impairment. Seventy percent of hereditary hearing impairments are nonsyndromic, whereas a minority of 15–30% are syndromic (Figure 54–1).
Causes and distribution of genetic deafness.
Nonsyndromic Hereditary Hearing Impairment
Nonsyndromic hereditary hearing impairment is classified by the mode of inheritance. Autosomal recessive transmission (designated by prefix DFNB) is implicated in approximately 80% of cases, autosomal dominant transmission (DFNA) is present in approximately 20% of cases, and X-linked (DFN) and mitochondrial transmission are responsible for <2% of cases (see Figure 54–1). One single gene, GJB2 (Gap-Junction Beta 2 encoding for connexin 26), has emerged to be the most common cause of recessive deafness, and up to 40% of the onset of sporadic prelingual hearing impairment can be attributed to defects in this gene both in Europe and the United States. The prevalence is higher in southern Europe than in northern Europe, mainly owing to one single gene mutation, c.35delG. In a stretch of six guanines extending from position 30 to 35, one base pair is deleted. The high incidence of this mutation seems to be due to a common ancestor. Other common mutations include c.167delT in Ashkenazi Jews and c.235delC in the Japanese population. Also, a common digenic pattern of inheritance involving GJB2 and GJB6 has been detected. Patients with a monoallelic mutation in GJB2 harbor in addition a deletion of GJB6.
Mitochondrial genes constitute a small and unique group. Inheritance is entirely through the mother, because the maternal oocyte is the sole contributor of mitochondria. Although hearing loss occurs frequently in mitochondrial diseases, it is much more seldom the only symptom. The c.A1555G mutation in the MT-RNR1 gene is the most important one among inherited hearing impairment with mitochondrial transmission.
Syndromic Hereditary Hearing Impairment
Syndromic hearing impairment ...