Infectious Causes

The most common infectious cause of congenital SNHL is cytomegalovirus (CMV), which infects 1/100 newborns in the USA (Chapters 247 and 630). Of these, 6,000-8,000 infants each year have clinical manifestations, including approximately 75% with SNHL. Congenital CMV warrants special attention because it is associated with hearing loss in its symptomatic and asymptomatic forms, and the hearing loss may be progressive. Some children with congenital CMV have suddenly lost residual hearing at 4-5 yr of age. Much less common congenital infectious causes of SNHL include toxoplasmosis and syphilis. Congenital CMV, toxoplasmosis, and syphilis also can manifest with delayed onset of SNHL months to years after birth. Rubella, once the most common viral cause of congenital SNHL, is very uncommon because of effective vaccination programs. In utero infection with herpes simplex virus is rare, and hearing loss is not an isolated manifestation.

Other postnatal infectious causes of SNHL include neonatal group B streptococcal sepsis and bacterial meningitis at any age. Streptococcus pneumoniae is the most common cause of bacterial meningitis that results in SNHL after the neonatal period and has become less common with the routine administration of pneumococcal conjugate vaccine. Haemophilus influenzae type b, once the most common cause of meningitis resulting in SNHL, is rare owing to the Hib conjugate vaccine. Uncommon infectious causes of SNHL include Lyme disease, parvovirus B19, and varicella. Mumps, rubella, and rubeola, all once common causes of SNHL in children, are rare owing to vaccination programs.

Genetic Causes

Genetic causes of SNHL probably are responsible for as many as 50% of SNHL cases (see Tables 629-2 and 629-3). These disorders may be associated with other abnormalities, may be part of a named syndrome, or can exist in isolation. SNHL often occurs with abnormalities of the ear and eye and with disorders of the metabolic, musculoskeletal, integumentary, renal, and nervous systems.

Autosomal dominant hearing losses account for about 10% of all cases of childhood SNHL. Waardenburg (types I and II) and branchio-otorenal syndromes represent 2 of the most common autosomal dominant syndromic types of SNHL. Types of SNHL are coded with a 4-letter code and a number, as follows: DFN = deafness, A = dominant, B = recessive, and number = order of discovery, for example DFNA 13. Autosomal dominant conditions in addition to those just discussed include DFNA 1-11, 13, 15, 17, 20, 22, 28, 36, 48 and mutations in the crystallin gene (CRYM).

Autosomal recessive genetic SNHL, both syndromic and nonsyndromic, accounts for about 80% of all childhood cases of SNHL. Usher syndrome (types 1, 2, and 3), Pendred syndrome, and the Jervell and Lange-Nielsen syndrome (one form of the long Q-T syndrome) are 3 of the most common syndromic recessive types of SNHL. Other autosomal recessive conditions include Alström syndrome, type 4 Bartter syndrome, biotinidase deficiency and DFNB1-4, 6-9, 12, 16, 18, 21-23, 28-31, 36, 37, 67.

Unlike children with an easily identified syndrome or with anomalies of the outer ear, who may be identified as being at risk for hearing loss and consequently monitored adequately, children with nonsyndromic hearing loss present greater diagnostic difficulty. Mutations of the connexin-26 and -30 genes have been identified in autosomal recessive (DNFB 1) and autosomal dominant (DNFA 3) SNHL and in sporadic patients with nonsyndromic SNHL; up to 50% of nonsyndromic SNHL may be related to a mutation of connexin-26. Mutations of the GJB2 gene co-localize with DFNA 3 and DFNB 1 loci on chromosome 13, are associated with autosomal nonsyndromic susceptibility to deafness, and are associated with as many as 30% of cases of sporadic severe to profound congenital deafness and 50% of cases of autosomal recessive nonsyndromic deafness. Sex-linked disorders associated with SNHL, thought to account for 1-2% of SNHL, include Norrie disease, the otopalatal digital syndrome, Nance deafness, and Alport syndrome. Chromosomal abnormalities such as trisomy 13-15, trisomy 18, and trisomy 21 also can be accompanied by hearing impairment. Patients with Turner syndrome have monosomy for all or part of 1 X chromosome and can have CHL, SNHL, or mixed hearing loss. The hearing loss may be progressive. Mitochondrial genetic abnormalities also can result in SNHL (see Table 629-2).

Many genetically determined causes of hearing impairment, both syndromic and nonsyndromic, do not express themselves until some time after birth. Alport, Alström, and Down syndromes, von Recklinghausen disease, and Hunter-Hurler syndrome are genetic diseases that can have SNHL as a late manifestation.

Physical Causes

Agenesis or malformation of cochlear structures including the Scheibe, Mondini (Fig. 629-1

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