- A baby girl for whom you provide care has been referred on her newborn hearing screen.
Physical examination results are within normal limits. When she is referred on a follow-up
hearing screen at 2 weeks of age, you recommend auditory brainstem response testing, which
subsequently reveals absent hearing in both ears. A careful family history is negative for any
individuals who have deafness. The parents ask you what could have caused deafness in their
baby.
Of the following, the likelihood that this infant has a genetic cause for deafness is CLOSEST to
A. <1%
B. 5%
C. 25%
D. 50%
E. 75% - The incidence of prelingual, moderate-to-profound sensorineural hearing loss (>40 dB) is 1
in 500 in developed countries, making it the most common birth defect in these regions.
Approximately 1 in 1,000 newborns is deaf (hearing loss in the severe-to-profound range of 71
to 90 dB).
Fifty percent of all prelingual, moderate-to-profound hearing loss is genetic, 25% is
nongenetic (eg, having bacterial and viral causes), and 25% is idiopathic (Item C183). Of the
genetic forms of deafness, 30% are syndromic, and 70% are nonsyndromic. Syndromic causes
of deafness include conditions such as Usher, Waardenburg, and Treacher-Collins syndromes.
Nonsyndromic causes can be autosomal recessive (75% to 85%), autosomal dominant (15 % to
24%), and X-linked (1% to 2%). - A critical aspect of the evaluation of any child who has prelingual hearing loss is defining
the type of loss as clearly as possible. Careful audiologic evaluation should be undertaken in
concert with examination by an otolaryngologist. Affected individuals should undergo computed
tomography scan of the temporal bones to look for malformations of the inner ear. The
information gleaned from these studies helps to guide the evaluation because different gene
mutations are associated with different clinical manifestations. It is important to obtain a careful,
three-generation pedigree, with attention to any individuals who have hearing loss, vision loss,
dysmorphic features, early death, birth defects, and intellectual disabilities. Referral to a
geneticist is important to determine if there is a recognizable pattern of features or any genetic
testing that could be helpful. - The gene loci for nonsyndromic deafness are designated “DFN” (for DeaFNess). The loci
are categorized as DFNA (autosomal dominant), DFNB (autosomal recessive), and DFN (Xlinked).
Some genes can be involved in both dominant and recessive forms of deafness,
depending on the specific mutation(s), and some mutations cause both sensorineural and
conductive hearing loss. Although there are exceptions, most autosomal dominant loci cause
postlingual hearing impairment, and most autosomal recessive loci cause severe-to-profound
prelingual deafness. X-linked (DFN) loci are associated with both pre- and postlingual hearing
loss. - Because the many molecular genetic testing options can be somewhat confusing,
consultation with a clinical geneticist is strongly recommended. Numerous laboratories offer
panels of common mutations. In many populations worldwide, approximately 50% of individuals
who have prelingual, autosomal recessive nonsyndromic hearing loss have GJB2 mutations; the
remainder may have a mutation in any number of genes that may have been described as
causing deafness in only one or two families.
Dr. Jayaprakash
Superintendent, ICH.
Institute of Child Health.
Gov. Medical College Kottayam.
Kerala, India.