Biology Reference
In-Depth Information
Studying hearing loss in aneuploidies does not lend itself easily to a better
understanding of hearing and deafness in general. Because of the wide spec-
trum of malformations in most patients with chromosomal aneuploidies,
and the various stages of development that are affected, it is difficult to
determine which genes on the extra or missing chromosome influence the
normal development or function of any particular organ system. However,
because of the prevalence of hearing loss in individuals with aneuploidy
syndromes, it represents a significant aspect of health care management of
such patients. Undiagnosed hearing loss adds another invisible burden for
individuals who are already challenged with mental retardation or physical
difficulties.
2.2.1.1 Trisomy 21 and Hearing Loss
Down syndrome occurs in about 1 in 700 newborns (Fig. 5.3). Individuals
with Down syndrome have a host of clinical findings, including mental retar-
dation, short stature, hypotonia, characteristic facial features, cataracts,
heart defects, thyroid disorders, an increased incidence of leukemia, and
premature aging (Jones 1997). Hearing loss is found in about 40% to 80%
of patients, depending on the threshold level used and method of testing
(Roizen et al. 1993). The hearing loss is generally mild, bilateral conduc-
tive; however, sensorineural hearing loss was detected in 34% of children
evaluated in one study (Roizen et al. 1993). The external ears of Down syn-
drome patients tend to be simple and low set with an overfolded upper helix
and small or absent lobes (Jones 1997). Children tend to have dysfunctional
Eustachian tubes, leading to a high incidence of otitis media and account-
ing for some of the conductive hearing loss (Roizen et al. 1993). The
pathogenesis of the sensorineural hearing loss in individuals with Down
syndrome has not been determined, although a gene dosage effect of a gene
on chromosome 21 must be considered a likely mechanism.
2.2.1.2 Trisomy 13 and Hearing Loss
With an incidence of 1 in 12,000 (Hook 1980), trisomy 13 is much rarer in
liveborns than is trisomy 21, and the phenotype is much more severe. Ninety
percent of trisomy 13 infants die before 6 months of age. The infants are
severely retarded due to various types of forebrain defects. More than 50%
of cases are noted to have eye defects ranging from anopthalmia to
micropthalmia, cleft lip and/or cleft palate, polydactyly, microcephaly, heart
defects, renal anomalies, and deafness (Jones 1997).
Individuals with trisomy 13 mosaicism may show a less severe phenotype
than those with a full trisomy. Presumably, the presence of a normal cell
line, especially in particular tissues and organs, mitigates the effects of the
trisomic cells. There have been at least two reports of hearing loss or deaf-
ness in patients with mosaic trisomy 13 (Delatycki and Gardner 1997), but
the mechanism of this pathology remains unclear.
