Biology Reference
In-Depth Information
2.2.1.3 Turner Syndrome and Hearing Loss
Turner syndrome, as illustrated in Figure 5.4, is characterized by monosomy
X (45,X) in 50% of cases and various mosaicisms or structural abnormali-
ties of the other X chromosome in the remaining cases. The incidence is
about 1 in 2,500 live births, but the frequency at conception is much higher.
Greater than 95% of 45,X concepti result in spontaneous loss (Gardner and
Sutherland 1996). Females with Turner syndrome are characterized by
small stature, streak gonads, short or webbed neck, broad chest with widely
spaced nipples, prominent ears, epicanthal folds, and a high-arched palate.
Coarctation of the aorta and renal anomalies are common findings. Intelli-
gence is usually within normal limits (Jones 1997). About half of Turner
syndrome females have moderate sensorineural hearing loss, often com-
bined with conductive hearing loss. Chronic or recurrent ear infections are
common in childhood and may account for some of the conductive hearing
loss (Gorlin et al. 1995). The nature of the sensorineural hearing loss is
unknown.
2.2.2 Unique Chromosomal Rearrangements Associated
with Hearing Loss
There are a number of reports in the literature of various chromosomal
rearrangements and hearing loss. Unlike aneuploidies and microdeletion
syndromes, these are not disorders that have been seen repeatedly. Instead,
they are rearrangements that are either unique, or reported in only a few
individuals. In each case, the chromosomal rearrangement was discovered
either at amniocentesis, or in the evaluation of a dysmorphic, develop-
mentally delayed child. Hearing loss is usually just one finding of many
physical abnormalities. All the children have various degrees of mental
retardation and dysmorphic features. Hearing loss is probably underdiag-
nosed among infants born with unusual chromosomal rearrangements, but
given the severity of the physical problems present at birth a hearing test
may not be considered a high priority.
2.2.2.1 An Unusual Marker Chromosome 15
Marker chromosomes are chromosomes that are structurally abnormal.
They are often difficult to identify, and specific tests are required to deter-
mine the origin of the chromosomal material. The prevalence of marker
chromosomes is less than 0.7 per 1,000 births (Gardner and Sutherland
1996). An inverted duplication of chromosome 15 is among the more
common markers. Generally, the centromere and the proximal portion of
the q arm are duplicated and inverted. Small inverted duplications may
have no phenotypic effect, whereas larger ones produce characteristic
mental retardation and dysmorphic features (Gardner and Sutherland
1996). Hearing loss among individuals with the “common” inverted dupli-
cation of chromosome 15 is not a typical finding. However, Huang et al.
