Biology Reference
In-Depth Information
childhood, ototoxicity is one of the commonest causes of deafness in Mon-
golia and China (Pandya et al. 1997; Hu et al. 1991).
A dramatic example of temporal variation in the incidence of deafness
is provided by the last rubella pandemic. In the United States alone, more
than 5,000 children were deafened by prenatal exposure to the virus. Exam-
ination of the birthdates of children at the Maryland School for the Deaf
(Figure 4.1) reveals a remarkably high concentration during the fall and
spring of 1964 to 65. During this period, there was also a marked increase
in the proportion of sibships with only one deaf child and a corresponding
decrease in the estimated proportion of genetic cases from about 0.5 to 0.1
(Bieber 1981). Rubella deafness is thus an entity in which the birth date
can be an important clue to the etiology. Clearly, any attempt to partition
deafness into its genetic and environmental causes may lead to inconsistent
results if there is unrecognized secular or geographic variation in either the
environmental or genetic causes of deafness.
3.1 Syndromic Deafness
During the past three decades, dramatic progress has been made in the clin-
ical delineation of more than 150 forms of syndromic deafness (Konigsmark
1976). In a comprehensive clinical survey, Fraser (1976) estimated that a
specific form of syndromic deafness could be recognized in 20 to 25% of
the genetic cases. In many important deafness syndromes, such as those of
FIGURE 4.1. Birth dates of students at Maryland School for the Deaf by year and
month showing cases resulting from phrasing rubella epidemic born in fall of 1964
and spring of 1965. (Data from Bieber 1981.)
