Biology Reference
In-Depth Information
40 dBHL), moderate (41 to 55 dBHL), moderately severe (56 to 70 dBHL),
severe (71 to 90 dBHL), or profound (greater than 90 dBHL). Thresholds
less than 25 dBHL are considered normal. The degree of hearing loss is
often not uniform across the tested frequencies (typically 250 Hz to 8 kHz).
For example, there may be normal hearing at low and middle frequencies,
with severe hearing loss at high frequencies. This type of hearing loss is
described as high-frequency or down-sloping sensorineural hearing loss,
since the line connecting the pure-tone threshold levels slopes down in the
higher frequencies, which are recorded on the right side of an audiogram.
Sensorineural hearing loss may also be low-frequency or up-sloping
(Lesperance et al. 1995), or it may be U-shaped, predominantly affecting
middle frequencies. A flat audiogram refers to relatively similar thresholds
across the tested frequencies.
Figure 6.2 illustrates several of the many possible distinct audiometric
patterns of hearing loss. These descriptions provide concise and useful cat-
egorizations of audiometric findings, but in practice many audiograms do
not fit neatly into any one category. Furthermore, the degree and type of
hearing loss between two ears of a given patient may be asymmetric. In a
family segregating a gene causing hearing loss, subjective evaluations of
audiometric data are sometimes required to distinguish between an
affected and an unaffected individual. This distinction needs to be made
before genotyping begins, since mistakes in assigning affection status can
jeopardize a genetic analysis, particularly in families with progressive or
late-onset deafness. Based on data of hearing thresholds of age- and sex-
matched normal-hearing individuals, Govaerts and coworkers (1998) have
proposed an algorithm to more objectively differentiate between affected
and unaffected persons. Furthermore, age- and sex-specific reference ranges
for hearing levels and longitudinal changes are available for a Caucasian
population of 681 men and 416 women (Morrell et al. 1996). The subjects
were rigorously screened for otological disorders and noise exposure in
order to remove these confounding variables and provide a normative
data set.
2.2 Clinical
It is important to distinguish between syndromic and nonsyndromic hearing
impairment. Specifically, it should be noted whether the hearing loss is
inherited in association with any other disorders or abnormalities, even if
they do not fit neatly into any previously described syndromes. An addi-
tional distinction regards the onset of hearing loss, which is often described
in relation to the development of speech because adequate auditory func-
tion is important for the development of spoken language. Hearing impair-
ment occurring prior to speech development is called prelingual, whereas
that occurring after speech development is called postlingual. Given the
wide range of age of onsets of postlingual hearing loss, this term is impre-
