Biology Reference
In-Depth Information
Usher syndrome type 1D; USH1E, Usher syndrome type 1E; USH1F, Usher
syndrome type 1F; USH2A, Usher syndrome type 2A; USH2B, Usher syndrome
type 2B; USH3, Usher syndrome type 3; VBCH, van Buchem disease; VCFS,
Velocardiofacial syndrome; WFS, Wolfram syndrome; WS1, Waardenburg syndrome
type 1; WS2, Waardenburg syndrome type 2; WS3, Waardenburg syndrome type 3;
WS4, Waardenburg syndrome type 4; XPA, Xeroderma pigmentosum.
component (Fischel-Ghodsian 1998). Since hearing loss can be acquired, it
is to be expected that, among hearing impaired members of a large family
with a high incidence of deafness, some individuals may have hearing loss
due to a nongenetic cause. These individuals are said to be phenocopies
(Hadorn 1961). Audiological evaluations may not be able to distinguish
between hereditary and acquired deafness, in which case the information
derived from a thorough clinical history is invaluable.
The ongoing identification and inevitable elucidation of the functions of
the genes for hearing loss provides entry points toward an integrated under-
standing of auditory system structure, function, and development. From
a public health perspective, the insights gained from molecular genetic
studies may guide the development of strategies to slow the rate of pro-
gressive hearing loss. Even without specific medical treatment, knowledge
of impending hearing loss allows timely speech and communication reha-
bilitation. It is also anticipated that results of genetic tests will be used to
counsel individuals who are at risk for hearing loss.
2. Classification and Evaluation of Hearing Loss
2.1 Audiometric
Pure-tone audiometry can be used to classify hearing loss as conductive,
sensorineural, or mixed, which is a combination of conductive and sen-
sorineural. Conductive hearing loss is associated with pathology affecting
any of the anatomic components that mechanically transduce sound to the
cochlea. Thus, abnormalities of the external ear, ear canal, tympanic mem-
brane (eardrum), ossicles, oval window (the interface of the ossicular chain
with the cochlea), round window, or middle ear space may cause conduc-
tive hearing loss. Sensorineural hearing loss may be associated with dys-
function of any of the components of the auditory pathway that convert the
physical stimulus of sound into an electrical stimulus that is transmitted to
the auditory cortex. Sensorineural hearing loss may therefore be caused by
lesions of the cochlea, auditory (cochlear; eighth cranial) nerve, auditory
brainstem, or even higher order auditory structures within the brain.
The severity of hearing loss is also routinely categorized. Measured pure-
tone hearing thresholds may be used to classify hearing loss as mild (26 to
