C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
983
setting for different listening situations. The development
of microcircuitry has also made it possible for hearing
aids to be miniaturized to the point that, in many cases,
they can be placed deep in the ear where they take
advantage of the normal shape of the external ear and
ear canal. Although modern hearing aids have improved
greatly, they cannot replicate the person’s ability to hear
both soft and loud noises. They also fail consistently to
filter out distorted or background noise.
57
Surgically implantable cochlear prostheses for the pro-
foundly deaf have been developed and are available for
use in adults and children.
67
These prostheses are inserted
into the cochlea and work by providing direct stimu-
lation to the auditory nerve. For the implant to work,
the auditory nerve must be functional. Although early
implants used a single electrode, current implants use
multielectrode placement, enhancing speech perception.
Much of the progress in implant performance has been
achieved through improvements in the speech proces-
sors that convert sound into electrical stimuli. Advances
in the development of the multichannel implant have
improved performance such that cochlear implants have
been established as an effective option for adults and
children with profound hearing impairment.
67,68
Hearing Loss in Infants and Children
Even mild or unilateral hearing loss can have a detrimental
effect on the language development and hearing-associated
learning of the young child.
69,70
In the United States, the
average incidence of hearing loss is 1.1 per 1000 infants.
70
The cause of hearing impairment in children depends on
whether the hearing loss is conductive or sensorineural.
Most conductive hearing loss is caused by middle ear infec-
tions. Causes of sensorineural hearing impairment include
genetic, infectious, traumatic, and ototoxic factors.
Genetic causes are probably responsible for as much as
50% of sensorineural hearing loss in children. The most
common infectious cause of congenital sensorineural
hearing loss is cytomegalovirus (CMV), which infects 1
out of every 100 newborns in the United States each year;
of these, about 1200 to 2000 have sensorineural hearing
loss.
70
Of particular concern is the fact that congenital
CMV infection can cause both symptomatic and
asymptomatic hearing loss in the newborn. Some children
with congenital CMV infection, who were asymptomatic
as newborns, have suddenly lost residual hearing at 4
to 5 years of age. Postnatal causes of sensorineural
hearing loss include
β
-hemolytic streptococcal sepsis
in the newborn, and although less frequent with the
routine administration of the conjugate pneumococcal
vaccine,
S. pneumoniae
bacterial meningitis is the most
frequent cause after the neonatal period. Other causes of
sensorineural hearing loss are toxins and trauma.
Hearing impairment can have a major impact on the
development of a child; therefore, early identification
through screening programs is strongly advocated.
70–72
The currently recommended screening techniques are
either the transient evoked otoacoustic emissions (OAE)
or the ABR.
70
Both methodologies are noninvasive,
relatively quick (<5 minutes), and easy to perform. The
OAE measures sound waves generated in the inner ear
(cochlea) in response to clicks or tone bursts emitted
and recorded by a minute microphone placed in the
external ear canals of the infant. The ABR uses three
electrodes pasted to the infant’s scalp to measure the
EEG waves generated by clicks. Because many children
become hearing impaired after the neonatal period
and are not identified by neonatal screening programs,
it is recommended that all infants with risk factors for
delayed onset of progressive hearing loss receive ongoing
audiologic and medical monitoring for 3 years and at
appropriate intervals thereafter.
72
Once hearing loss has been identified, a full
developmental and speech and language evaluation
is needed. Parental involvement and counseling are
essential. Children with sensorineural hearing loss should
be evaluated for possible hearing aid use by a pediatric
audiologist. Hearing aids may be fitted for infants as
young as 2 months of age.
70
Infants and young children
with profound congenital or prelingual deafness have
benefited frommultichannel cochlear implants.
70
Because
of the increased risk of pneumococcal meningitis, children
who receive implants should receive age-appropriate
immunization against pneumococcal disease.
Hearing Loss in the Elderly
The term
presbycusis
is used to describe degenerative
hearing loss that occurs with advancing age.
73,74
The
degenerative changes that impair hearing may begin in
the fifth decade of life and may not be clinically apparent
until later. The hearing loss is typically gradual, bilateral,
and characterized by high-frequency hearing loss. It is
further characterized by reduced hearing sensitivity and
speech understanding in noisy environments, slowed
central processing of acoustic information, and impaired
localization of sound sources. The disorder first reduces
the ability to understand speech and, later, the ability to
detect, identify, and localize sounds. The most common
complaint of persons with presbycusis is not that they
cannot hear, but rather that they cannot understand what
is being said. High-frequency warning sounds, such as
beepers, turn signals, and escaping steam, are not heard
and localized, with potentially dangerous results.
Given the high prevalence of hearing loss in people
of retirement age and its adverse effects on well-being,
screening for hearing loss should be performed at annual
health care visits. Clinical measures for hearing loss
such as whispered voice tests and finger friction tests
are reportedly imprecise and are not reliable methods
for screening. Screening audiometry administered by
someone trained in its use is a practical and cost-effective
method for detecting significant hearing loss.
The majority of hearing loss in the elderly is
sensorineural. In mild to severe loss, the most effective
treatment is hearing amplification with hearing aids, lip
reading, and assistive listening devices (e.g., hearing aids
with the telephone, captioning on televised programs,
flashing alarms). Cochlear implants are indicated at
any age for people with bilateral hearing losses not
materially helped by hearing aids.
