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Nervous System
Conductive Hearing Loss
Conductive hearing loss occurs when auditory stimuli
are not adequately transmitted through the auditory
canal, tympanic membrane, middle ear, or ossicle chain
to the inner ear. Temporary hearing loss can occur as
the result of impacted cerumen in the outer ear or fluid
in the middle ear. Foreign bodies, including pieces of
cotton and insects, may impair hearing. More permanent
causes of hearing loss are thickening or damage of
the tympanic membrane or involvement of the bony
structures (ossicles and oval window) of the middle ear
caused by otosclerosis or Paget disease.
Sensorineural Hearing Loss
Sensorineural, or perceptive, hearing loss occurs with
disorders that affect the inner ear, auditory nerve, or
auditory pathways of the brain.
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With this type of
deafness, sound waves are conducted to the inner ear,
but abnormalities of the cochlear apparatus or auditory
nerve decrease or distort the transfer of information to
the brain. Abnormal function resulting from damage
or malformation of the central auditory pathways and
circuitry is included in this category.
Sensorineural hearing loss can vary with respect to
onset and severity, and can affect one or both ears. It may
have a genetic cause or result from an infection, other
illness, trauma, or exposure to loud noise. Hearing loss
that has its onset before speech-language acquisition is
typically defined as prelingual, and that known to have
developed after speech development as postlingual.
Genetic hearing loss may result from mutation in a
single gene (monogenetic) or from a combination of
mutations in different genes and environmental factors
(multifactorial). Genetic forms of hearing loss also can
be classified as being part of a syndrome in which other
abnormalities are present, or as nonsyndromic, in which
deafness is the only abnormality.
Environmentally induced deafness can occur through
direct exposure to excessively intense sound, as in
the workplace or at a concert. Sustained or repeated
exposure to noise pollution at sound intensities greater
than 100 to 120 dB can cause corresponding mechanical
damage to the organ of Corti. Severe damage can result
in permanent sensorineural deafness to the affected
sound frequencies.
A number of infections can cause sensorineural hearing
loss. Deafness or some degree of hearing impairment is the
most common serious complication of bacterial meningitis
in infants and children. The mechanism causing hearing
impairment is thought to be a suppurative labyrinthitis
or neuritis resulting in the loss of hair cells and damage
to the auditory nerve. Sudden sensorineural hearing
loss represents an abrupt loss of hearing that occurs
instantaneously or on awakening. It is most commonly
caused by viral infections or circulatory disorders.
Among the neoplasms that impair hearing are
acoustic
neuromas.
Acoustic neuromas are benign Schwann cell
tumors affecting CN VIII. These tumors usually are
unilateral and cause hearing loss by compressing the
cochlear nerve or interfering with blood supply to the
nerve and cochlea.Other neoplasms that can affect hearing
include meningiomas and metastatic brain tumors. The
temporal bone is a common site of metastases.
Drugs that damage inner ear structures are labeled
ototoxic.
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Vestibular symptoms of ototoxicity include
light-headedness, giddiness, and dizziness; if toxicity
is severe, cochlear symptoms consisting of tinnitus or
hearing loss occur. Hearing loss is sensorineural and may
be bilateral or unilateral, or they may be transient, as
often is the case with salicylate toxicity, or they may be
permanent. Several classes of drugs have been identified as
having ototoxic potential, including the aminoglycoside
antimicrobials and some other basic antimicrobials,
antimalarial drugs, some chemotherapeutic drugs, loop
diuretics, and salicylates (aspirin). The risk of ototoxicity
depends on the total dose of the drug and its concentration
in the bloodstream. It is increased in persons with
impaired kidney functioning and in those previously or
currently treated with another potentially ototoxic drug.
Diagnosis andTreatment of Hearing Loss
Diagnosis of hearing loss is aided by careful history of
associated otologic factors such as otalgia, otorrhea,
tinnitus, and self-described hearing difficulties; physical
examination to detect the presence of conditions such as
otorrhea, impacted cerumen, or injury to the tympanic
membrane; and hearing tests. Testing for hearing loss
involves a number of methods, including a person’s
reported ability to hear an observer’s voice, use of a
tuning fork to test air and bone conduction, audioscopes,
and auditory brain stem evoked responses (ABRs).
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Tuning forks are used to differentiate conductive and
sensorineural hearing loss. Audioscopes can be used
to assess a person’s ability to hear pure tones at 1000
to 2000 Hz (usual speech frequencies). The ABR uses
electroencephalographic (EEG) electrodes and high-gain
amplifiers to produce a record of brain wave activity
elicited during repeated acoustic stimulations of either
or both ears. It involves subjecting the ear to loud
clicks and using a computer to analyze nerve impulses
as they are processed in the midbrain. Imaging studies
such as computed tomography (CT) scans and magnetic
resonance imaging (MRI) can be done to determine the
site of a lesion and the extent of damage.
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Untreated hearing loss can have many consequences.
Social isolation and depressive disorders are common
in the hearing-impaired. Hearing-impaired people may
avoid social situations where background noise makes
conversation difficult to hear. Safety issues, both in and
out of the home, may become significant. Treatment
measures for hearing loss range from simple removal
of impacted cerumen in the external auditory canal to
surgical procedures, such as those used to reconstruct
the tympanic membrane.
Hearing aids remain the mainstay of treatment for
many persons with conductive and sensorineural hearing
loss. With the advent of microcircuitry, hearing aids are
now being designed with computer chips that allow
multiple programs to be placed in a single hearing aid.
The various programs allow the user to select a specific
