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Nervous System
The treatment of otosclerosis can be medical or
surgical. A carefully selected, well-fitting hearing aid
may allow a person with conductive deafness to lead a
normal life. Sodium fluoride has been used with some
success in the medical treatment of otospongiosis.
55
Because much of the conductive hearing loss associated
with otosclerosis is caused by stapedial fixation,
surgical treatment involves stapedectomy with stapedial
reconstruction using the patient’s own stapes or a
stapedial prosthesis.
Disorders of Inner Ear
and Central Auditory Pathways
Located in the bony wall of the petrous part of the
temporal bone, the inner ear contains a bony labyrinth
(meaning ‘maze’), or system of intercommunicating
channels and the receptors for hearing and position
sense.
2,57,22,46
A thin-walled, membranous labyrinth floats
inside the outer bony wall, or the bony labyrinth (see
Fig. 38-17). Two separate fluids are found in the inner
ear: the perilymph or periotic fluid, which separates the
bony labyrinth from the membranous labyrinth, and
the endolymph or otic fluid, which fills the membranous
labyrinth. Localized dilatations of the labyrinth develop
into three main sensory areas: the semicircular canals,
the vestibule, and the cochlea (see Fig. 38-17). The
receptors in the cochlea are sensitive to sound, and those
in the semicircular canals and vestibule are sensitive to
changes in head position and maintenance of balance.
The cochlea is a shell-shaped part of the bony
labyrinth that consists of three parallel compartments:
the scala media or middle compartment of the cochlear
canal, the scala vestibuli, and the scala tympani
(Fig. 38-20). The scala media is an endolymph-
containing space that is continuous with the lumen of
the saccule and contains the spiral organ of Corti, the
receptor organs for hearing. The scala vestibuli and scala
tympani are perilymph spaces that communicate with
each other at the apex of the cochlea through a small
channel called the
helicotrema
. The scala vestibuli begins
at the oval window, and the scala tympani ends at the
round window.
The organ of Corti is composed of supporting cells; a
flexible, fibrous “floor” called the basilar membrane; and
several long rows of cochlear hair cells. Sound waves,
delivered to the oval window by the stapes footplate,
are transmitted to the perilymph and vibrate the basilar
membrane. Transduction of sound stimuli occurs when
the cilia of the cochlear hair cells are bent by this sound-
induced movement.
Afferent fibers from the organ of Corti have their
cell bodies in the spiral ganglion of the cochlear nerve.
Auditory information travels along the vestibulocochlear
nerve (CN VIII) and then to the cochlear nuclei in the
caudal pons and midbrain to the thalamus. On the way
many secondary nerve fibers pass to the opposite side
of the brain. From the thalamus, the auditory pathway
passes to the primary auditory cortex (area 41) and the
auditory association cortex (areas 42 and 22) where the
meaningfulness of sound is perceived (see Fig. 38-14).
Because some of the fibers from each ear cross, each
auditory cortex receives impulses from both ears.
Tinnitus
Tinnitus is the perception of abnormal ear or head noises
not produced by an external stimulus.
58–60
Although it
often is described as “ringing of the ears,” it may also
assume a hissing, roaring, buzzing, or humming sound.
Tinnitus may be constant, intermittent, and unilateral
or bilateral. The condition affects both sexes equally,
is most prevalent between 40 and 70 years of age, and
occasionally affects children.
Although tinnitus is a subjective experience, for clinical
purposes it is subdivided into objective and subjective
tinnitus.
Objective tinnitus
refers to those rare cases in
which the sound is detected or potentially detectable by
another observer. Typical causes of objective tinnitus
include vascular abnormalities or neuromuscular
disorders. For example, in some vascular disorders, sounds
generated by turbulent blood flow (e.g., arterial bruits
or venous hums) are conducted to the auditory system.
Vascular disorders typically produce a pulsatile form of
tinnitus.
Subjective tinnitus
refers to noise perception when
there is no sound stimulation of the cochlea. A number of
causes and conditions have been associated with subjective
Cochlear nerve Spiral ganglion
Scala tympani (perilymph)
Basilar membrane
Organ of Corti
Tectorial membrane
Scala media
(endolymph)
Vestibular membrane
Stria vascularis
Scala vestibuli (perilymph)
Oval window
Round window
Middle ear
FIGURE 38-20.
Path taken by sound waves reaching the inner ear.
