Porth's Essentials of Pathophysiology, 4e

980

Nervous System

U N I T 1 0

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. 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. Disorders of Inner Ear and Central Auditory Pathways

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

Scala vestibuli (perilymph)

Oval window

Vestibular membrane

Stria vascularis

Tectorial membrane Scala media (endolymph)

Organ of Corti

Basilar membrane

Middle ear

Round window

Scala tympani (perilymph)

Cochlear nerve Spiral ganglion

FIGURE 38-20. Path taken by sound waves reaching the inner ear.