Several types of disorders can affect the auditory and vestibular system and result in deafness, dizziness (vertigo), or both. Auditory disorders are classified as either sen-sorineural or conductive. Conductive hearing loss results when sound waves are mechanically impeded from reaching the auditory sensory receptors within the internal ear. This type of hearing loss principally involves the external ear or structures of the middle ear. Conductive hearing loss is the second most common type of loss after sensorineural hear-ing loss, and it usually involves a reduction in sound level or the inability to hear faint sounds. A conductive hearing loss may be caused by otitis media (ear infection); in fact, this is the most common cause of temporary hearing loss in chil-dren. Fluid that collects in the tympanic cavity can also cause significant hearing problems in children. Other com-mon causes of conductive hearing loss include excess wax or foreign bodies in the external acoustic meatus or dis-eases that affect the ossicles in the middle ear (otosclero-sis; see also Folder 25.1). In many cases, conductive hearing loss can be treated either medically or surgically and may not be permanent. Sensorineural hearing impairment may also occur after injury to the auditory sensory hair cells within the inter-nal ear, cochlear division of cranial nerve VIII, nerve path-ways in the CNS, or auditory cortex. Sensorineural hearing loss accounts for about 90% of all hearing loss. It may be congenital or acquired. Causes of acquired sensorineural hearing loss include infections of the membranous labyrinth (e.g., meningitis, chronic otitis media), fractures of the tem-poral bone, acoustic trauma (i.e., prolonged exposure to excessive noise), and administration of certain classes of antibiotics and diuretics. Another example of sensorineural hearing loss often re-sults from aging. Sensorineural hearing loss not only in-volves a reduction in sound level; it also affects the ability to hear clearly or to distinguish speech. A loss of the sensory hair cells or associated nerve fibers begins in the basal turn of the cochlea and progresses apically over time. The char-acteristic impairment is a high-frequency hearing loss termed presbycusis (see presbyopia, page 915). In selected patients, the use of a cochlear implant can partially restore some hearing function. The cochlear  implant is an electronic device consisting of an external microphone, amplifier, and speech processor linked to a  receiver implanted under the skin of the mastoid region. The receiver is con-nected to the multielectrode intracochlear implant  inserted along the wall of the cochlear canal. After considerable train-ing and tuning of the speech processor, the  patient’s hearing can be partially restored to various degrees ranging from recognition of critical sounds to the ability to converse.
suspended within the bony labyrinth (Fig. 25.8b), and the remaining space is filled with perilymph. The membranous labyrinth is composed of two divisions: the cochlear labyrinth and the vestibular labyrinth (Fig. 25.8c).
FIGURE 25.7 • Photograph of a cast of the bony labyrinth of the internal ear. The cochlear portion of the bony labyrinth appears blue-green; the vestibule and semicircular canals appear orange-red. (Courtesy of Dr. Merle Lawrence.)
The vestibular labyrinth contains the following:  Three semicircular ducts lie within the semicircular canals and are continuous with the utricle.  The utricle and the saccule, which are contained in recesses in the vestibule, are connected by the membranous utriculosaccular duct.
The cochlear labyrinth contains the cochlear duct, which is contained within the cochlea and is continuous with the saccule (see Fig. 25.8b and Fig. 25.8c).
Sensory Cells of the Membranous Labyrinth
Specialized sensory cells are located in six regions in the membranous labyrinth.