Table 122 Ototoxic Medications

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Antibioties

Mainly vestibulotoxic

Gentamicin

Streptomycin

Tobramycin

Mainly cochleotoxic

Vancomycin

Kanamycin

Neomycin

Amikacin

Diuretics (mainly cochleotoxic)

Furosemide

Bumetanide

Ethacrynic acid

Quinine (cochleotoxic)

Aspirin and sodium salicylate (cochleotoxic, reversible)

Chemotherapy (mainly mixed toxicity)

C/s-platinum (mainly cochleotoxic)

Nitrogen mustard

Actinomycin

Bleomycin

TABLE 12-3 -- CLINICOANATOMICAL CORRELATIONS OF DISORDERS OF CRANIAL NERVE Vlil-AUDITORY SYSTEM

Anatomical Location

Signs/Symptoms

Associated Neurological Findings

Cerebral cortex

Aphasia

Normal hearing

Auditory hallucinations

Brain stem

Unilateral hearing loss

Vestibular disorder

Tinnitus

Poor word recognition

Cerebellopontine angle

Unilateral hearing loss

Vertibular disorder

Tinnitus

Facial weakness

Poor word discrimination

Facial numbness

Cochlear nerve

Unilateral hearing loss

Vestibular disorder

Tinnitus

Facial weakness

Cochlea

Hearing loss

Aural fullness

Tinnitus

Hyperacusis

Middle ear

Hearing loss

Tinnitus

ossicular chain, ending at the stapes. Sound energy is then converted into changes in neural firing, which is passed more centrally through a complex cross-connected network of neurons. This neural network can be considered as a series of four orders of neurons. First order neurons are derived from the auditory (spiral) ganglion, second order neurons are those in the neighborhood of the cochlear nuclei in the brain stem. Third order neurons are found at the level of the inferior colliculus, and fourth order neurons travel from the medial geniculate body in the thalamus to the auditory cortex. Considerably more detail about these pathways can be found in Webster and colleagues. 2

FIRST ORDER NEURON

The vestibular and cochlear divisions of the inner ear are depicted in Figure 12-1 (Figure Not Available) A. Longitudinally (Figure 12-1 (Figure Not Available) B), the labyrinth is composed of an outer compartment containing perilymph (similar to extracellular fluid), and an inner compartment containing endolymph (similar to intracellular fluid). The hair cells are embedded in the

TABLE 12-4 -- CLINICOANATOMICAL CORRELATIONS OF DISORDERS OF CRANIAL NERVE VIII-VESTIBULAR SYSTEM

Anatomical Location

Signs/Symptoms

Assoeiated Neurological Findings

Cerebral cortex

Quick spins

Confusion

Brain stem

Vertigo

Nystagmus

Ataxia

Weakness

Numbness

Dysarthria

Diplopia

Cerebellopontine angle

Vertigo

Nystagmus

Ataxia

Hearing loss

Facial weakness

Facial numbness

Vestibular nerve

Vertigo

Nystagmus

Ataxia

Labyrinth

Vertigo

Hearing symptoms

Ataxia

Oscillopsia

Pressure sensitivity

floor of the inner compartment, the basilar membrane. Two types of hair cells are in the cochlea, inner and outer. When sound is transmitted to the cochlea, oscillations are set up in the basilar membrane, and this stimulates the inner hair cells. Place-code mapping occurs between frequency of sounds to areas of the cochlea called a tonotopic distribution, which results in registration of higher frequencies at the base of the cochlea and lower frequencies at the apex.

The more numerous outer hair cells are motile and may help to dynamically tune the system. Appreciation of the motile nature of outer hair cells has led to a new test of hearing called otacoustical emissions or OAEs (see later). Reissner's membrane roofs the compartment containing the hair cells. In certain conditions in which endolymph pressure is increased (e.g., Meniere's disease), Reissner's membrane may bow out or rupture. The s piral ganglion of the cochlea is wrapped around the cochlea and receives output from the hair cells to create the auditory nerve, which runs with the vestibular nerve and facial nerve in the internal auditory canal (IAC). In addition to the afferent fibers in the vestibulocochlear nerve, there are also e fferent fibers, which supply the outer hair cells.

Figure 12-1 (Figure Not Available) The cochleaA) and vestibular labyrinth(B). (From Kandel ER, Schwartz JH, Jessell TM [eds]: Principles of Neural Science, 3rd ed. East Norwalk, CT, Appleton & Lange, 1991.)

SECOND ORDER NEURON

The cochlear nerve enters the brain stem at the pontomedullary junction, where it bifurcates and terminates in the two major subdivisions of the cochlear nucleus--the dorsal and ventral cochlear nuclei (..Fig 12-2 ). The most important outflow is to the trapezoid body, which contains fibers destined for the bilateral superior olivary nuclei in the brain stem. The superior olive is concerned with sound localization, based in interaural differences in sound timing and intensity. The superior olive is also an essential part of the stapedius reflex, which is a protective reflex in the middle ear. The simplest stapedius reflex arc involves the spiral ganglion neurons, the cochlear nucleus, superior olive, and facial nerve nucleus. Output from the superior olive joins crossed and uncrossed axons from the cochlear nucleus to form the lateral lemniscus. This pathway ascends to the inferior colliculus. Because the lateral lemniscus contains second order neurons from the cochlea and third and fourth order neurons from the superior olive, it contributes to three waves of the auditory brain stem response (described later). The tonotopic arrangement of the cochlea is maintained in the cochlear nucleus, lateral lemniscus and reticular formation. Because of the extensive crossing that occurs early in the central auditory circuitry, central auditory lesions usually do not cause monaural hearing loss.

THIRD ORDER NEURON

The inferior colliculus is a major integrating center for the auditory system and serves as a center for feedback pathways to the lower auditory system. Output from the

Figure 12-2 Central auditory pathways(Reprinted with permission from Kandel ER, Schwartz JH, Jessell T: Principles of Neural Science, 3rd ed. East Norwalk, CT, Appleton & Lange, 1991, p 495.)

inferior colliculus ascends to the medial geniculate body of the thalamus. FOURTH ORDER NEURON

The medial geniculate body is the major auditory nucleus of the thalamus. Parts of the medial geniculate are hypothesized to function in directing auditory attention. The medial geniculate body sends output to the primary auditory cortex, also known as the transverse temporal gyri of Heschl (Brodmann areas 41 and 42) and auditory association cortex (areas 22 and 52) (Fig. 12-3 (Figure Not Available) ). The medial geniculate also sends output to the auditory motor cortex, which controls body responses to sound. The auditory cortex is divided into three areas, including a primary area (Al), a secondary area (All), and a remote projection region (Ep). Investigators have varied in assigning area 42 to the primary or secondary auditory cortex. The ventral medial geniculate projects almost entirely to Al, whereas the surrounding auditory areas receive projections from the rest of the geniculate body. As with the lower auditory systems, tonotopic relationships are maintained.

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