INTRODUCTION
Many common medications, including salicylates, quinines and the aminoglycoside antibiotics can cause ototoxicity, or ear poisoning.
The onset of toxicity is not as dramatic as anaphylactic shock nor is it visible, like a vibrant red rash. But the impact of this quiet damage on patients’ well-being is no less profound. It can produce anything from mild hearing loss to profound bilateral deafness and can cause balance problems severe enough to prevent employment.
This course will help the reader understand ototoxicity, its causes and the appropriate nursing care. Topics include anatomy and physiology, ototoxic substances, signs and symptoms of ototoxicity, risk factors, assessment and nursing interventions.
Chapter 1: Anatomy and Physiology
After studying the information presented in this chapter, you will be able to —
- List the three areas of the inner ear: cochlea, vestibule and canals.
- Identify the two inner ear functions.
- Name the three sensory systems needed for balance.
- List the two reflexes dependent upon the inner ear.
To understand ototoxicity and its effects on hearing and balance, you must have a basic understanding of anatomy and physiology. This chapter looks at the anatomy of the ear, cranial nerve VIII and the brain as they apply to hearing and balance and then discusses the physiology of hearing and balance.
Ear Anatomy
A human ear has three general areas: the outer ear, the middle ear and the inner ear. (See Figure 1) Both the outer and middle ears are designed, and used exclusively, for hearing.
Figure 1: The Human Ear
Source: Reprinted with permission from the Vestibular Disorders Association.
The outer ear is composed of the pinna (auricle), the external auditory canal and the tympanic membrane (eardrum).1 There are three middle ear sections: the middle ear cavity, ossicles (little ear bones) and eustachian tube. (See Figure 2) The middle ear cavity contains the ossicles and the eustachian tube opening. Sound vibrations are transmitted from the outer to the inner ear via the ossicles (malleus, incus, stapes).
The Middle Ear
The inner ear is the sensory end organ for both hearing and balance (vestibular function) and composed of the snail-shaped cochlea, the vestibule and the three semicircular canals. These three areas are continuous with one another and share the two inner ear fluids, endolymph and perilymph. Each of these inner ear areas contains specialized hair cells or cilia, the basic unit of sensory reception for both hearing and balance.2
The cochlea is the end organ of hearing with hair cells present along its entire length from the base to the apex. (See Figure 3.) These hair cells are found in an area called the organ of Corti and occur in two rows, the inner hair cells and the outer hair cells. Endolymph, an inner ear fluid, is produced here in the stria vascularis.
Figure 3: Cochlear Hair Cells in the Organ of Corti
Source: Reprinted with permission from the Vestibular Disorders Association.
Each of the three semicircular canals lies in a different plane. An enlargement, the ampulla, is visible at the end of each canal. This ampulla contains the canal’s sensory hair cells used for movement detection. Within the vestibule are the saccule and utricle. These two areas are at right angles to each other and contain many hair cells. The hair cells are surrounded by a jelly-like substance and the top sprinkled with calcium carbonate crystals, known as otoliths or otoconia. (Patients sometimes refer to them as “ear rocks.”)
Although the vestibule is only one of two areas involved in sensing balance, the function is referred to as vestibular function. This entire area is also referred to as the labyrinth.
Cranial Nerve VIII
Hearing and balance information are both sent directly to the brain via the afferent fibers of cranial nerve VIII, also known as the auditory, acoustic or vestibulocochlear nerve. This sensory nerve has two branches, the cochlear and the vestibular. (See Figure 4.) The cochlear nerve carries sound signals from the cochlea to the auditory pathway in the brain.
Balance (vestibular) information is sent along the vestibular nerve from the semicircular canals and the vestibule to the vestibular nucleus of the medulla.
Figure 4: Cranial Nerve VIII
Source: Reprinted with permission from the Vestibular Disorders Association.
Brain
Sound and balance information are sent to the brain via cranial nerve VIII. Once in the brain, this information travels along different routes to separate areas.
Sound travels along a specific pathway on its way to the brain’s primary auditory cortex in the transverse temporal gyri of the temporal lobes. There are several “relay” areas at which neurons join and certain functions are carried out. These areas include the ventral cochlear nucleus, dorsal cochlear nucleus, olivary complexes, nucleus of the lateral lemniscus, inferior colliculus and the medial geniculate body.
Balance input is transmitted from the inner ear to the vestibular nuclei in the medulla. There are lines of communication between the vestibular nuclei and several structures. Information flows along tracts from the inner ear, eyes, neck receptors and peripheral proprioceptors to the vestibular nuclei of the brain. Nerve fibers leave the vestibular nuclei bound for the vestibulospinal tracts, cervical cord motor neurons, oculomotor nuclei, cerebellum, reticular formation, hypothalamus, opposite vestibular nuclei (from left to right and right to left) and on to the postcentral gyrus of the cerebral cortex via the thalamus.
