Audition Case File

Audition Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 22

A 25-year-old woman has been having difficulty for several months understanding what people are saying over the phone when she holds the receiver to her left ear. She has no problem with her right ear, and no other symptoms. On examination, when a vibrating tuning fork is placed in the center of her forehead, she hears the sound louder in her right ear than in her left ear. The base of a vibrating tuning fork is then placed on her left mastoid process. When she is no longer able to hear the sound through the bone, the vibrating tuning fork is held close to her left ear. She is still able to hear the vibrating tuning fork in the air after she can no longer hear it on bone. Based on the examination findings and further workup, the patient is diagnosed with an acoustic neuroma.

• Based on the examination findings, is the hearing loss more likely to be conductive or sensorineural in nature?
• What are other possible diagnoses?

ANSWERS TO CASE 22: AUDITION

Summary: A 25-year-old woman with several month history of left-sided hearing loss.

• Interpretation of examination findings: The Weber test indicates a sensorineural etiology for the patient’s hearing loss. The Rinne test is not suggestive of a conductive hearing loss. Both of these findings are consistent with the diagnosis of an acoustic neuroma, which affects the eighth cranial nerve.
• Other possible diagnoses: Sensorineural hearing loss can be caused by a variety of etiologies in addition to a tumor involving the eighth cranial nerve or the cerebellopontine angle. Drugs such as aminoglycosides and salicylates can have toxic side effects to the hair cells of the cochlea. Infectious, immune-mediated, or traumatic nerve injury can also lead to hearing deficits similar to those that the patient exhibits.

CLINICAL CORRELATION

The Weber test is performed by holding the tuning fork to the center of her forehead. Normally, the sound is heard in the midline. In air conduction hearing loss, the sound is lateralized toward the abnormal side. In sensorineural hearing loss, the sound is lateralized away from the abnormal side. This patient’s examination lateralized away from her abnormal side. The Rinne test was performed by comparing bone with air conduction on the affected side. Normally, hearing by air conduction will continue after bone conduction ceases. A person with conductive hearing loss will hear bone conduction better than air conduction. This patient was able to hear air conduction after the bone conduction had ceased. An MRI was obtained which revealed an acoustic neuroma of the left eighth cranial nerve localized at the cerebellopontine angle.

The capture and interpretation of sound is a key element used in communication and interpretation of our surroundings. Sound waves travel through the external auditory canal where they are translated into mechanical energy through the middle ear. Auditory information is then transmitted to the inner, fluid-filled ear where it is detected and organized tonotopically by the hair cells of the cochlea. This information is then transmitted through the cochlear nerve to the brain stem and auditory cortex, where the information is processed and interpreted. An acoustic neuroma impairs the ability of the cochlear nerve to function properly, resulting in a sensorineural hearing loss. Other types of hearing loss can be congenital or acquired, and can be because of conductive or sensorineural etiologies. The ability to communicate and interact socially can be significantly impaired with diminished or absent hearing. Treatment options for acoustic neuromas include surgical resection stereotactic radiosurgery.

APPROACH TO AUDITION

Objectives

1. Understand the basic anatomic structures involved in audition.
2. Review the central auditory pathways.
3. Be familiar with the diagnostic tests used to differentiate between conductive and sensorineural hearing loss.

Definitions

Hertz: A unit of frequency equal to one cycle of a wave per second. The frequency of a sound wave determines the pitch of the sound.

Decibels: The unit for measuring the amplitude of a sound wave. The amplitude determines the loudness of a sound.

Oval window: Oval opening in the cochlea which separates the air-filled middle ear from the fluid-filled inner ear cavity. Vibrations of the stapes bone against the oval window transmit sound to the inner ear.

Round window: Round opening in the cochlea connecting the inner ear to the middle ear. Movement of the membrane covering the round window allows for sound waves to be dissipated into the air-filled middle ear.

Sensorineural hearing loss: Partial or complete deafness that occurs as a result of injury to the cochlear nerve or the sensory structures of the inner ear.

Conductive hearing loss: Partial or complete deafness that occurs as a result of disruption of auditory conduction through the bones of the middle ear or mechanical obstruction of the external ear.

DISCUSSION

The structures of the auditory system can be divided into three components: the external, middle, and inner ear. The external ear consists of the auricle and external auditory canal, and is separated from the middle ear by the tympanic membrane. The middle ear is filled with air and contains three bony ossicles: the malleus, incus and stapes. The malleus attaches to the tympanic membrane, and the stapes attaches to the oval window through a ligamentous membrane. The oval window separates the middle ear cavity from the inner ear space, which is filled with fluid. Sound waves traveling through air strike the tympanic membrane and create motion which is conveyed through the ossicles to the oval window. This chain of ossicles functions not only as an amplifier, but as an impedance matcher as well. The impedance of the tympanic membrane is matched to the higher impedance of the oval window, thus decreasing energy lost as the sound waves travel from air to fluid. Sound consists of sinusoidal waves of air molecules. The frequency of a wave is measured in hertz (Hz) and determines the pitch of a sound. The amplitude of a wave is measured in decibels (dB) and determines the loudness of a sound. The human ear can detect sound frequencies between 20 and 20,000 Hz and loudness between 1 and 120 dB.

The oval window opens up into the vestibule of the inner ear, which is filled with perilymph. The cochlea and the semicircular canals lie on either side of the vestibule. The cochlea is a perilymph-filled tube which wraps around itself approximately 2.5 times. The semicircular canals are also filled with perilymph and are important for providing information about the position and movement of the head. These three chambers of the inner ear lie within the temporal bone and form the bony labyrinth. The membranous labyrinth, suspended within the bony labyrinth, is filled with endolymph, and contains the sensory organs.

The bony cochlea turns around a central bony modiolus, which forms the axis of the cochlear turns. The spiral lamina is a ridge of bone that divides the cochlear cavity into two chambers: the scala vestibule and the scala tympani. The scala media, or membranous cochlear duct, lies between the scala vestibule and scala tympani. The organ of Corti is suspended in the endolymph within the scala media. It rests on the basilar membrane as it spirals within the cochlear turns.

As the footplate of the stapes moves against the oval window, a pressure wave is produced in the perilymph of the scala vestibule. This pressure wave travels within microseconds to the apical connection between the scala vestibule and scala tympani, called the helicotrema. The vibrations from this pressure wave are transmitted through the fluid to the basilar membrane of the organ of Corti. The cytoarchitecture of the organ of Corti is such that specific portions resonate harmonically with each audible frequency, allowing for tonotopical organization of sound. The width of the basilar membrane is greater and more flexible toward the apex than at the base where it is stiffer. This allows lower frequencies to resonate near the apex and helicotrema and higher frequencies near the base and oval window.

The organ of Corti contains two types of receptors: inner and outer hair cells. The base of each inner hair cell is indirectly attached to the basilar membrane and functions as an auditory receptor cell. Stereocilia extend above the apical surface of the cell and lie just below the tectorial membrane, which is attached separately to the wall of the scala media. As sound waves travel within the cochlea, the basilar and tectorial membranes move independently of each other. The stereocilia touch the tentorial membrane and bend, opening ionic channels that create changes in the potential of the hair cell membrane. Specifically, the stereocilia bend when they come into contact with the tectorial membrane and produce a change in the K+ conductance inward. The inward K+ current depolarizes the cell and activates Ca2+ conductance that further contributes to the depolarization. Transmitter is released at the base of the hair cell and binds to spiral ganglion cells to trigger an action potential. The base of each hair cell synapses with dendritic processes from up to ten spiral ganglion cells. These ganglion cells are bipolar cells which synapse with only one inner hair cell each. They form the spiral ganglion, and their axons form the cochlear division of the eighth cranial nerve. The outer hair cells contain stereocilia embedded in the tectorial membrane. They have contractile properties which allow them to control the apposition of the tectorial membrane to the inner hair cells, and therefore the sensory response properties of the organ of Corti.

The pressure waves that initially travel in the scala vestibule traverse the scala media, vibrate the basilar membrane, and induce pressure waves in the scala tympani. These then induce movements of the elastic diaphragm covering the round window within the scala tympani. The round window opens up into the air-filled middle ear where the pressure waves are finally dissipated.

The cochlear nerve travels to the brain stem where it enters at the medullary pontine junction. Each entering nerve fiber bifurcates and synapses with neurons in both the dorsal and ventral cochlear nuclei. These nuclei contain tonotopically organized neurons. The dorsal, intermediate, and ventral acoustic striae project from the cochlear nuclei and relay information to central and rostral structures. The dorsal acoustic stria projects from the dorsal cochlear nucleus to the contralateral lateral lemniscus. The intermediate acoustic stria projects from the ventral cochlear nucleus, taking a similar course as the dorsal stria. The ventral acoustic stria projects from the ventral cochlear nucleus as well, and travels to the ipsilateral and contralateral nuclei of the trapezoid body and superior olivary nuclei. These nuclei then project to the ipsilateral and contralateral lateral lemnisci. The superior olivary nucleus is the first point where information from both ears converges. It is sensitive to small changes in time of arrival and intensity of stimulus, which helps to localize the sound.

The monaural central auditory pathway carries information about the frequency of sound. It consists of neuronal fibers projecting from the dorsal and ventral cochlear nuclei through the dorsal and intermediate striae to the contralateral inferior colliculus. The binaural pathway ascends bilaterally with the ventral acoustic stria and carries information about the location of origin and direction of auditory stimuli. The pathway includes synapses in the trapezoid body, superior olivary complex, and lateral lemniscus nuclei before ending in the inferior colliculus. Axons then travel to the medial geniculate nucleus via the brachium of the inferior colliculus. The medial geniculate nuclei are the final sensory relay stations of the hearing pathway. The auditory radiation is formed by the efferent projections from the medial geniculate nucleus to the transverse temporal gyri and the adjacent planum temporale in the temporal lobe. The primary and secondary auditory cortices are located within these areas of the temporal lobe.

Sound is perceived when auditory impulses arrive at the primary auditory cortex. The processing and interpretation of sound, however, involves a combination of structures involved in audition. Information about the location of a sound is initially processed in the superior olive and inferior colliculus; however, precisely locating a sound requires processing up to the auditory association areas in the superior temporal gyrus and posterior parietal cortex. The interpretation of combinations of different frequencies in sequence begins in the cochlear nuclei and continues to the inferior colliculus and medial geniculate

nucleus and primary auditory cortex. Tonotopic organization is continuous throughout these structures.

Representation of sound is achieved bilaterally in each temporal lobe. The ascending pathways from the brain stem to the auditory cortex consist of both crossed and uncrossed fibers (see Figure 22-1). Each lateral lemniscus conducts stimuli from both ears, and an ipsilateral lesion above the level of the cochlear nuclei does not usually interfere with impulses from both ears. Because of this, deafness in one ear usually implies a lesion below the brain stem at the level of the cochlear nerve, cochlea, or middle ear.

Figure 22-1. The ascending auditory pathways.

COMPREHENSION QUESTIONS

[22.1] A 75-year-old man comes into your office complaining of increasing difficulty hearing, particularly understanding conversation in noisy places. On audiometric testing you note a loss of high-frequency hearing from the upper end of the normal spectrum down into the speech range. The hearing loss is symmetric bilaterally. Further workup for this patient is negative and you diagnose him with presbyacousis, an age-related degeneration of cochlear hair cells. Based on his pattern of hearing loss, in which area of the cochlear duct would you expect the hair cells to be most degenerated?

A. At the base of the cochlear duct

B. At the apex of the cochlear duct

C. In the middle of the cochlear duct

D. Evenly throughout the cochlear duct

[22.2] A 27-year-old man presents to your office following an industrial accident in which a large explosion occurred on his right side. He complains of pain and loss of hearing in his right ear. On examination you note a completely destroyed tympanic membrane on that side, and audiometric testing reveals near complete hearing loss in all frequencies on that side. Which of the following best describes the thalamic response to sound in this man?

A. Stimulation of the right lateral geniculate body only

B. Stimulation of the right medial geniculate body only

C. Stimulation of bilateral medial geniculate bodies

D. Stimulation of bilateral lateral geniculate bodies

[22.3] A 68-year-old man with a prior stroke 3 years ago is brought into your office by family members who state that he had a sudden, complete hearing loss that occurred yesterday evening. Prior to the event he had been well, and did not have any difficulties hearing. On examination he has a complete failure to respond to any auditory stimuli. Otologic examination is normal, as are otoacoustic emissions and auditory brainstem responses. All of this information indicates that the peripheral and central auditory pathways are intact up to the level of the thalamus. Which location is the most likely site of this man’s lesion?

A. Bilateral parietal lobes

B. Right superior temporal lobe

C. Left superior temporal lobe

D. Bilateral superior temporal lobes

Answers

[22.1] A. Loss of high-frequency hearing results from damage to the hair cells located at the base of the cochlear duct. Presbycusis is an age-related degeneration of hair cells in the organ of Corti, beginning at the base of the cochlear duct and slowly progressing toward the apex. The tonotopic organization of the cochlear duct predicts the frequencies of hearing affected. The cochlear duct is stiffer at the base, causing it to resonate at a high frequency, and more lax at the apex where it responds to lower frequency vibrations. Loss of hair cells at the base of the cochlear duct therefore cause a loss of high-frequency hearing, and as the degeneration progresses up the duct, the hearing loss descends the normal hearing spectrum, until it reaches the speech range (roughly 200-6000 Hz).

[22.2] C. Because of the decussation in the ascending auditory pathway, sound will continue to stimulate bilateral medial geniculate bodies in this patient. The thalamic nucleus involved in the ascending auditory pathway is the medial geniculate body, which receives input via the brachium of the inferior colliculus. Additionally, sound from one ear is represented bilaterally in the CNS, so even though this man has no response from his right ear, sound waves affecting his left ear will cause stimulation of bilateral structures above the cochlear nuclei. The point of initial decussation in the ascending auditory pathway is the trapezoid body. The lateral geniculate body is the thalamic relay for vision.

[22.3] D. The primary auditory cortex is located on the superior aspect of the temporal lobe bilaterally. This area is also known as the transverse temporal gyrus of Heschl. Each lobe receives input from both ears, although the contralateral ear is more highly represented. Since the auditory cortex receives bilateral input, a lesion must affect both temporal lobes in order to cause cortical deafness.

NEUROSCIENCE PEARLS ❖ Sound is transmitted through the external, middle, and inner ear via air, bony ossicles, and fluid respectively. ❖ The organ of Corti within the cochlea is organized tonotopically with higher pitches at the base and lower pitches at the apex, enabling us to differentiate sound tones. ❖ The stereocilia touch the tentorial membrane and bend, opening ionic channels that create changes in the potential of the hair cell membrane. ❖ The upper auditory pathways contain both crossed and uncrossed fibers creating bilateral representation of information from each ear.

REFERENCES

Hudspeth AJ. Hearing. In: Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science. 4th ed. New York, NY: McGraw-Hill; 2000. 

Martin JH. The auditory and vestibular systems. Neuroanatomy, Text and Atlas. 2nd ed. Stamford, CT: Appleton & Lange; 1996. 

Ropper AH, Brown RH. Deafness, dizziness, and disorders of equilibrium. Adam’s and Victor’s Principles of Neurology. 8th ed. New York, NY: McGraw-Hill; 2005.

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