3. Introduction
• The auditory system is the
sensory system for hearing
• Antonio Scarpa was an
Italian surgeon and
anatomist, who first
described many structures of
the ear
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5. Sound
• Sound wave is a form of energy
• It is produced by vibrations that create a
sinusoidal wave of alternating condensations
and rarefactions
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9. Acoustic reflex
• It is an involuntary
muscle contraction that
occurs in the middle ear
in response to loud
sound stimuli
• This reflex decrease the
transmission of
vibrational energy to the
cochlea
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10. Acoustic reflex threshold
• It is the sound pressure level from which the
sound stimulus with a given frequency will
trigger the acoustic reflex
• It is the function of sound pressure level and
frequency
• Normal ART is 70-100db SPL
• It is usually 10-20db below the discomfort
threshold
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11. Labyrinth
• It is a complex of
interconnecting cavities,
tunnels, ducts, and
canals that lies in the
petrous portion of the
temporal bone
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12. Cont’d
• The vestibule, cochlea,
and semicircular canals
form the bony, or
osseous, labyrinth
• It is filled with
perilymph, a thin watery
fluid similar to CSF
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13. Cont’d
• The membranous
labyrinth is an
arrangement of sacs and
ducts that lies within the
bony labyrinth
• It is filled with
endolymph (Scarpa’s
fluid)
• It has two major
components: the
vestibular apparatus and
the cochlear duct (scala
media)
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15. Cochlea
• Double walled ,fluid filled tube which curl to
snail shaped structure
• It turns around 2.5-2.75 to reach its apex
• It transforms pressure vibration to neural
signals
• It has 2 major chambers; the scala tympani and
scala vestibuli, which are filled with the
perilymph
• Oval window opens into the scala vestibuli
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18. Cont’d
• Frequency processing occurs in the cochlea
• The base of the cochlea process higher
frequencies and the apex lower frequencies
• Auditory nerve fibers are tuned to different
center frequencies
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19. Organ of corti
• Helical band between the outer bony cochlea and
modiolus
• It contains the hair cells, which produces nerve impulse
in response to sound vibration
• contains around 16000-20000 hair cells
• Each hair cell has many cilia which bend with
vibrations of the basilar membrane
• Layers
– Basilar membrane
– Reticular lamina
– Tectorial membrane
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21. Cont’d
• The organ of Corti rests on the basilar
membrane and contains inner and outer hair
cells
• The inner hair cells are the receptors, or end
organs, of the cochlear nerve.
• From the apex of each inner hair cell, a
stereocilium extends to just beneath the
tectorial membrane
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22. Cont’d
• Axons of the spiral
ganglion cells form the
cochlear nerve, which
contains around 30,000
fibers
• The spiral ganglion
consists of type I and
type II bipolar neurons
that lie in the modiolus
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23. Cont’d
• Axons from type I cells are myelinated and
form the bulk of the nerve, which make up
95% of the ganglion
• The type II cells connect with the outer hair
cells and modulate the activity of the inner hair
cells
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24. Acoustic (Vestibulocochlear)
Nerve
• It has two components, the vestibular and the
cochlear branches
• The cochlear portion subserves hearing; the
vestibular nerve subserves equilibration,
coordination, and orientation in space
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26. THE COCHLEAR NERVE
• Sound waves converge on the tympanic
membrane and are transmitted by the auditory
ossicles (malleus, incus, and stapes) to the
inner ear, or labyrinth
• The inner ear transforms pressure vibration to
neural signals
• The spiral ganglion of the cochlear nerve lies
in the spiral canal of the modiolus
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28. Cont’d
• The acoustic nerve traverses the internal
auditory canal (IAC), where it lies lateral and
inferior to the facial nerve
• Each entering fiber bifurcates to synapse in
both the dorsal (posterior) and ventral
(anterior) cochlear nuclei
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29. Cont’d
• In the cochlear nuclei, low-frequency tones are
processed ventrally and high frequencies
dorsally.
• Second-order neurons in the cochlear nuclei
give rise to the dorsal, ventral, and
intermediate acoustic stria.
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30. Cont’d
• Fibers of the ventral acoustic stria are both
crossed and uncrossed
• This fibers may synapse in the nuclei of the
trapezoid body, superior olive, or lateral
lemniscus.
• The binaural pathway, especially the superior
olivary complex component, can determine the
time difference between the two ears and aid in
the localization of sound
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31. Cont’d
• Fibers from the lateral lemniscus ascend to
synapse in the central nucleus of the inferior
colliculus, an auditory reflex center that is also
tonotopically organized.
• The inferior colliculus is the central relay
nucleus of the auditory pathway and receives
both ascending and descending input.
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33. Cont’d
• Descending auditory projections run parallel to
the ascending fibers and are concerned with
auditory reflexes
• Descending pathways include the
corticogeniculate, corticocollicular,
geniculocollicular, and collicular efferents.
• The efferent cochlear bundle projects from the
superior olive to the cochlea
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35. Cont’d
• There is extensive crossing of the central
auditory pathways above the level of the
cochlear nuclei
• Commissures connect the nuclei of the lateral
lemniscus and the inferior colliculi
• The corpus callosum contains fibers that
connect the auditory cortices of the two
hemispheres
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38. Cont’d
• Whispered voice has been recommended as an
excellent screening test.
• Inability to perceive a whispered voice has a
likelihood ratio (LR) of 6.1 (95% CI, 4.5 to
8.4) for clinically significant hearing loss;
normal perception has an LR of 0.03 (95% CI,
0 to 0.24).
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39. Cont’d
• When using whispered voice, certain tones are
heard better and at a greater distance than
others
• Sibilants, and the short vowels such as a, e,
and i, are heard at a greater distance than broad
consonants such as 1, m, n, and r, and such
vowels as o and u.
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40. Cont’d
• “Seventy-six” and “sixty-seven” can be heard
at a greater distance than “ninety-nine” and
“fifty-three.”
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41. Tuning fork
• Tuning forks typically 128, 256, or 512 Hz are
used are used
• It gives more specific information about to air
conduction (AC) and bone conduction (BC)
• In evaluating BC, be certain the patient hears
rather than feels the tuning fork
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42. Cont’d
• How useful tuning fork tests are for general
screening has been questioned
• The primary usefulness of both the Weber and
Rinne tests is not as a screening tool but to
make an initial differentiation between SNHL
and CHL in a patient complaining of unilateral
symptoms of hearing loss or tinnitus
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43. Rinne test
• The air conduction is compared to bone
conduction for each ear.
• It measures air conduction by holding a
vibrating tuning fork just outside each ear, and
bone conduction by placing a tuning fork
handle on each mastoid process
• Normal individuals hear the tone better by air
conduction
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44. Weber test
• In the Weber test the tuning fork is placed on
the vertex of the skull in the midline, and the
patient is asked to report the side where the
sound is louder
• Normally, the tone sounds equal on both sides.
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45. Cont’d
Types of HL Auditory
Acuity
Rinne Test Weber Test
CHL Decreased BC > AC (Rinne
negative or
abnormal)
Lateralizes to the
abnormal side
SNHL Decreased AC > BC (Rinne
positive or normal)
Lateralizes to the
normal side
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46. Audiometry
• It can assess the hearing in a detailed manner
• The range of human hearing is 20 to 20,000 Hz
• Speech usually falls in the 300 - 3,000-Hz
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47. Cont’d
• There are many different audiologic
techniques; pure tone and speech audiometry
• The pure tone audiogram displays the severity
of any hearing loss in relation to established
reference values, and the pattern may suggest
the etiology
• Speech audiometry uses spoken words and
sentences instead of pure tones
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48. Cont’d
• The speech reception threshold is considered
the intensity level at which the patient can
correctly understand 50% of the material
presented
• Speech discrimination, is the proportion of the
material the patient can understand when
presented at a level that should be easily heard
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49. Cont’d
• Poor speech discrimination, out of proportion
to pure tone hearing loss, is characteristic of a
retrocochlear lesion, such as cerebellopontine
angle tumor.
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50. Cont’d
• A tympanogram measures the impedance of
the tympanic membrane.
• An abnormal tympanogram is seen in such
conditions as otitis media, tympanic membrane
perforation, ossicular dislocation, otosclerosis,
cerumen impaction, and eustachian tube
dysfunction.
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51. Auditory evoked Potential
• It is a potential produced by auditory stimuli
and recorded using electroencephalogram
(EEG) electrodes
• They are classified according to three main
characteristics: their latency range, time
course, and dependency on cognitive
processing
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52. Imaging
• An MRI scan with fine cuts through the
auditory canal should be performed when
disorders of the eighth nerve are suspected
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54. Case scenario
• A 41-year-old woman was referred to an
otolaryngologist for dizziness and progressive
hearing loss in the left ear
• One year ago the patient began having episodes of mild
dizziness, which felt like the room was spinning
when she moved her head. Two months ago she
noticed greatly reduced hearing in her left ear,
making it impossible to use the telephone receiver
unless it was on her right ear. In addition, she had some
left facial pain and decreased taste on the left side of
her tongue. Past medical history was notable for a
melanoma resected from the right hip region 6 months
previously, with one positive lymph node.
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55. PHYSICAL EXAMINATION
• Vital signs: Stable
• Ears: Normal otoscopic exam of the external auditory canals and
tympanic membranes.
• NS
– MENTAL STATUS: Alert and oriented × 3. Mildly anxious, but
otherwise normal.
– CRANIAL NERVES: Pupils equal mid sized and reactive to light.
– Normal fundi. Visual fields full. Extraocular movements intact.
– Facial sensation intact to light touch, but decreased corneal reflex on
the left.
– Face symmetrical.
– Hearing greatly diminished to finger rub or whispering in the left
ear.
– A vibrating tuning fork sounded louder when held just outside the
left ear than when the handle was touched to the left mastoid
process (air conduction greater than bone conduction).
– Taste was not tested. Voice and palate elevation normal.
– Tongue midline.
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57. Hearing loss
• It can be unilateral or bilateral
• Unilateral hearing loss can be caused by
disorders of the EAC, middle ear, cochlea,
eighth nerve, or cochlear nuclei
• once the auditory pathways enter the
brainstem, information immediately crosses
bilaterally at multiple levels
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58. Cont’d
• It can be divided into
– Conductive HL
– Sensorineural HL
– Central HL
• As a generality, CHL affects low frequencies
and SNHL affects high frequencies
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60. Conductive hearing loss
• It is that due to impaired conduction of sound
to the cochlea
• Commonly caused by cerumen in the external
auditory canal, otitis, tympanic membrane
perforation, and sclerosis of the middle ear
ossicles
• Disease of the nasopharynx with obstruction of
the eustachian tube
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61. Sensorineural hearing loss
• It is due to disease of the
cochlea or eighth CN
• Caused by exposure to
loud sounds, infections,
ototoxic drugs, head
trauma, aging, Meniere’s
disease, cerebellopontine
angle tumors, and, rarely,
internal auditory artery
infarct
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62. Cont’d
• Cerebellopontine angle tumors include
acoustic neuroma (vestibular schwannoma),
meningioma, cerebellar astrocytoma,
epidermoid, glomus jugulare, and metastases
• The most common tumor by far in this location
is acoustic neuroma, accounting for about 9%
of intracranial neoplasms
63. Acoustic neuroma
• The mean age of onset is 50 years
• It is nearly always unilateral
• This slow growing tumor develops at the
transitional zone between Schwann cells and
oligodendrocytes, which occurs at the point
where CN VIII enters the internal auditory
meatus
64. Cont’d
• Common early symptoms are unilateral
hearing loss, tinnitus and unsteadiness
• The next cranial nerve to be affected is usually
the nearby trigeminal nerve, with facial pain
and sensory loss
• Often the first sign of trigeminal involvement
is a subtle decrease in the corneal reflex
65. Cont’d
• With large tumors, cerebellar and corticospinal
pathways are compressed, causing ipsilateral
ataxia and contralateral hemiparesis.
• Impairment of swallowing and the gag reflex
(CN IX and X) and unilateral impaired eye
movements (CN III and VI) occur only in very
large tumors.
66. Cont’d
• Ultimately, if left untreated, the tumor will
compress the fourth ventricle, causing CSF
outflow obstruction, hydrocephalus,
herniation, and death.
• With appropriate clinical evaluation and MRI
scanning, acoustic nerve tumors can be
detected at an early stage, when they still lie
entirely within the auditory canal
67. Treatment
• Treatment has traditionally been by surgical
excision, but more recently there is a shift
towards stereotactic radiosurgery with gamma
knife or CyberKnife.
• Some smaller tumors may be monitored by
MRI, especially in older patients
68. cont’d
• Conventional surgery requires a posterior
fossa approach, often involving collaboration
between a neurosurgeon and
otolaryngologist. Surgeons strive to spare
facial nerve function during the procedure,
and with smaller tumors some hearing may
even be spared in the affected ear
69. Central hearing loss
• Central hearing loss is that due to disease of
the central pathways
• Central hearing loss is very rare because of the
bilaterality and redundancy of the auditory
system
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70. Tinnitus
• Tinnitus is spontaneous noise in the ears or
inside the head in the absence of auditory
stimuli
• In many cases, no precise etiology can be
established.
• It can be due to an abnormal excitation of the
auditory apparatus or its afferent pathways, but
the exact mechanism is often unclear
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71. Cont’d
• It can be objective or subjective
• However, most tinnitus is subjective tinnitus
• It may vary in pitch and intensity and may be
continuous or intermittent
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72. Cont’d
• Tinnitus is commonly associated with deafness
• It is common in presbycusis and in other types
of SNHL and is a fairly constant feature of
otosclerosis
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73. Cont’d
• Pulsatile tinnitus is synchronous with the
pulse; it is in reality a bruit
• It can be caused by carotid stenosis, AV
malformations, glomus tumors; venous hums;
and hypertension
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74. Cont’d
Other Causes
Cerumen impaction
Medications (particularly ototoxic drugs)
Ménière’s disease
Acoustic neuroma
Acute or chronic acoustic trauma
Paget’s disease
Labyrinthitis
Anemia
Arnold-Chiari malformation.
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75. Cont’d
• Diagnosis is usually by history
• Its severity can be rated from slight to severe
according to its effects
• Treat the underlying cause if it is identified
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The auditory system is a sound analyzer
It includes both the sensory organs and the auditory parts of sensory system
It is produced by vibrations that create a sinusoidal wave of alternating condensations and rarefactions in a conductive medium, usually air but sound travels in any elastic medium
Amplitude – Loudness or volume (Physical strength of a wave)
Fequncy - Pitch
All waves has 3 basic characteristics
Amplitude - Loudness or volume (Physical strength of a wave)
Frequency – Pitch (high or low) .Unit is cycle/sec (complete wave/second) measured in Hz (humans with normal hearing can hear 20-20k Hz) and
Wave length – the distance b/n 2 successive waves
FIGURE 17.1 The right osseous labyrinth in the temporal bone viewed from above.
FIGURE 17.3 The membranous labyrinth.
Membranous labyrinth lies inside a complex of interconnected tunnels within the petrous temporal bone, called the “bony labyrinth.”
Each membranous labyrinth is composed of a spiral cochlear division and a larger vestibular division that includes the vestibule (saccule and utricle) and three semicircular canals oriented approximately at right angles to each other.
FIGURE VIII–1 Overview of the vestibulocochlear nerve, cranial nerve VIII. The membranous labyrinth
(blue) contains endolymph.
FIGURE 17.4 Structure of the cochlear duct and the spiral organ of Corti
Basilar membrane – hair cells rest here
Reticular lamina – rigid surface that supports te sterocilia of the hair cells
Tectorial membrane – gelatinous mass with internal fibers that sit on the top of sterociclia
Sound waves induce vibrations in the cochlea, which cause movement of the basilar and tectorial membranes.
This movement flexes the stereocilia, which activates the hair cell, causing impulses in the spiral ganglion
FIGURE VIII–3 Type 1 and type 2 hair
cells in vestibular apparatus.
The vestibulocochlear, acoustic, or eighth cranial nerve (CN VIII) has two components, the vestibular and the cochlear, blended into a single trun
FIGURE VIII–2 Bony and membranous labyrinths of the vestibulocochlear nerve. cranial nerve VIII (removed
from the surrounding petrous temporal bone).
It transforms pressure vibration to neural signals
FIGURE 17.6 The acoustic nerve and its connections.
The primary auditory cortex is tonotopically organized with high frequencies medial and low frequencies lateral
The auditory association cortex (Wernicke’s area in the dominant hemisphere) lies just posterior to the primary auditory cortex
history
tendency to turn the head when listening
lip reading
speaking with a loud voice
Inorder to ensure the tympanic membrane is intact and to exclude the presence of wax, pus, blood, foreign bodies, and exudate
(some argue that only 512 Hz or higher is useful for testing hearing)
Detailed assessment of hearing is done with audiometry, which is usually performed as a battery of tests.
The loss of discrimination is proportional to the severity of the hearing loss in patients with cochlear lesions.
In cranial nerve (CN) VIII lesions, discrimination may even paradoxically decline as intensity is raised
IMAGE 12.5A,B Left Acoustic Neuroma (Vestibular Schwannoma) Axial T1-
weighted MRI images with intravenous gadolinium contrast. (A) and (B) are
adjacent sections progressing from inferior to superior.
Distal to the cochlear nuclei
Therefore, unilateral hearing loss is not caused by lesions in the central nervous system proximal to the cochlear nuclei.
Ménière’s disease is a notable exception, causing predominantly low-frequency hearing loss, at least early in the course
It is due to disease of the cochlea (e.g. Ménière’s disease) or eighth CN (e.g., acoustic neuroma).
It may be described in many ways, such as ringing, buzzing, blowing, whistling, swishing, or roaring
