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Anatomy of the Auditory System:
A Review
Ozarks Technical Community CollegeHIS 125
» Two Major Subsystems˃ Peripheral Auditory System
+ Outer, Middle, Inner Ear+ Housed in the temporal bone of the skull
˃ Central Auditory System+ Auditory Nerve, Brainstem, Brain
˃ Note: We will focus on the peripheral auditory system in this presentation
The Auditory System
The Outer Ear
» Also referred to as the auricle or pinna
» Funnels sounds into the ear canal
» Pinna Effect=Most efficient at picking up high-frequency sounds due to the geography of the outer ear (folds, crevasses)
» Also aids in localization to sound (tell what direction sounds are coming from)
» Image from www.wetcanvas.com
External Auditory Canal
» Aka external auditory meatus or ear canal» ~1 inch (2.5cm) long in adults» Actually an elliptical shape, NOT round» The outer 1/3 of the external auditory canal
(EAC) in cartilaginous» The inner 2/3 (closest to eardrum) is bony» Cerumen, or wax, is made in the ear canal
Outer Ear Disorders
» Anotia=absence of the outer ear» Microtia=malformation of the outer ear» Wax build-up» Otalgia=ear pain» Otitis Externa=aka swimmer’s ear or outer ear infection
(bacterial)» Atresia=absent or closed ear canal» Stenosis=narrowing of EAC» Otomycosis=fungal infection of EAC» Exostoses=bony growth in EAC, common in cold water
swimmers» Osteoma=bony tumor in EAC
Microtia of Right Earhttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Occluding Cerumenhttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Otitis Externahttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Stenosishttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Exotoseshttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Osteomashttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Tympanic Membrane
» The Tympanic Membrane (TM) or eardrum separates the outer ear from the middle ear˃ Concave, disc-like structure which vibrates in response to sound entering the EAC˃ This vibration sets into motion the middle ear bones ˃ Composed of 3 layers
» The TM is attached to the malleus, the largest and first middle ear bone
» The TM is visualized using an otoscope. The process of viewing the TM through an otoscope is called otoscopy, and should ALWAYS be performed before a hearing test.
Parts of the TM
» Annulus=ring of tissue surrounding the TM which attaches it to the EAC
» Pars tensa=largest surface area of TM that is taut or tense; composed of medial (middle) and inferior (bottom-most) portion of TM
» Pars flaccida=looser and smaller area of TM; most superior or topmost part of TM
» Umbo=the point on the TM where the greatest retraction/movement occurs
Disorders involving TM
» Myringitis=inflammation of TM» Tympanosclerosis=thickening and scarring of
the TM» Perforation=a hole or tear in the TM
Bullous Myringitishttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
TympanosclerosisHawkelibrary.com
TM PerforationHawkelibrary.com
The Middle Ear
» Air-filled space between outer and inner ear˃ acts as the impedance-matcher between outer and inner ear˃ overcomes the loss of energy when sound is transmitted from one
medium (air in the outer ear) to another (fluid in the inner ear)
» Contains the ossicles, the 3 smallest bones of the body: ˃ malleus, incus, stapes ˃ AKA hammer, anvil, and stirrup
» The stapedius and tensor tympani muscles attach to the ossicles
The Middle Ear
» Eustachian tube˃ Connects the middle ear to the back of the throat (nasopharynx)
+ Approximately 1.5 inches long˃ Purpose is to equalize pressure between the middle ear and regular,
atmospheric pressure in ear canal˃ Also serves to drain fluid from middle ear that may have accumulated˃ More horizontally oriented in young children=more likely to get ear
infections
Middle Ear Disorders
» Otitis media (OM)=middle ear infection˃ Otitis media with effusion (fluid)˃ May be acute (sudden onset) or chronic (long-lasting)
» Eustachian tube dysfunction=malfunction of Eustachian tube, often swollen shut (popping, crackling, pressure, pain)
» Patulous Eustachian tube=Eustachian tube is stuck open (patent)˃ Autophony, hearing one’s own voice in head, is common symptom
» Otosclerosis=Bony growth over stapes footplate and fixation to oval window
» Ossicular chain discontinuity=Loss of connection between the ossicles; usually due to head injury
» Cholesteatoma=An erosive tumor composed of skin, protein, and fats
Serous Otitis Media http://wps.prenhall.com/chet_martin_audiolo
gy_11/194/49689/12720513.cw/index.html
Cholesteatomahttp://wps.prenhall.com/chet_martin_audiology_11/194/49689/12720513.cw/index.html
Disorders of the Outer & Middle Ear
» Cause a conductive hearing loss (CHL)˃ On the audiogram, you would find normal bone conduction thresholds
and abnormal air conduction thresholds˃ Tympanograms (test of middle ear function) will be abnormal
Conductive HL in Right Ear
Abnormal Tympanograms
Type B=flat Type C=negative pressure
Abnormal Tympanograms
Type As=stiff, hypocompliant Type AD=flaccid, hypercompliant
Conductive HL Treatment
» Medical˃ Antibiotic eardrops, oral antibiotics for bacterial infections
» Surgical˃ Myringotomy (incision in TM), pressure-equalization tubes, ossicular
repair, stapedectomy (removal of stapes and prosthesis placement)
» Amplification
The Inner Ear
» Contains our organs of hearing and balance˃ Note: we will discuss the vestibular system later
» Cochlea˃ Snail-shaped, fluid-filled organ of hearing˃ Tonotopically organized from high to low pitch
+ High frequencies at the base (nearest the stapes) and low frequencies at the apex
˃ The oval and round windows are most medially located/closest to the middle ear
+ The footplate of the stapes is attached to the oval window of the cochlea
˃ Divided into three chambers by Reissner’s membrane (aka vestibular membrane) and the basilar membrane
+ Scala vestibuli (perilymph)+ Scala media, aka cochlear duct
(endolymph)+ Scala tympani (perilymph)
Image from: medsci.indiana.edu
» This is where the magic happens!!!» Lies atop the basilar membrane, which is the “floor” of
scala media» Contains three rows of outer hair cells and one row of
inner hair cells» The tectorial membrane forms the upper border of the
Organ of Corti and hair cells deflect on this membrane with the vibration of the basilar membrane and, in turn, electrically stimulate the auditory nerve (Cranial Nerve VIII)
˃ Note: Please view the “Auditory Transduction” YouTube video
The Organ of Corti
Inner Ear Disorders
» Presbycusis˃ Age-related hearing loss; effects the high frequencies first
» Noise-Induced Hearing Loss (NIHL)˃ Exactly like it sounds; dependent on intensity, duration, and type of noise
exposure, classic “noise notch” seen from 3 to 6 kHz, with recovery at 8 kHz
» Meniere’s disease˃ Overacummulation of endolymph in the cochlea; characterized by attacks
of vertigo, tinnitus (roaring), low-frequency SNHL (usually unilateral), nausea/vomiting
» Ototoxicity˃ High-frequency SNHL due to damaging effects of certain drugs (most
notably aminoglycoside antibiotics and platinum-based chemo drugs)
» Perilymph fistula˃ Hole (fistula) at the oval window, in which perilymph leaks into middle ear.
Fluctuating HL (SNHL or Mixed HL) and dizziness common.
Meniere’s Disease Cochlear Cross-Section
*Note the displacement of the vestibular membrane
due to the overabundance of endolymph in scala media
Hawkelibrary.com
Ototoxicity
Partial Loss of Outer Hair Cells Complete Loss of Outer Hair CellsOne row of Inner Hair Cells unaffected
3 ro
ws
of o
uter
hai
r cel
ls
Hawkelibrary.com
» Result in a sensorineural hearing loss (SNHL)˃ On the audiogram, you would find abnormal bone conduction and air
conduction thresholds˃ Tympanograms (test of middle ear function) will usually be normal
Inner Ear Disorders
Bilateral SNHL
Normal Tympanogram
Type A=normal
The Otologic Exam
» Many professionals work together when it comes to the diagnosis and treatment of hearing disorders˃ Audiologists˃ Hearing Instrument Specialists˃ Otolaryngologists/ENTs˃ Speech Pathologists/Therapists
Signs of Ear Pathology
» Hearing Loss» Otalgia
˃ Ear pain
» Otorrhea˃ Ear discharge/drainage
» Tinnitus» Dizziness
FDA Regulations
» A hearing test is required within 6 months of fitting a hearing aid
» In adults, medical clearance by a physician is recommended within 6 months of fitting a hearing aid; however, the adult may sign a medical waiver if they do not wish to see an MD
» In children (<18y.o.), a medical examination by a physician is required within 6 months of fitting a hearing aid
» If any of the following conditions exist, a patient must be referred for a medical evaluation by a physician (preferably an ENT):˃ Visible congenital or traumatic deformity of the ear.˃ History of active drainage from the ear in the previous 90 days.˃ History of sudden or rapidly progressive hearing loss within the previous 90
days.˃ Acute or chronic dizziness.˃ Unilateral hearing loss of sudden or recent onset within the previous 90 days.˃ Audiometric air-bone gap equal to or greater than 15 decibels at 500 Hz,
1,000 Hz, and 2,000 Hz.˃ Visible evidence of significant cerumen accumulation or a foreign body in the
ear canal.˃ Pain or discomfort in the ear.
FDA Regulations
Components of a Medical Ear Exam
» Otoscopy˃ Many ENTs perform pneumatic otoscopy to visualize mobility of TM˃ Any cerumen that blocks the view of the TM is removed
» Tuning Fork Tests˃ See next slide
˃ Note: Please view “The ENT Exam” YouTube video
» Often utilize a 512 Hz or 1024 Hz tuning fork to help determine symmetry, type, and degree of hearing loss
» Most common:˃ Rinne˃ Weber
» Less common:˃ Schwabach˃ Bing
˃ Refer to pages 38-41 in your Hearing Disorders text
Tuning Fork Tests