Overview Of The Anatomy Of The Ear

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The structure of the External Auditory Canal consists of

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an outer half made of cartilage, and an inner half comprised of bone.

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Cerumen glands are found in the

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outer half

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The canal has a

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downward and forward course

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in an adult one must pull the auricle

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backward and upward to straighten it

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There is a more superior course in infants, therefore one must pull the auricle

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backward and downward to straighten it.

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An infection of the external auditory canal is

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otitis externa.

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The parts of the middle ear include the

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auditory ossicles and tympanic cavity and membrane.

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The auditory ossicles are connected by

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synovial joints which may undergo ankylosis resulting in otosclerosis

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The middle ear is connected to the

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auditory (pharyngotympanic) tube, mastoid antrum and mastoid air cells.

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Embryologically, it is an outpouching from the pharynx, therefore it is lined with mucosa like the

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pharynx; and is connected to the nasopharynx by the auditory tube.

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The normal function of the pharyngotympanic (auditory tube) is to

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regulate pressure within the middle ear, and allow drainage of secretions from the middle ear.

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The middle ear receives its sensory innervation from

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cranial nerve IX (glossopharyngeal).

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Which two muscles dampen sounds and protect the ear?

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stapedius (CN VII, Facial) and tensor tympani (CN V3, mandibular branch of Trigeminal)

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The middle ear can be thought of as

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a room with plumbing (blood vessels), electric wires (nerve), and heating/ac ducts (openings to other rooms) in its walls

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The Lateral Wall is comprised of the

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tympanic membrane which is divided into two parts, the larger pars tensa, and the smaller more superior pars flaccida.

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The tympanic membrane faces

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outward, forward, and downward.

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The lateral process and handle (manubrium) of the malleus are attached to the

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tympanic membrane.

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The most indrawn point on the tympanic membrane is the

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umbo.

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The Chorda tympani nerve crosses the

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medial surface of the handle of the malleus (medial surface of the tympanic membrane).

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The Medial Wall contains the

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promontory (prominence formed by the cochlea), oval and round windows.

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The Anterior Wall is a

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bony separation between the tympanic cavity and the carotid canal containing the internal carotid artery.

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The pharyngotympanic (auditory) tube and the tensor tympani muscle are located in its

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superior portion.

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Posterior Wall. The aditus (entrance to the mastoid antrum) is located

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superiorly.

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The stapedius muscle and facial canal (containing CN VII) are located in the

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posterior wall.

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Roof. The tegmen tympani is the

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thin layer of bone that separates the middle ear from the middle cranial fossa.

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Floor. Contains the

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superior bulb of the internal jugular vein.

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Facial Nerve (CN VII) (branches):
Greater petrosal nerve:

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parasympathetic to the lacrimal gland and mucous glands of the nasal and oral cavities.

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Chorda tympani:

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parasympathetic to submandibular and sublingual glands; and sensory (taste) from anterior 2/3 of tongue (sweet and salt).

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Retrograde infection from the pharynx via the auditory tube can result in

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otitis media

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This occurs when the auditory tube is blocked which results in

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resorption of air and a consequent negative pressure in the tympanic cavity.

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The negative pressure may result in

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aspiration of nasopharyngeal secretions and bacteria.

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Infants and young children are more susceptible to this because

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their auditory tube is shorter and positioned in a horizontal plane, as compared to the greater downward slope in older children and adults.

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Surgery to relieve the fluid accumulation in chronic otitis media includes

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myringotomy.

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It involves creating a curvilinear incision in the inferior portion of the tympanic membrane just below the handle of the malleus (clinical umbo)

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to facilitate drainage.

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A tympanostomy (myringotomy) tube may be inserted to

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ensure the opening remains patent, and allow proper ventilation of the tympanic cavity.

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Complications of otitis media include

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mastoiditis, an infection of the mastoid air cells; facial nerve palsy, and more rarely intracranial complications such as brain abscess, meningitis, extradural and subdural abscesses, and thrombosis of the sigmoid sinus.

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Common pathogens in acute otitis media (AOM) are:

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Haemophilus influenzae , Streptococcus pneumoniae, and Moraxella catarrhalis.

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S. pneumoniae was the most common pathogen prior to the

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availability of the Prevnar vaccine.

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S. pneumonia is an encapsulated gram positive cocci that

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colonizes the nasopharynx.

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It commonly causes the following infections:

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otitis media, sinusitis, pneumonia, tracheobronchitis, and meningitis.

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According to the American Academy of Pediatrics and the American Academy of Family Practice, the treatment protocol for children less than 6 months of age with AOM is

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amoxicillin 80-90 mg/kg BID x 3-5 days.

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H. influenzae is a

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gram negative coccobacillus.

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Serotype “b” is

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encapsulated and has a high risk of systemic infection until 6 years of age.

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It commonly causes the following infections:

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meningitis, cellulitis, epiglotitis and pneumonia.

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Non-typable H. influenzae does not have a

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capsule and is the cause for otitis media and sinusitis

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M. catarrhalis is a gram negative cocci that commonly causes

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otitis media, sinusitis, tracheobronchitis, and pneumonia.

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Chronic otitis media includes otitis media with effusion (OME) usually lasting

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longer than 3 months

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recurrent otitis media which is AOM that resolves and then recurs usually

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3 or more times in 6 months.

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Biofilms of the pathogens listed above are thought to be the cause of

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chronic otitis media

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A biofilm is a

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polymicrobial cluster of pathogenic bacteria encased within an extracellular matrix, adherent to a surface, and resistant to antibiotics.

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The biofilm is the

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predominant form of most bacteria in their natural habitat.

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Bacterial colonies similar to those found in lab cultures are found in

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acute infections, but nearly all chronic infections are biofilms.

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The pathogens associated with chronic infections are better identified by

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molecular markers than cultures because many of the bacteria in a biofilm are not amenable to culture.

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Unilateral hearing loss is classified as either

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conductive or sensorineural.

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Conductive hearing loss results from a

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disorder of the external acoustic meatus or middle ear, such as excess cerumen, a perforated tympanic membrane, or otosclerosis.

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Sensorineural hearing loss results from a

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disorder of a structure distal to the cochlear nuclei in the brainstem. It could be a disorder of the cochlea or cochlear division of CN VIII.

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Some causes include

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Meniere’s disease, ototoxic drugs such as antiseizure medications or aminoglycoside antibiotics, viral infections and cerebellopontine angle tumors of which an acoustic neuroma (aka vestibular schwannoma) is the most common type.

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When unilateral hearing loss is detected on physical examination, it is assessed via the

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Weber and Rinne tests.

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The Weber test will demonstrate

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lateralization

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In conductive hearing loss the sound will lateralize to the

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“bad” ear, i.e. the one with a conductive loss.

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sound vibration simply travels through the bones of the cranium and bypasses the

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external and middle ear structures, thus allowing it to be heard more clearly.

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plug one ear with your finger and hum. You will note that the sound is heard better in the

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plugged ear (simulating a conductive loss).

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With a sensorineural loss, the sound lateralizes to the

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“good” ear

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The Rinne test distinguishes

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which problem is producing the lateralization

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In a conductive loss: bone conduction is

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greater than air conduction (again, the sound bypasses the external and middle ears by traveling through the cranium to the inner ear).

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In a sensorineural loss, air conduction is

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greater than bone conduction, but there is reduction in the normal 2:1 air to bone conduction ratio.

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True vertigo is

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a sense of “spinning” that occurs as a result of a lesion of the vestibular pathway. This includes any structure from the labyrinth in the inner ear, the vestibular nerve and nucleus, cerebellum and parietal cortex.

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It is important to distinguish between

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peripheral versus central causes of vertigo.

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Central causes can include disorders of the

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posterior cranial fossa which are considered medical emergencies

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Associated symptoms include

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diplopia, dysarthria, incoordination, and episodes of unconsciousness.

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The Nylan-Barany or Dix-Hallpike maneuver can help to distinguish between

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central and peripheral vertigo.

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vertical nystagmus that occurs immediately and does not adapt with repetition is highly associated with

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central causes of vertigo