Chapter 31 - Otology anatomy, embryology, radiology Flashcards by Ben Walters

Where outer, middle and inner ear structures come from embryologically

Outer and middle: 1/2 arches, 1 pouch/groove

Inner ear: bilateral otic placodes

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Detection of angular and linear acceleration

Semicircular canals- angular

Utricle, saccule- linear (horiz, vert, respectively)

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Pathway of auditory information to brain

Hair cells
Auditory nerve
Cochlear nucleus
Superior olivary complex (crossover here)
lateral lemniscus
inferior colliculi
medial geniculate body
auditory cortex (Upper Temporal Lobe/Heschl)

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Cochlea basilar membrane vs apex

BM: thick, stiff, narrow, for high frequencies
Apex: thin, flexible, wide for low frequencies

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Anatomy of ear canal

Lateral third cartilaginous, hair follicles, cerumen/sebaceous
Medial 2/3 osseus

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Length and resonance frequency of EAC

2.5cm adults

3-4kHz

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6 Hillocks of His

Mesenchyme buds surrounding dorsal end of 1st branchial cleft. 1-3 mandibular (1st) arch, 4-6 from hyoid (2nd) arch

1- tragus
2-3- helix
4-5- antihelix
6- antitragus

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Function of auricle

Collect, direct sound toward TM

Shape creates small high-pitched frequency resonances helping you localize sound vertically

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From which branchial structure does the EAC develop?

1st branchial groove

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Treacher-Collins

ALso called mandibulofacial dysostosis
AD, complete penetrance, variable expression
down-slanting palperal fissures
auricular malformations (tag, stenosis, atresia)
ossicular abnormalities
malar hypoplasia, mandibular hypoplasia
flat nasal bridge
cleft palate
dental

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Goldenhar syndrome (Oculo-Auriculo-Verteral Synd)

1/2 arch malformation
Unilateral craniofracial malf
hemifacial microsomia, eye, strabismus, anotia, tags, atresia
Scoliosis

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Branchio-oto-renal syndrome

AD
Hypoplastic/absent kidneys
pit/tag
middle ear malformation
branchial cleft cyst/fistula

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CHARGE

Coloboma
Heart
Atresia choana
Retard growth/develop
Genital (hypogonad)
ear (low set, lop ears, asymmetric pinnae)

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DiGeorge sequence

22q11 deletion
Hypoplasia thymus/parathyroid
Cardiovascular
Low ears, micrognathia, hypertelorism, short philtrum, cleft palate, choanal atresia

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Crouzon syndrome

AD
Craniosynostosis
exophthalmos, hypotelorism, strabismus, beak-shaped nose, hypoplastic maxilla, low ears, atresia/stenosis canal

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Middle ear anatomy: what it contains, and is continuous with

1-2cm^3
ossciles, stapedius, tensor tympani, chorda
continuous with mastoid air cells via antrum, nasopharynx via ET

1st branchial arch: ossicles

head/neck malleus

body incus

2nd arch: ossicles

long process malleus, incus

stapes

Where does stapes footplate derive from

2nd arch and otic capsule

Embryology of annulus

NCC-origin, mesenchyme, ossifies during 3rd month gestation

Embyrology of TM

outer ectoderm
middle mesoderm
inner endoderm

Stapedius and tensor tympani

CN VII, V3
contracts with high intensity sound, especially low frequency
Stiffen ossicular chain to decrease sound transmission
Protect cochlea from damage
reduce intensity of low-frequency background noise to preserve higher frequency speech info

Four sections of temporal bone

squamous
petrous
tympanic
mastoid

Describe course of CN VII through T bone

Exit IAC
Labyrinthine- superior to cochlea, angle forward just sup/lat to cochlea to reach geniculate ganglion, then posterior/inferior turn (first genu)
Tympanic- extend down from genu posteriorly, laterally along medial wall of tympanic cavity, above oval window, below lateral SCC, reach pyramidal eminence, then drop sharp inferior for second genu
Mastoid- after genu, downward in posterior wall of tympanic cavity, anterior wall of mastoid, exit SMF

Imaging necessary to evaluate facial nerve

CT - see osseus facial nerve canal integrity MRI - nerve itself Need both!

Endolymph vs perilymph

Perilymph- within osseus labyrinth (surrounds membranous labyrinth aka cochlea/vestibular organs), similar to ECF (high Na, low K) Endolymph- within cochlea/vestibular organs, High K, low Na, like ICF Difference in ions --> gradient of 80-100 mV allowing transduction of acoustic energy into neural impulse

What maintains the endocochlear potential?

Stria Vascularis, which is in outer wall of membranous labyrinth

Inner vs outer hair cells

Both within the organ of corti (which is within the cochlea), overlaid by tectorial membrane Inner are afferent Outer are efferent (from brain to hair cells)

How hair cells are stimulated

Stereocilia (evaginations of apical surface, look like hair) Sound moves tectorial and basilar membranes differentially --> shear force --> bends stereocilia --> opens/closes ion channels --> receptor potential --> neurotransmitters onto afferent fibers

Embryology of inner ear

Otic placodes (ectoderm lateral to neural tube rim) Invagine to otic pits...enveloped by mesenchyme (otocysts) Ossify labyrnthine between 16-24 wk Hair cell, auditory neural development complete by 26-28 wk, so fetus can hear 2-3 months prior to birth

How much of congenital deafness can be attributed to membranous dysplasia? How would you test for this?

90% Can only detect histopathologically Only 5-15% congen deaf have otic capsule anl (would show on IMG)

Describe SCC dehiscense

``` CHL Sound/pressure induced vertigo autophony Absence of bone over superior SCC May be due to incomplete ossification of otic capsule IMG with High Res CT ```

Cochlear Aplasia: location, age of arrest, appearance

osseus and membranous labyrinth 5th wk Only see a vestibule and SCCs

Cochlear hypolasia: location, age of arrest, appearance

Osseus and membranous 6th wk Single turn or less

Incomplete partition - Mondini: location, age of arrest, appearance

Osseus/Memb 7th wk 1.5 turns, partial or complete lack of interscalar septum

Common cavity: location, age of arrest, appearance

Entire osseus and membranous labyrinth 4th week Cochlea and vestibule are confluent, forming ovoid cystic space, no internal architecture

Complete labyrinthine aplasia (Michel): location, age of arrest, appearance

Entire osseus and membranous lab Prior to 4th wk complete absence inner ear structures

Which vestibular organ also responds to auditory stimuli?

Saccule | Because of this the vestibular-evoked myogenic potential EP test works, to evaluate balance function

Where is the auditory info processed in brain?

Heschl's gyrus, on superior surface of temporal lobe, close to sylvian fissure High frequency medially, low laterally Auditory association cortex is lateral to this, part of Wernicke's area