FACTS & HINTS

High-Yield Facts

Clinical Points

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General Testing of Extraocular Muscles

Extraocular muscles act as synergists and antagonists and are responsible for multiple movements of the eye. It can be difficult to test each eye muscle individually.

Ageneralist, however, can gain a general idea of extraocular muscle (or nerve) impairment bychecking the abilityof individual muscles to elevate or depress the globe with the eye abducted or adducted, therebyaligning the globe with the pull (line of contraction) of the muscle Ask patient to "Follow myfinger with just your eyes" and move your finger in the form of the letter H.

Superior rectus is tested bymoving your finger superiorlyand mediallyto the eye (to counteract the interaction of the inferior oblique)

a.Inferior rectus is tested bymoving your finger inferiorlyand mediallyto the eye (to counteract the interaction of the superior oblique)

b.The medial and lateral rectus muscles are tested bymoving your finder mediallyand laterallyto the eye.

c.The inferior oblique is tested bymoving your finger superiorlyand laterallyto the eye

d.The inferior oblique is tested bymoving your finger inferiorlyand laterallyto the eye.

Remember that because all the muscles are involved in the continuous movement of the eye, it is difficult to isolate the action of just one with absolute clinical certaintyvia this test.

Clinical Points

Cataract

Most common clinical condition of the eye worldwide

Involves opacification or cloudiness of the lens

Risk factors include: smoking, age, alcohol, diabetes, steroid use, and exposure to ultraviolet (UV) rays

Treatment involves surgical removal of the lens

Eyesight is corrected with an implant, glasses, and/or contact lens

Clinical Points

Glaucoma

One of most common cause of blindness world wide

Build-up of pressure in anterior and posterior chambers of the eye

Usuallya result of resistance to outflow of aqueous humor via angle of eye and through canal of Schlemm

Results in compression of neural layer of retina

Leads to visual field defects and ultimatelyblindness

Increased intraocular pressure classified as open or closed angle

a.Open angle-develops graduallywith blocking of canal of Schlemm or obstruction of angle

b.Closed angle-occurs rapidlywhen iris and lens block passage of aqueous humor through pupil

Clinical Points

Orbital "blow-out" Fractures

Medial and inferior walls of orbit are verythin, so a blow to the eye can fracture the orbit

Indirect trauma that displaces walls is called a "blow-out" fracture

Fractures of medial wall mayinvolve ethmoid and sphenoid sinuses

Fracture of the floor mayinvolved the maxillarysinus

Fractures can result in intraorbital bleeding

Blood puts pressure on eyeball, causing exophthalmos

Blood and orbital structures can herniate into maxillarysinus

Clinical Points

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Conjunctivitis

Common condition often referred to as "pink eye"

An inflammation of the conjunctiva

Symptoms include redness, irritation, and watering of the eyes and sometimes discharge and itching

Can be triggered byinfection

a.Highlycontagious

b.Caused bybacteria or viruses

c.Sexuallytransmitted diseases (STDs), such as gonorrhoea and chlamydia, can cause it

d.Viral conjunctivitis is common with several viral infections and can arise as a result of or during a common cold or flu

Can be triggered byallergies

a.More freqentlyoccurs in children with other allergic conditions, e.g., hayfever, animal fur

b.Typicallyaffects both eyes at the same time

Can be triggered byan external irritant

a.Can be caused bypollutants such as traffic fumes, smoke

b.Can be caused bychemicals such as soap, chlorine

Mnemonics

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Memory Aids

Formula for the innervation of extraocular muscles:

LR6-SO4

Rest 3

LR6 L ateral R ectus → CNVI (abducent)

SO4 S uperior Oblique → CNIV (trochlear nerve)

Rest:Remaining 4 muscles → CNIII (oculomotor)

Superior rectus

Inferior rectus

Medial rectus

Inferior Oblique

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10 Ear

STUDYAIMS

At the end of your study, you should be able to:

Define the external, middle, and inner ear

Describe the anatomical features of the external ear

Describe the tympanic membrane (ear drum)

Describe the walls of the middle ear

Outline the structures located within the middle ear

Understand the transmission of sound through the middle ear

Describe the bonylabyrinth and its components

Describe the membranous labyrinth and its components

Understand the organization of the semicircular canals and their relationship to equilibrium

Understand the organization of the Organ of Corti and its relationship to hearing

Understand how sound is perceived

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GUIDE

Head and Neck: Ear

Ear

Is divided into three parts

External ear

Auricle

External acoustic meatus

Middle ear

Tympanic cavityand its contents

Epitympanic recess

Inner ear

Vestibulocochlear organ

Membranous labyrinth

Bonylabyrinth

Functions are equilibrium (balance) and hearing

External Ear

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Auricle or pinna

Skin-covered elastic cartilage

Collects sound and directs it to external auditorymeatus

Features

Deep depression: concha

Lobule: earlobe External auditory(acoustic) meatus

From the deepest part of the concha to the tympanic membrane

Through the tympanic part of the temporal bone

Lateral one third is cartilaginous and lined with normal skin

Medial two thirds are bonyand lined with thin skin continuous onto tympanic membrane

Ceruminous glands (waxproducing) and sebaceous glands found in subcutaneous tissue of lateral one third Tympanic membrane

Forms medial wall external ear, lateral wall of middle ear

Oval and semitransparent

Covered with thin skin externallyand mucous membrane internally

Has shallow, cone-like depression at its center (seen otoscopically) with the umbo at the center of the depression

Lateral process (handle) of the malleus embedded in tympanic membrane and tip extends to the umbo on the internal surface

Pars tensa

Part of membrane below handle of malleus

Contains circular and radial fibers

Pars flaccida

Part of membrane above handle of malleus

Contains no fibers

Moves in response to air vibrations

Transmits vibrations to auditoryossicles of middle ear via handle of malleus

Middle Ear

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Contained within petrous part of temporal bone

Includes tympanic cavityand epitympanic recess

Is connected anteriorlywith nasopharynxbypharyngotympanic tube (auditorycanal)

Contains

Auditoryossicles (bones of middle ear)

Malleus (hammer)

Incus (anvil)

Stapes (stirrups)

Stapedius and tensor tympani muscles

Chorda tympani nerve

Tympanic plexus (nerves)

Walls of tympanic cavity

Roof

Thin bone-tegmen tympani

Separates tympanic cavityfrom middle cranial fossa

Floor

Bony

Separates tympanic cavityfrom superior bulb of internal jugular vein

Medial wall

Separates tympanic cavityfrom inner ear

Features

a.Promontory: round projection overlying basal turn of cochlea

b.Oval window into which footplate of stapes fits-covered bysecondarytympanic membrane

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c. Fenestra cochlea or round window Anterior wall

Separates tympanic cavityfrom carotid canal

Superiorlyhas opening of auditorytube and canal for tensor tympani Posterior wall

Superiorly, aditus (opening) to mastoid antrum, connecting to mastoid air cells

Between posterior wall and aditus, prominence of canal of facial nerve Pyramidal eminence

a.Tinycone-shaped prominence

b.Contains proximal attachment of stapedius muscle

Lateral wall

Tympanic membrane

Lateral wall of epitympanic recess

Handle of malleus

Ossicles

Chain of bones across tympanic cavity

Extend from tympanic membrane to oval window on medial wall

Handle of malleus attaches to tympanic membrane and head articulates with incus

Incus articulates with head of malleus and end of long limb with stapes

Head of stapes articulates with long limb of incus and base (foot-plate) fits into oval window on medial wall

Increase the force but decrease amplitude of vibrations from tympanic membrane Auditory(pharyngotympanic) tube

Formerlycalled Eustachian tube

= Communication between middle ear and nasopharynx

Allow equalisation of atmospheric pressure in middle ear

Activelyopened bycoordinated contractions of levator and tensor veli palatine muscles Muscles

Function to dampen movement of auditoryossicle

Tensor tympani

Stapedius Nerves

Chorda tympani

Branch of facial nerve (cranial nerve [CN]) VII

Emerges from facial canal through canaliculus in posterior wall of middle cavity

Carries taste fibers to anterior two thirds of tongue

Crosses medial surface of neck of malleus

Exits anteriorlyvia canaliculus to petrotympanic fissure

Tympanic plexus

On the promontoryof the medial wall

Contributions from tympanic branch of glossopharyngeal (CN IX)

Twigs from internal carotid plexus

Twig from facial nerve Supplies

a.Mucous membrane of tympanic cavity

b.Mastoid antrum

c.Mastoid air cells

d.Auditorytube

Inner Ear

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Concerned with the reception of sound and maintenance of balance

Buried within petrous portion of temporal bone

Membranous labyrinth suspended within bonylabyrinth

Bonylabyrinth

Occupies lateral region of petrous ridge of temporal bone

= Space filled with perilymph

Surrounded bybonycapsule harder than petrous bone

Cochlea

Concerned with hearing

Contains the membranous cochlear duct

Consists of

a.Spiral canal

b.Bonycore, the modiolus

Canal spirals around modiolus

Basal turn forms promontoryof medial wall of tympanic cavity

At basal turn, bonylabyrinth communicates with subarachnoid space above jugular foramen via cochlear aqueduct Vestibule

Small oval chamber

Contains membranous utricle and saccule

Oval window is on lateral wall Continuous with

a.Cochlea anteriorly

b.Semicircular canals posteriorly

Communicates with posterior cranial fossa via aqueduct of vestibule a. Extends to posterior surface of petrous ridge of temporal bone

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b. Contains membranous endolymphatic duct Semicircular canals

Anterior, posterior, and lateral

Set at right angles to each other in three planes

Lie posterosuperior to vestibule

Each opens into vestibule

Swelling at one end of each canal: ampulla

Contain membranous semicircular ducts Membranous labyrinth

Collection of ducts and sacs

Suspended within bonylabyrinth

Filled with endolymph

Vestibular labyrinth Utricle

a.Has specialized area of sensoryepithelium: macula

b.Hair cells in macula innervated byvestibular division of vestibulocochlear nerve (CN VIII)

c.Hairs respond to tilting of head and linear acceleration and deceleration

Saccule

a.Communicates with utricle

b.Continuous with cochlear duct

c.Contains macula, identical in structure and function to that of utricle

Semicircular ducts

a.Within semicircular canals

b.Each has ampulla at one end

c.Ampullarycrest in each ampulla senses movement of endolymph in plane of duct

d.Hair cells in crest innervated byvestibular division of vestibulocochlear nerve (CN VIII)

e.Detect rotational (tilting) movements of head

Cochlear labyrinth

Spiral ligament suspends cochlear duct from external wall of spiral canal

Cochlear duct

a.Triangular in shape

b.Filled with endolymph

c.Spans spiral canal, dividing it into two channels, each filled with perilymph

d.Two channels: scala tympani and scala vestibule, meet at apexof cochlea (helicotrema)

e.Third, middle channel = cochlear duct

f.Roof of duct = vestibular membrane

g.Floor of duct = basilar membrane

Spiral organ of Corti

a.Found on basilar membrane

b.Covered bygelatinous tectorial membrane

c.Contains hair cells-tips embedded in tectorial membrane

d.Hair cells innervated bycochlear division of vestibulocochlear nerve (CN VIII)

Propagation of sound

Initiated bysound waves

Transmitted as vibrations byossicles

Stapes vibrates in oval window

Creates waves of pressure in perilymph of vestibule

Waves ascend in spiral canal in scala vestibule

Transfer to scala tympani at helicotrema

Pass down scala tympani to round window

Dissipated via secondarytympanic membrane in round window to air of middle ear cavity

Reception of sound

Deformation of cochlear duct bypressure waves in perilymph

Stimulates hair cells of spiral organ embedded in tectorial membrane

Base of spiral organ receives high frequencysounds and apexreceives low-frequencysounds Internal auditory(acoustic) meatus

1 cm long tinycanal in petrous ridge of temporal bone

Opening in posteromedial aspect of ridge in posterior cranial fossa Transmits facial (CN VII) and vestibulocochlear (CN VIII) nerve

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[Plate 98, Auditory (Pharnygotypmpanic, Eustachian) Tube]

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