Functional Anatomy of the Eye

Question 1

Describe the structures forming the walls of the orbital cavity

Answer 1

• Roof - frontal bone, anterior cranial fossa, sphenoid bone
• Lateral wall - zygomatic bone, sphenoid bone
• Floor - maxilla, part of zygomatic bone
  
  • Below the floor of the orbit is the maxillary paranasal sinus - air filled cavity
• Medial wall - ethmoid, maxilla, lacrimal, part of sphenoid
  - Ethmoid bone contains ethmoid air cells

Question 2

What are the openings at the apex of the orbital cavity

Answer 2

• Superior orbital fissure
• Inferior orbital fissure
• Optic canal

Question 3

Outline which walls of the orbital cavity are most susceptible to fracture

Answer 3

• Medial wall and floor of the orbit are the weakest parts of the orbital cavity as they contain paranasal air sinuses behind the bone
• Fracture can break into sinus and cause infection to spread to and from orbit

Question 4

What are signs and symptoms of orbital blow out fracture

Answer 4

• History of trauma to the eye/orbit
• Periorbital swelling, painful
• Double vision - especially on vertical gaze
• Impaired vision - restricted eye movement
• Anesthesia over affected cheek (upper teeth and gums) on affected side
  
  • Infra-orbital nerve from maxillary nerve runs on orbital floor

Question 5

What are the implications of a blow out fracture on the orbital contents

Answer 5

• Sudden increase in intra-orbital pressure fractures floor of orbit
  
  • Retropulsion (forceful backward movement) of eyeball by fist or ball
• Orbital contents and blood can prolapse into maxillary sinus
  
  • Fracture site can trap structures
  • Extra orbital muscle located near floor or orbit
• Eye cannot look up due to trapped contents - prevents movement

Question 6

State the connective tissue that form the eyelid

Answer 6

• Eyelid (palpebrae) formed from superior and inferior tarsus
  
  • Provide a connective tissue skeleton to the eyelid - firmness and shape
• Fibrous ring surrounding it - orbital septum

Question 7

Explain the role of fibrous ring in the eyelid

Answer 7

• Orbital septum
• Thin sheet of fibrous tissue originating from orbital rim
• Blends with tendon of LPS and tarsal plates
• Separates intra-orbital contents from eyelid fat and orbicularis oculi muscle
• Act as a barrier against infection spreading from the pre-septal space to post-septal space

Question 8

Explain the presentation and complications of periorbital cellulitis

Answer 8

• Secondary to infection from bites, periorbital trauma, sinuses (fronto-ethmoidal sinuses)
• Pre-septal orbital cellulitis - in front of orbital septum
  
  • Localized infection and swelling
• Complication include abscess formation and spread of infection intracranially
  
  • Cavernous sinus thrombosis - veins of orbit drain to cavernous sinus, pterygoid venous plexus and facial veins
• Post-septal orbital cellulitis - infection can spread into orbital cavity

Question 9

Describe the glands of the eyes and the pathology associated with each

Answer 9

• Meibomian glands and glands of Zeis located on the edge of eyelids
• Meibomian glands secrete oily substance onto edge of eye
  
  • Help prevent evaporation of tear film and tear spillage
  • Can block causing Meibomian cyst
• Glands of Zeis are eyelash follicles
  - Can block causing styes

Question 10

Outline the secretion and drainage of tears

Answer 10

• Lacrimal apparatus involved in secretion of tears into conjunctival sac
• Lacrimal gland - arranged around edge of levator palpebrae superioris
• Lacrimal punctum drains conjunctivae into the lacrimal sac and down the nasolacrimal duct
  
  • Lacrimal sac located within the lacrimal fossa

Question 11

Describe what conjunctivae is and how its tears affect it

Answer 11

• Conjunctivae is a secretory mucosa lubricating the conjunctival and corneal surfaces
• Blinking washes tear film across front of eye, rinsing and lubricating the conjuctivae and cornea

Question 12

Explain the conjunctiva

Answer 12

• Transparent mucous membrane that produces mucous and tears
• Covers white of eye (sclera) and lines inside of eyelids (forming a conjunctival sac)
  
  • Does not cover over cornea
• Highly vascular with small blood vessels within the membrane

Question 13

Outline pathology of conjunctiva

Answer 13

• Conjunctivitis - inflammation and infection
  
  • Pink eye
• Haemorrhage from blood vessel readily visible as a subconjunctival haemorrhage
  - No treatment needed - just a bruise without skin

Question 14

Describe the blood supply to and from the orbit

Answer 14

• Main arterial supply through ophthalmic artery branching off internal carotid artery
• Ophthalmic veins drain venous blood into cavernous sinus, pterygoid plexus and facial vein

Question 15

State the nerves which innervate the orbit

Answer 15

• General sensory from the eye through ophthalmic division of trigeminal nerve
• Special sensory vision from retina through optic nerve
• Motor nerves to muscle through occulomotor, trochlear and abducens

Question 16

Describe the layers of the eyeball

Answer 16

• Outer - fibrous, tough sclera (white of eye) continuous anteriorly as transparent cornea
• Middle - vascular consisting of choroid, ciliary body and iris
• Inner - retina (inner photosensitive layer lying on an outer pigmented layer)

Question 17

Describe the iris and lens

Answer 17

• Iris is a muscle that can dilate and constrict under autonomic control to help focus
• Lens are suspended by suspensory ligament to ciliary muscle
• Under parasympathetic control where contraction of muscle reduces tension of ligament and causes lens to become fatter

Question 18

Outline how light is perceived in the retina

Answer 18

• Light must reach the photosensitive retina to be detected by photoreceptors (rods and cones)
• Action potentials generated in response to light
• Pass via ganglion cells whose axons collect in area of optic disk to optic nerve
• Rods are for vision in low intensity light
• Cones are for vision in high intensity light and colour vision - concentrated in macula

Question 19

Describe refraction and where it occurs

Answer 19

• Refraction is the change in direction of light on passing through boundary of two different mediums
• Light will be refracted from air into liquid tear film
• Cornea is where most light refraction occurs (air-cornea interface)
• Refraction through lens and vitreous humour before reaches retina

Question 20

Explain the accommodation reflex

Answer 20

• Focusing near objects requires greater refraction of light
• Pupil constricts - increase focus so lens don’t have to refract as much light
• Eyes converge - image is brought to focus on same point of retina in both eyes
• Lens become more biconcave (fatter) - ciliary muscles contract

Question 21

Describe problems of lens

Answer 21

• As we age, the lens become stiffer and less able to change shape
  
  • Presbyopia - age related inability to focus near objects
• Progressive opacities can also occur within lens
  - Cataracts - non-transparent lens

Question 22

Explain what the macula and optic disk are

Answer 22

• Macula - area of the retina upon which an object in your visual field falls if you gaze direct at it (center of vision)
• Optic disk - accumulation of retinal axons that leave the eye as the optic nerve
  - No photoreceptors present (blind spot)

Question 23

Explain what is found on a retinal scan

Answer 23

• Macula is a dark spot and optic disk is a light spot
• Veins are thicker than arteries due to larger lumen
• Optic disk always medial to macula (to tell which eye)

Question 24

Levator palpebrae superioris action, innervation, damage presentation

Answer 24

• Action - elevates upper eyelid
• Innervation - CN III
• Damage - ptosis

Question 25

Palpebral part of orbicularis oculi action, innervation, damage presentation

Answer 25

• Action - blink/close eyes
• Nerve - temporal branch of CN VII
• Damage - can’t close eyes properly

Question 26

Superior tarsal muscle action, innervation, damage presentation

Answer 26

• Action - keeps upper eyelid elevated
• Innervation - sympathetic
• Damage - partial ptosis

Question 27

State the extra-ocular muscle actions and innervation

Answer 27

• Lateral rectus
  
  • Action - abduction of eye
  • Innervation - CN VI
• Medial rectus
  
  • Action - adduction of eye
  • Innervation - CN III
• Superior rectus
  
  • Action - elevation, intorsion, adduction of eye
  • Innervation - CN III
• Inferior rectus
  
  • Action - depression, extorsion, adduction of eye
  • Innervation - CN III
• Inferior oblique
  
  • Action - elevation, extorsion, abduction of eye
  • Innervation - CN III
• Superior oblique
  
  • Action - depression, intorsion, abduction of eye
  • Innervation - CN IV
• LR6SO4

Question 28

State what extra-ocular muscles are involved in looking left/right, up and down

Answer 28

• Looking left and right due to medial and lateral rectus muscles
• Looking upwards due to superior rectus and inferior oblique muscles
• Looking downwards due to inferior rectus and superior oblique

Question 29

How are lateral and medial rectus tested

Answer 29

Abduction and adduction of eye

Question 30

How are inferior rectus and superior oblique tested

Answer 30

• Superior oblique - move eye into medial position (looking towards nose) then move eyeball down
• Inferior rectus - move eye into lateral position then move eyeball down

Question 31

How are inferior oblique and superior rectus tested

Answer 31

• Inferior oblique - move eye into medial position then move eyeball up
• Superior rectus - move eye into lateral position then move eyeball up

Question 32

Describe the presentation of CN III damage

Answer 32

• Eyeball in down and out position
  
  • CN III innervates all muscles except lateral rectus and superior oblique
• Severe ptosis
  
  • CN III innervates levator palpebrae superioris
• Pupil dilation
  - CN III carries parasympathetic function to sphincter pupillae

Question 33

Describe the presentation of CN IV

Answer 33

• Eyeball is held extorted, and up and in
  
  • CN IV innervates superior oblique muscle
• Compensate for the slight extortion of eyeball by tilting the head slightly
• Difficulties when looking down and medially

Question 34

Describe the presentation of CN VI

Answer 34

• Unopposed pull of medial rectus muscle
  - Innervates lateral rectus
• Unable to abduct the eye on affected side
• Diplopia

Question 35

Describe the route of aqueous humour in the eye

Answer 35

• Aqueous humour secreted by ciliary processes within ciliary body
• Flows from posterior chamber through pupil into anterior chamber
  
  • Anterior chamber - between cornea and iris
  • Posterior chamber - between iris and lens
• Drains through iridocorneal angle - angle between iris and cornea
• Trabecular meshwork helps Schlemm’s canal drain the aqueous humour into the venous circulation

Question 36

Describe the role of aqueous humour

Answer 36

• Nourishes lens and cornea
• Lens and cornea are avascular to allow light to pass through
• Need aqueous humour to supply oxygen and nutrients

Question 37

Describe the type of glaucoma

Answer 37

• Drainage of aqueous humour from anterior chamber can be blocked
  
  • Trabecular meshwork deteriorates - old age
    
    • Open angle glaucoma
    • Blockage within the trabecular meshwork
  • Narrowing of iridocorneal angle - acute
    
    • Close angle glaucoma
    • Access to travecular meshwork is blocked off
• Rise in intra-ocular pressure and damage to optic nerve
  
  • Optic disk cupping
  • Visual field loss
• Sight threatening
• Treatment - topical medications (eye droplets) that reduces production of aqueous humour or increase its drainage

Question 38

Explain the principal of colour blindness

Answer 38

• Colour blindness - 3 types of cones (red, green, blue sensitive)
• Absence or dysfunction of one of these three cones leads to colour blindness

Question 39

Describe papiloedema

Answer 39

• Optic disk swelling due to increased intracranial pressure
• Usually occurs in both eyes
• Increased size of the blind spot means patients may lose vision briefly

Question 40

Describe blepharitis

Answer 40

• Inflammation of the eyelids
• Inflamed, irritated, itchy, and reddened eyelids
• Can be due to bacterial infection, skin disease, blockage of Meibomian glands

Question 41

Outline the steps for a clinical examination of the eye

Answer 41

• IVA FROM
• Inspect - general, eyelids, conjunctiva, sclera
• Visual Acuity
  
  • Snellen chart with one eye
  • 6/18 means that standing 6 meters away, the patient can only see the letter size that should be seen from 18 meters away in a person with normal vision
• Visual Fields
  
  • Move hand slowly towards the centre and see where the patient first sees your finger
• Reflexes - pupil light reflex, accommodation reflex
• Ophthalmoscopy
  
  • Dim lights and sit opposite the patient and shine light
    
    • Pupil should appear pink/red (retina)
  • Slowly move the light towards the eye at an angle to observe the optic disk on the medial side
  • Follow blood vessels out from the optic disk and check macula
• Movements of the eye
  - Keeping head still, follow fingers with your eye