H&N17 - Anatomy of the Eye & Orbit

Question 1

6 features of the orbital cavity

Shape
Borders (roof, floor, lateral and medial walls)
Contents x5
Blood Supply (arterial and venous drainage)
Nerve Innervation (GS, SS, motor)
Anatomical Relations x3

Answer 1

1.) Shape - pyramidal shaped with apex pointing posteriorly

2. ) Borders
   - roof: frontal bone and sphenoid (lesser wing)
   - floor: maxilla, palatine, zygomatic bones
   - medial wall: ethmoid, maxilla, lacrimal, sphenoid (body)
   - lateral wall: zygomatic bone and sphenoid (greater)

3.) Contents - eyeball (globe), extra-ocular muscles, fat, neurovasculature, lacrimal apparatus

4. ) Blood Supply - main arterial supply is the ophthalmic artery (ICA) and its subsequent branches
   - ophthalmic veins (superior and inferior) drain into the cavernous sinus, pterygoid plexus and facial vein
5. ) Nerve Innervation
   - general sensation: ophthalmic nerve (Va)
   - special sensation (vision): optic nerve
   - motor innervation: CN III, IV, VI
6. ) Anatomical Relations - paranasal sinuses (maxillary and ethmoid), nasal cavity, anterior cranial fossa
   - implications for orbital trauma and infection spread

Question 2

6 openings into the orbital cavity and their contents

4 Main
2 Others

Answer 2

1. ) Optic Canal - optic nerve and ophthalmic artery
2. ) Superior Orbital Fissure - CN III, IV, Va, VI
3. ) Inferior Orbital Fissure - maxillary nerve (Vb), inferior ophthalmic vein, sympathetic nerves
4. ) Nasolacrimal Canal - medial wall of the orbit
   - drains tears from the eye to the nasal cavity
5. ) Others - supraorbital foramen and infraorbital canal
   - carry small neurovascular structures

Question 3

4 features of orbital blow out fractures

Location
Cause
Complications x2
Signs x4

Answer 3

1.) Location - floor of the orbit and medial is the weakest part of the orbital cavity so most susceptible to fractures

2. ) Cause - sudden increase in intra-orbital pressure
   - often retropulsion of the globe by fist or a ball
3. ) Complications - orbital contents can prolapse and bleed into the maxillary or ethmoidal sinus
   - fracture site can trap structures near orbital floor e.g. extra-ocular muscles or soft tissue
4. ) Signs - periorbital swelling and pain
   - can’t fully elevate eyeball (inferior rectus entrapment)
   - diplopia worse on vertical gaze
   - maxillary nerve damage causes numbness over cheek, lower eyelid, and upper lip on affected side

Question 4

5 features of the eyelids

Function
Contents x5
Meibomian Glands (and related pathology)
Sebaceous Glands (and related pathology)
Blepharitis

Answer 4

1.) Function - protects the front of the eye

2. ) Contents - skin, SC tissue, tarsal plate, glands
   - muscles: orbicularis oculi (palpebral part) and LPS
3. ) Meibomian Glands - secrete oily substance onto lid edges to prevent evaporation of tear film and spillage
   - sit within the tarsal plate
   - if blocked, produces a Meibomian cyst (painless)
4. ) Sebaceous Glands - associated w/ eyelash follicle
   - if blocked during infection causes a stye
5. ) Blepharitis - inflammation of the eyelids
   - includes skin, lashes and Meibomian glands

Question 5

What is the orbital septum and its function

2 types of cellulitis within the eye

Answer 5

1.) What is it? - thin fibrous sheet originating from the orbital rim periosteum, blends w/ the tarsal plates

2. ) Function - separates SC tissue and orbicularis oculi muscle from intra-orbital contents
   - prevents deeper spread of superficial infections
3. ) Periorbital (pre-septal) Cellulitis - infection within the eyelid tissue superficial to the orbital septum
   - secondary to superfical infections e.g. bites, wounds, or bacterial sinusitis in children
   - ocular function is unaffected
4. ) Orbital (post-septal) Cellulitis - infection within the orbit, posterior to the orbital septum
   - symptoms: exopthalmous, reduced +/- painful eye movements, reduced visual acuity (optic nerve affected)
   - complication: can spread intracranially (venous drainage) –> cavernous sinus thrombosis or meningitis

Question 6

3 features of the lacrimal apparatus

Tear Production
Tear Drainage
Effect of Blinking

Answer 6

1. ) Tear Production - by the lacrimal gland
   - sits lateral and above the eye
2. ) Tear Drainage - drained into the nasal cavity
   - lacrimal ducts are canaliculi and nasolacrimal duct
   - obstruction leads to epiphora (overflow of tears over the lower eyelid)
3. ) Blinking - orbicularis oculi (palpebral part)
   - distributes tear film across the front of the eye, rinsing and lubricating conjunctiva and cornea

Question 7

3 features of the anatomy of the eyeball

3 Layers
Maintaining Eyeball Position x3
Conjunctiva

Answer 7

1. ) Layers
   - outer: sclera which is continuous anteriorly as the cornea and posteriorly as dura mater covering the CN II
   - middle - iris, ciliary body, and choroid
   - inner: retina
2. ) Eyeball Position - maintained by:
   - suspensory ligament, extra-ocular muscles, orbital fat
3. ) Conjunctiva - transparent mucous membrane producing the mucous component of tear film
   - covers the sclera but doesn’t cover the cornea
   - subconjunctival haemorrhage is readily visible but is often painless and self limiting

Question 8

3 features of focusing light onto the back of the eye (macula)

Transparent Structures
Refractive Structures
Eyeball Shape

Answer 8

1. ) Transparent Structures - so light can pass through
   - lens and cornea are avascular in order to be transparent so they get their nutrients from aq. humour
2. ) Refractive Structures - bend light to a focal point
   - cornea (main), lens, aqueous and vitreous humour
3. ) Eyeball Shape - affects ability to see
   - short-sighted: myopia (eyeball too long)
   - long-sighted: hypermetropia (eyeball too short)

Question 9

What is the accommodation reflex and why is it needed?

3 ways the eye ‘accommodates’

Presbyopia

Answer 9

What is it? - reflex action of the eye, in response to focusing on a near object

• light rays from near objects are more divergent so more refraction is required
• cornea is fixed in shape so can’t bring into focus

1. ) Pupils Constrict - limits amount of light entering
2. ) Eyes Converge - ensures image remains focused on same point of retina in both eyes
3. ) Lens Becomes Fatter - becomes more biconvex
   - caused by contraction of the ciliary muscle relaxing the suspensory ligament

Presbyopia - age-related inability to focus near objects
- as you get older, the lens become stiffer

Question 10

4 features of photoreceptors

Rods
Cones
Fovea
Optic Disc

Answer 10

1. ) Rods - see in black and white, active in low-light
   - abundant in peripheral parts of the retina
2. ) Cones - see in colour, active in high light levels
   - concentrated within the macula of the retina

3.) Fovea - region in retina only containing cones

4. ) Optic Disc - where retinal ganglion cells collect to go on to form the optic nerve
   - ‘blind spot’ because it has no photoreceptors

Question 11

blurred vision (decreased acuity) is caused by pathologies affecting ….

3 things

Answer 11

1. ) Transparency - of structures anterior to the retina
   - e.g. cataracts causes the lens to be more opaque
2. ) Refraction - ability of structures to refract light
   - astigmatism: irregularity of corneal surface
   - shape of eyeball or presbyopia
   - refractive error has no effect on light travelling perpendicular to the cornea (pin-hole testing)

3.) Retina/Optic Nerve - e.g. retinal detachment, age-related macular degeneration, optic neuritis

Question 12

4 features of aqueous humour

Production
Function
Flow
Drainage

Answer 12

1.) Production - by ciliary processes in the ciliary body

2. ) Function - nourishes the lens and cornea
   - lens and cornea are avascular to be transparent

3.) Flow - from posterior chamber (bathing the lens), through pupil into anterior chamber (cornea)

4. ) Drainage - through iridocorneal angle
   - via trabecular meshwork into canal of Schlemm

Question 13

4 features of glaucoma

Definition
Cause
Chronic
Acute

Answer 13

1. ) Definition - optic nerve damage secondary to raised intraocular pressure
2. ) Cause - blocked drainage of aqeuous humour from the anterior chamber
3. ) Chronic - open angle glaucoma (most common)
   - trabecular meshwork deteriorates with age
   - increased IOP –> optic disc cupping –> gradual loss of peripheral vision
4. ) Acute - closed angle glaucoma (emergency)
   - narrowing of iridocorneal angle
   - occurs in older patients (55+, 70-80s most common)
   - symptoms: painful red eye, oval shaped pupil, blurred vision, halo around eyes, nausea and vomiting
   - treament: drugs to reduce IOP then surgery