L14: applied anatomy of the orbit and eye

Question 1

Describe the orbit

Answer 1

Pyramidal shaped bony cavity which contains the eyeball, its muscles, nerves, vessels & most of the lacrimal apparatus
4 walls
Ethmoid bone contributes to medial wall & maxillary bone contributes to floor
Floor of the orbit and medial wall are the weakest parts of the orbital cavity

Question 2

Describe anatomical relations of the orbit

Answer 2

1) Anterior cranial fossa
2) Ethmoidal air cells (air sinuses) – convey an added strength to the medial wall
3) Maxillary air sinus
4) Nasal cavity

Question 3

Describe the pathophysiology of orbital cellulitis

Answer 3

Air cells (Ethmoidal) can become infected and this can break through the thin lamina papyracea (thin part of the ethmoid bone which forms medial wall of orbital cavity) & track into the orbit

Question 4

Describe an orbital blowout fracture

Answer 4

Inferior and medial walls of the orbit are the weakest -> vulnerable to fracture when there is direct impact to front of the eye
Sudden increase in intraorbital pressure (usually floor that is fractured)
Orbital contents prolapse & bleeding max. sinus, soft tissue & muscles near orbital floor can ‘trap’ in fracture site
Entrapment prevents upward gaze

Question 5

Describe the management of orbital blowout fractures

Answer 5

CT orbit & referral to ophthalmology
Antibiotics
Follow up in 1 week, surgical repair 1-2 weeks post injury if symptoms persist

Question 6

List the series of openings at the orbital apex

Answer 6

1) Optic canal – optic nerve & ophthalmic artery
2) Superior orbital fissure – CN III’s branches, IV, VI, Va & superior ophthalmic vein
3) Inferior orbital fissure – infraorbital nerve & inferior ophthalmic vein

Question 7

Describe the blood supply to the eye

Answer 7

Branches of ophthalmic artery supplies eye structures
Retina supplied by central retinal artery & draws supply from underlying choroid layer
Ciliary arteries feed extensive capillary bed within choroid layer
Retina requires both circulations to function properly

Question 8

Describe the eyelids

Answer 8

Protects the front of the eye
Consist of skin, subcutaneous tissue, muscles & tarsal plate
Muscles running within the eyelid – orbicularis oculi (palpebral part): closes eye lid & levator palpebrae superioris (retracts eye lid)

Question 9

List glands within the eyelids

Answer 9

Meibomian glands within tarsal plate – modified sebaceous, provide lipid layer of tear film & prevent tear evaporation and spillage over lid
Glands associated with lash follicle – sebaceous (oily substance)
Blockage of a gland can cause a lump within the eyelid

Question 10

Describe a stye

Answer 10

Outer part of lid
Painful
Red with a white punctum 
Infected (staphylococcus)
Treatment: warm compress +/- oral abx

Question 11

Describe a Meibomian cyst

Answer 11

Deeper within lid
Painless 
Firm lump palpable – enlarges gradually 
Blocked duct (not infected)
1/3 resolve spontaneously, surgical incision if persists

Question 12

Describe blepharitis

Answer 12

Inflammation of eye lid margin
Causes: staphylococcus, meibomian gland dysfunction
Crusting, dry eyelids +/- swollen + red
Not serious
Treatment: warm compress & lid hygiene

Question 13

Describe the orbital septum

Answer 13

Thin fibrous sheet originating from orbital rim
Separates intra-orbital contents from muscle & subcutaneous tissue of eyelid
Barrier against infection from the superficial eyelid region (pre-septal) into the orbital cavity proper (post-septal)

Question 14

Describe periorbital (pre-septal) cellulitis

Answer 14

Infection confined to skin & tissues of eyelid, superficial to orbital septum
Secondary to superficial infections, painful
Ocular function remains UNAFFECTED
If any doubt, refer

Question 15

Describe orbital (post-septal) cellulitis

Answer 15

Infection within the orbit posterior/deep to the orbital septum (spread of infection from paranasal air sinuses - specifically Ethmoidal sinus)
Proptosis/exophthalmos (bulging eyeballs)
Reduced +/- painful eye movements
Reduced visual acuity
Ophthalmic veins drain to cavernous sinus – potential route for infection to spread intracranially -> cavernous sinus thrombosis & meningitis

Question 16

List the contents of the orbital cavity

Answer 16

Nerves
Blood vessels
Fat
Lacrimal apparatus
Eyeball
Extra-ocular muscles

Question 17

Describe tear film & lacrimal apparatus

Answer 17

Tear film consists of three layers (oil, water, mucus) -> meibomian glands (oily), lacrimal gland (water) & goblet cells in conjunctiva (mucus)
Blinking distributes tear film across surface of eye
Lacrimal apparatus -> series of structures that collect & drain tear fluid
Obstruction to drainage causes epiphora (overflow of tears over lower eyelid)

Question 18

How is the eyeball maintained in position?

Answer 18

Suspensory ligament
Extra-ocular muscles
Orbital fat

Question 19

Describe conjunctivitis

Answer 19

Uncomfortable
Watery +/- discharge
Infectious (typically viral)
Contagious & self-limiting (don’t give abx)

Question 20

Describe subconjunctival haemorrhage

Answer 20

Red eye – small conjunctival blood vessel ruptures
Painless
No other symptoms
Often no cause: spontaneous

Question 21

Describe the three layers of the eyeball

Answer 21

Outer: sclera (continuous as cornea anteriorly)
Middle: choroid (vascular): continuous with ciliary body and iris = uveal tract
Inner: retina (photosensitive layer)

Question 22

Describe the retina

Answer 22

Photosensitive and non-photosensitive parts
Neurosensory cell layer – senses light & where the photoreceptors are found (rods & cones)
-cones: high visual acuity & colour vision and many are concentrated in an area called the macula
-rods: vision in low intensity light & do not discern colours
Pigmented cell layer – lies between the choroid & neurosensory layer of the retina & contains melanin -> helps to absorb scattered light, reducing reflection & allows us to focus

Question 23

Describe central retinal artery occlusion

Answer 23

Sudden painless loss of sight in one eye, developing over seconds 
Pale retina (ischaemia), ‘cherry red spot’ = macula 
Underlying choroidal layer, blood supply unaffected = remains perfused

Question 24

Describe production and drainage of aqueous humour

Answer 24

Secreted by ciliary processes within the ciliary body
Flows from posterior chamber, through pupil into anterior chamber
Nourishes lens and cornea
Drains through the iridocorneal angle (between iris and cornea) via trabecular meshwork into canal of Schlemm

Question 25

Describe glaucoma

Answer 25

Optic nerve damage secondary to raised intraocular pressure
Drainage of aqueous humour from anterior chamber blocked causing rise in intra-ocular pressure
Sight-threatening

Question 26

Describe chronic glaucoma

Answer 26

Open-angle glaucoma
Trabecular meshwork deteriorates as age
Many asymptomatic (picked up on routine eye checks)
Increased IOP -> increased optic disc cupping
Gradual loss of peripheral vision

Question 27

Describe acute glaucoma

Answer 27

Close-angle glaucoma
Narrowing of iridocorneal angle
Ophthalmological emergency – acutely painful red eye, irregular oval-shaped pupil, blurring of vision, nausea & vomiting
Treatment: medical (drugs to reduce IOP) then surgical treatment

Question 28

Describe how light reaches and focuses into the macula

Answer 28

Transparent
Pupil – regulates light entry
Tear film, cornea, lens – refract light to bring into focus
Shape of eyeball – too long (short sighted: myopic) or too short (long-sighted: hypermetropic)

Question 29

Describe the accommodation reflex

Answer 29

Light from near-objects more divergent – greater refraction required to focus onto retina
Eye accommodates:
1) Pupil constricts
2) Eyes converge
3) Lens becomes more biconvex by contraction of ciliary muscle
Lens become stiffer by age -> presbyopia: age-related inability to focus near objects

Question 30

Describe phototransduction

Answer 30

Photoreceptors (rods & cones) convert light signals into action potentials
Action potentials propagated via retinal ganglion cells
RGC axons collect in area of optic disc (no photoreceptors = blind spot) forming the optic nerve
APs propagated along visual pathway to occipital lobe for interpretation

Question 31

How is visual acuity measured?

Answer 31

Snellen chart
Read set of letters increasingly smaller size – one eye at a time
Normal vision = 6/6

Question 32

List causes of decreased visual acuity

Answer 32

Transparency of structures anterior to retina – cataract
Refractive ability of structures anterior to retina – astigmatism, presbyopia, shape of eyeball
Retina (including macula) or optic nerve – age-related macular degermation, optic neuritis

Question 33

Describe how to test if transparency of structures is affected

Answer 33

Check for ‘red reflex’ (use ophthalmoscope)

Absent suggests light is prevented from reaching retina & reflecting back

Question 34

Describe how to differentiate between a refractive error and non-refractive error

Answer 34

Use of pin hole: only allows light to enter directly perpendicular to cornea and lens
Light does not need to be refracted to be brought into focus on macula – removes need to refract
If acuity does not improve -> problem at level of retina or optic nerve