L14: applied anatomy of the orbit and eye Flashcards
Describe the orbit
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
Describe anatomical relations of the orbit
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
Describe the pathophysiology of orbital cellulitis
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
Describe an orbital blowout fracture
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
Describe the management of orbital blowout fractures
CT orbit & referral to ophthalmology
Antibiotics
Follow up in 1 week, surgical repair 1-2 weeks post injury if symptoms persist
List the series of openings at the orbital apex
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
Describe the blood supply to the eye
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
Describe the eyelids
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)
List glands within the eyelids
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
Describe a stye
Outer part of lid Painful Red with a white punctum Infected (staphylococcus) Treatment: warm compress +/- oral abx
Describe a Meibomian cyst
Deeper within lid Painless Firm lump palpable – enlarges gradually Blocked duct (not infected) 1/3 resolve spontaneously, surgical incision if persists
Describe blepharitis
Inflammation of eye lid margin Causes: staphylococcus, meibomian gland dysfunction Crusting, dry eyelids +/- swollen + red Not serious Treatment: warm compress & lid hygiene
Describe the orbital septum
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)
Describe periorbital (pre-septal) cellulitis
Infection confined to skin & tissues of eyelid, superficial to orbital septum
Secondary to superficial infections, painful
Ocular function remains UNAFFECTED
If any doubt, refer
Describe orbital (post-septal) cellulitis
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
List the contents of the orbital cavity
Nerves Blood vessels Fat Lacrimal apparatus Eyeball Extra-ocular muscles
Describe tear film & lacrimal apparatus
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)
How is the eyeball maintained in position?
Suspensory ligament
Extra-ocular muscles
Orbital fat
Describe conjunctivitis
Uncomfortable
Watery +/- discharge
Infectious (typically viral)
Contagious & self-limiting (don’t give abx)
Describe subconjunctival haemorrhage
Red eye – small conjunctival blood vessel ruptures
Painless
No other symptoms
Often no cause: spontaneous
Describe the three layers of the eyeball
Outer: sclera (continuous as cornea anteriorly)
Middle: choroid (vascular): continuous with ciliary body and iris = uveal tract
Inner: retina (photosensitive layer)
Describe the retina
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
Describe central retinal artery occlusion
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
Describe production and drainage of aqueous humour
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
Describe glaucoma
Optic nerve damage secondary to raised intraocular pressure
Drainage of aqueous humour from anterior chamber blocked causing rise in intra-ocular pressure
Sight-threatening
Describe chronic glaucoma
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
Describe acute glaucoma
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
Describe how light reaches and focuses into the macula
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)
Describe the accommodation reflex
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
Describe phototransduction
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
How is visual acuity measured?
Snellen chart
Read set of letters increasingly smaller size – one eye at a time
Normal vision = 6/6
List causes of decreased visual acuity
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
Describe how to test if transparency of structures is affected
Check for ‘red reflex’ (use ophthalmoscope)
Absent suggests light is prevented from reaching retina & reflecting back
Describe how to differentiate between a refractive error and non-refractive error
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
