16 - Anatomy of the Eye Flashcards
What are the walls of the orbital cavity?
- Pyramid shaped
Which parts of the orbit are vulnerable to fracture and why?
- Impact to front of eye, e.g fist or ball, medial and inferior wals weakest so most susceptible
- Sudden increase in intraorbital pressure can lead to orbital blow out fracture of floor
Why is the medial wall of the orbit slightly stronger than the floor, but what are the implications of this?
- Ethmoid bone very thin but ethmoidal air cells add strength
- Air cells can become infected, break through thin lamina papyracea and cause orbital cellulitis
What are the nerves and blood vessels that run through the orbit?
- Optic canal: opthalmic artery and optic nerve
- Superior Orbital Fissure: CN II, VI, Va and superior opthalmic vein back to cavernous sinus
- Inferior Orbital Fissure: infraorbital nerve (Vb) and inferior opthalmic vein
What are some of the clinical presentations of an orbital blow out fracture?
- History of trauma to eye
- Periorbital swelling
- Double vision, worse on looking up
- Eye cannot gaze up
- Numbness over cheek, lower eyelid, upper lip on affected side (infraorbital nerve)
What is the stucture of the eyelids?
- Tarsal plate
- Skin, muscles (OO, LPS)
- Glands (meibomian and sebaceous on lash follicle)
What are the following pathologies of the eyelid?
Meibomian Cyst: oily substance that stops evaporation of tears gets blocked. Self limiting, not painful, not on edge
Stye: painful, usually lash edge as infected lash follicles sebaceous glands
Blepharitis: inflammation of lids including skin, lashes and meibomian glands
All need good eye hygeine
How does the anatomy of the orbit mean that superficial infetions don’t spread backwards?
- Orbital septum and tarsal plates separate subcut tissue and muscles from intra orbital contents
What is peri-orbital cellulitis?
- Due to superficial infection, e.g bite, wound, bacterial sinusitis
- Confined in front of orbital septum
- Oculomotor movements unaffected
What is orbital cellulitis?
- Infection within the orbit affecting the muscles or the eye itself
- Could spread intracranially via orbital veins
- Need emergency IV and surgical drainage
Label the different parts of the lacrimal apparatus and what is its function?
- Lacrimal gland produces tears and is under PS control of facial nerve
- Lacrimal fluid goes over conjunctival sac and passes to lacrimal lake at medial angle of the eye
- Then goes to lacrimal sac and nasolacrimal duct into nasal cavity
- Epiphora if blockage
What are the different layers of the eyeball?
- Outer: tough fibrous sclera with transparent cornea anteriorly. this layer is continuous posteriorly with dura mater covering optic nerve. Thin transparent layer called conjunctivae covers sclera up to cornea
- Middle: choroid which goes anteriorly as iris, ciliary body
- Inner: retina
What is the diagnosis and treatment for both of these?
- A: conjunctivitis, usually viral and due to blood vessels dilating. highly contagious. eye can feel gritty and uncomfortable
Hygeine and chloramphenicol eye drops
- B: subconjuctival haemorraghe. blood vessel ruptures but not painful. like a bruise but takes longer to heal
What may be a cause of conjunctivitis in the neonate and how would we treat it?
- Chlamydial infection from mother
- Systemic antibiotics like erythromycin
Label the following parts of the eye and shade the different chambers of the eye.
What is uveitis?
- Acute pain with red eye and painful to look at bright lights
- Inflammation of choroid layer (uvea = choroid, ciliary process and iris)
- Associated with autoimmune conditions like IBS and needs immediate referal to opthalmology
What are the layers of the retina?
- Pigmented epithelial cell layer lies between choroid and neurosensory layer. It contains melanin to absorb light and prevent reflection
Where are the different photoreceptors in the retina?
All in neurosensory layer and convert light into electrical impulses
- Cones (R,G and B responsible for colour blindness): high definition colour in the fovea of the macula, lateral to optic disc
- Rods: Low intensity light but no colour in peripheral retina
Where are the anterior and posterior chambers of the eye and where does the aqueous humour that nourishes them come from?
Anterior: between cornea and iris and communicates with posterior through pupil
Posterior: between iris and lens
Ciliary body and processes secrete aqueous humour to support shape of eyeball and provide nourishment to lens and cornea. Drains through to irido-corneal angle into the canal of Schlemm via trabecular meshwork back into venous circulation
What is glaucoma?
Rise in intra-ocular pressure due to blockage of drainage of aqueous humour, causes irreversible death and damage of optic nerve
Blindness or impairement of vision, open angle or closed angle
How can we screen for signs of glaucoma?
- Tonometry to measure IOP
- Check for visual field loss, especially peripheral
- Check for cupping of optic disc
How do we treat signs of raised intra-ocular pressure to stop glaucoma from occuring?
- Topical eye drops to stop production of aqueous humour, e.g beta blockers like timolol, or increase its drainage
- Surgical trabeculectomy
What is open angle glaucoma?
- Most common
- Trabecular meshwork blockage as it deteriorates with age
- Usually painless and develops overtime but can screen at eye test
- Gradual loss of peripheral vision and optic disc cupping
What is closed angle glaucoma?
- Narrowing of irido-corneal angle
- Rapid rise in IOP so sudden onset painful red eye, blurred vision, halos due to corneal oedema
- Eye is hard and tender to palpate
- Sight threatening* in few hour so rapid recognition and treatment
How can we treat acute closed angle glaucoma once we have recognised it?
- Diuretics to reduce humour production
- Muscarinic eye drops like pilocarpine to constrict pupil and open angle
- Strong analgesia
- Surgical
Long sighted middle aged people or elderly people with shallow anterior chambers at risk
Where is the lens of the eye?
Behind the iris, attached to the ciliary body by suspensory ligaments
No nerve innervation or blood supply but ciliary body under PS control alters suspensory ligaments to change shape of lens and its refractive power. Body relax, ligaments are taut keeping lens flat
How do we get light to focus on the retina?
- Need transparent structures and to refract light
- Main refractor is cornea and conjuctiva, also some refraction from aqueous/vitreous humour and the lens
- Refraction also based on shape of eye e.g myopia and hypermetropia
What is the accomodation reflex?
Focusing near objects requires greater refraction of light
- Contraction of the pupils
- Convergence of the eyes
- Thickening of the lens to become biconvex by contracting ciliary muscle so suspensory ligaments loosen
Why is our accomodation reflex impaired as we get older?
Lens becomes dense and less elastic so more difficult to change shape and refract light, presbyopia
Can be corrected with lenses
What are some differentials for blurred vision?
- Cataracts (degradation of proteins in lens)
- Inability to refract light e.g astigmatism, shape of eyeball
- Retinal detachment
- Age related macular degeneration
- Optic neuritis
How can you decide whether someone has blurred vision due to decreased visual acuity or a refractive error?
- Get patient to make a pinhole and look through, this only allows perpendicular light through so if refractive issue vision will improve as no light needs to be refracted
