Session 9: Functional Anatomy of the Orbit and Eye Flashcards
Main arterial supply of the eye and its orbit.
Ophthalmic artery and subsequent branches (retinal arteries)
Main venous drainage of the eye and its orbit.
Ophthalmic veins (superior and inferior),
What do the ophthalmic veins drain venous blood into?
Cavernous sinus
Pterygoid plexus
Facial vein
General sensory from the eye (including conjunctive and cornea).
Ophthalmic branch of the trigeminal nerve (CN Va)
Special sensory vision from retina.
Optic nerve
Motor nerves to muscles of the eye and orbit.
CN III
CN IV
CN VI
What parts of the orbit are the weakest and most susceptible to damage?
The inferior part (floor) which mainly comprise of the maxilla and the medial walls which mainly comprise of the ethmoid bone.
What is thinner; the medial wall or the floor?
The medial wall, even if the medial wall is thinner the floor is more susceptible to damage.
How come even if the medial wall is thinner (lamina papyracea) the floor is still more susceptible to damage?
Because the ethmoidal sinuses (air cells) act as buttresses and convey an added strength to the medial wall.
Give examples of injuries that is common to cause a fracture of the floor or medial wall of the orbit.
Direct impact to the front of the eye, e.g. by a fist or a ball.
This leads to a sudden increase in infraorbital pressure.
What is a fracture called that involves the floor of the orbit?
An orbital blow-out fracture.
More than making the orbit susceptible to trauma, what other complications might ensue due to the thin medial and inferior walls?
In acute sinusitis of the ethmoidal air cells the infection can break through the medial wall and cause orbital cellulitis.
Explain Orbital blow out fractures.
Sudden increase in intra-orbital pressure leads to fractures of the floor of the orbit (maxilla)
What happens in the event of a orbital blow out fracture to the orbital contents?
The orbital contents can prolapse and bleed into the maxillary sinus.
The fracture site can also trap structures like extra ocular muscles that are located near the orbital floor.
What will trapping of an extra ocular muscle cause?
Prevents an upward gaze on the affected side.
Common clinical presentation of an orbital blow out fracture.
Periorbital swelling
Pain
Double vision (diplopia) that worsens on a vertical gaze.
Numbness over cheek, lower eyelid and upper lip as well as upper teeth and gums on affected side.
Why might you get numbness over cheek etc… in an orbital blow out fracture?
Can cause damage to the infraorbital nerve as it runs through the infraorbital fracture.
What protects the front of the eye?
Skin, subcutaneous tissue, tarsal plate and muscles.
The orbicularis oculi (palpabral part) as well as the levator palpebrae helps close and open the eyelids respectively.
Glands like meobomian glands and sebaceous.
Function of the tarsal plates.
Eyelids are strengthened and given their shape by tarsal plates.
Functions of the meibomian glands.
Oily secretions from the glands lubricate the edge of the eyelids.
This secretion mix with the tear film and prevents the tears from evaporating too quickly.
What else adds secretion to the tear film?
The lacrimal apparatus.
What does the lacrimal apparatus consist of?
The lacrimal gland, the lacrimal ducts and the lacrimal canaliculi.
Where can the lacrimal glands be found?
In a fossa called the lacrimal fossa on the superolateral part of the orbit.
Explain the route of the lacrimal fluid.
Secreted by the lacrimal gland. Enters the conjunctival duct through the lacrimal ducts and into the lacrimal lake.
The fluid drains into the lacrimal sac and then into the nasal cavity via the nasolacrimal duct.
Explain the function of blinking in protecting the eye.
The eye blinks when the cornea becomes dry. This leads to a film of fluid covering the cornea.
This also sweeps any dust and other foreign material to the medial angle of the eye.
What is a stye?
Infected hair follicle or sebaceous gland. This can be blocked.
Usually a staphylococcus infection.
What is a meibomian cyst?
Blockage of the meibomian glands leading to a cyst.
Meibomian cysts are found deeper than styes.
What is blepharitis?
Inflammation of the eye lid.
This includes the skin, the lashes and the meibomian glands.
What is the orbital septum?
A thin sheet of fibrous tissue that originates from the orbital rim periosteum and then blends with the tarsal plates.
Function of the orbital septum.
Along with the tarsal plates they separate the subcutaneous tissue of the eyelid and orbicularis oculi muscle from the intra-orbital contents.
It acts as a barrier against superficial infection spreading from pre-septal to post-septal space.
What is periorbital cellulitis?
Also called pre-septal cellulitis.
An infection occurring within the eyelid tissue and superficial to the orbital septum.
Usual cause of periorbital cellulitis.
Secondary to superficial infections like from bites and wounds. Can also be secondary to bacterial sinusitis in children.
Clinical presentation of periorbital cellulitis.
Red around the eye
Pain
Swelling
Ocular functions like eye movements and vision remains unaffected.
It can be difficult to differentiate between peri-orbital cellulitis and more severe orbital ones.
What is orbital cellulitis?
Also called post-septal cellulitis.
This is an infection within the orbit posterior or deep to the orbital septum.
Clinical presentation of orbital cellulitis.
Proptosis/exopthalmous
Reduced and painful eye movement.
Reduced visiual acuity.
Complications of orbital cellulitis.
If not treated early it can lead to visual impairness.
Also as the orbital veins drain to the cavernous sinus, pterygoid venous plexus and the facial veins the infection can spread intracranially and lead to cavernous sinus thrombosis or meningitis.
What is epiphora?
Obstruction of the drainage of lacrimal tears.
This is overflow of tears over lower eyelid
Three layers of the eyeball.
Outer
Middle
inner
What does the outer layer contain?
Sclera and the transparent cornea which is a continuation of the sclera anteriorly.
What does the middle layer contain?
Choroid
Ciliary body
Iris
This layer is highly vascular
What does the inner layer contain?
The retina
Contains inner photosensitive layer and a outer pigmented layer.
What is the eyeball’s position maintained by?
Suspensory ligament
Extra-ocular muscles
Orbital fat ++
What is the anterior surface of the eyeball covered by?
A conjunctival membrane
What part of the anterior eyeball is not covered by the conjunctival membrane?
The cornea
What is the role of the conjunctiva?
A transparent mucous membrane that produces mucous component of the tear film.
It is highly vascular with small blood vessels.
What is the limbus?
The junction of conjunctivae with the cornea.
What is conjunctivitis?
An inflammation of the conjunctiva.
Usually of viral aetiology and highly contagious
What is this?
A subconjunctival haemorrhage
Also called red eye
Clinical presentation of conjunctivitis.
Uncomfortable eye
Gritty eye
Tearing of the eye
Treatment of conjunctivitis.
Reassurance
Hygiene advice
Topical eye drops
What causes conjunctivitis in the neonatal period?
Vertical transmission of chlamydia
Treated by erythromycin
Treatment of subconjunctival haemorrhage.
Not painful
Reassurance
Usually resolve without further treatment
If a patient present with red eye and acute eye pain.
What might this be?
E.g. uveitis.
What is uveitis?
An inflammation of the choroid layer (uvea is the collective term for the choroid, ciliary process and the iris).
Clinical presenation of uveitis.
Red and painful eye
Pain often worse at trying to focus or looking at bright lights.
There are two types of cells in the retina, which?
Neurosensory cells or pigmented epithelial cells.
Where can the pigmented epithelial cell layer be found and what does it contain?
Can be found between the choroid and the neurosensory layer of the retina.
It contains melanin.
Purpose of the melanin in the eye.
Absorbs scattered light
Reducing reflection and allowing a focused image appropriate for the retina.
Function of the neurosensory layer.
Sensing light and turning it into action potential.
What are the two types of photoreceptors?
Cones and rods
Function of the cones.
High visual acuity and colour vision.
What is the macula?
An area of the retina where a lot of the cones are highly concentrated.
What is the fovea?
The very centre of the macula where only cones can be found.
What are the three types of cones?
Red, green and blue sensitive ones.
Function of rods.
Responsible for vision in low intensity light like night vision.
They do not discern colours.
They are more abundant in numbers twoards the peripheral parts.
What is needed for light to reach the macula?
Transparent structures and medium.
Also need to refract light to bring the light to a focal point.
What refracts light?
Cornea and its associated tear film
The lens
Aqueous humour and vitreous humour
What else effects ability fo cous light appropriately onto the retina?
The shape of the eyeball
What is myopia?
Short-sightedness
Can occur if the eyeball is too long
What is hypermetropia?
Long-sighted
What is the accomodation reflex and why is it important?
When you focus on an object that requires greater refraction of light beyond capabilities of the cornea to bring focus on the retina.
Explain the accomodation reflex.
Pupil constriction to limit light coming through
Converging of the eyes to ensure the image remains focused on the same point.
The ciliary muscle contracts, this makes the suspensory ligament becoming less taught and fattens the lens.
What causes presbyopia?
Age-related inability to focus on near-objects because of the lens becoming stiffer.
Explain how an image is produced.
Light innervates the cones. This causes an actional potential to be generated and pass via retinal ganglion cells.
The RGC axons the collect at the optic disc to form the optic nerve.
The AP then goes via the visual pathway to the occipital lobe for interpretation.
What is the blind spot of the eye?
The optic disc where there are no photoreceptors.
Give causes of blurring of vision.
Cataracts
Astigmatism
Presbyopia
Shape of eyeball
Retinal detachment
Age-related macular degeneration
Optic neuritis
What is cataracts?
Opacity of the lens. Causing less transparent structures of light to reach the retina.
What is astigmatism?
Irregularity of the corneal surface
What is the most common cause of adult blindness in the UK?
Age-related macular degeneration
How can you test if a decreased visual acuity is due to a refractive error?
By pin-hole testing.
If there are no refractions and just light from a pin-hole then the acuity should improve.
The eye has three chambers.
Which?
Anterior chamber
Posterior chamber
Vitreous chamber
Where is the anterior chamber?
Between the cornea and iris
Where is the posterior chamber?
Between the iris and the lens
Where is the vitreous chamber?
Posterior to the lens within the retina
What can be found in the posterior chamber?
The ciliary body and ciliary processes.
What fluid can be found in the anterior and posterior chamber?
Aqueous humour
What fluid can be found in the vitreous chamber?
Vitreous humour
What is the aqueous humour secreted by?
The ciliary process within the ciliary body.
Explain the route of the aqueous humour.
From ciliary processes to the posterior chamber.
It then enters the anterior chamber via the pupil.
Then drains through the iridocorneal angle via the trabecular meshwork into canal of Schlemm and then eventually intot the venous circulation.
Functions of the aqueous humour.
Nourishment to the lens and cornea. This is important because the lens and cornea are avascular.
Also supports the shape of the eyeball and to maintain the intra-ocular pressure.
What happens if the drainage of the aqueous humour is obstructed?
It can lead to a rise in intra-ocular pressure.
What can an untreated rising or elevated IOP lead to?
Glaucoma and irreversible damage and death to optic nerve.
What is glaucoma?
Optic nerve damage that is secondary to raised IOP.
This is because of blocked drainage of aqueous humour from anterior chamber.
What are the two types of glaucoma?
Open-angle glaucoma also called chronic
Closed-angle glaucoma also called acute
What is the most common glaucoma?
Open-angled glaucoma
Explain open-angled glaucoma.
Trabecular meshwork deterioriates as we age and this leads to increasingly impaired drainage of the aqueous humour.
This is painless and develops insidiously over time.
There is a gradual loss of peripheral vision.
It can be picked up on a routine eye test.
Diagnosis of open-angled glaucoma.
Tonometry
Cupping of the optic disc upon fundoscopy
Visual field loss
Explain cupping of the optic disc.
When there is an increased optic cup:disc ratio.
Treatment of open-angled glaucoma.
Topical medication like eye drops.
B-blockers to reduce prod of aqueous humour
Increase its drainage
Explain acute closed angle glaucoma.
This is an ophthalmological emergency.
It is caused by a reduced angle of the irido-corneal angle as the iris for some reason pushes forward and closes this angle.
There is no problem with the trabecular meshwork.
Clinical presenation of closed-angle glaucoma.
Acute presentation
Painful red eye
Irregular oval-shaped pupil that is fixed
Blurring of vision
Halo around lights due to corneal oedema
Nausea and vomiting
Why is closed angle glaucoma a medical emergency?
Because it can lead to irreversible sight loss within a few hours as the IOP is rising very rapidly and can cause damage to optic nerve quickly.
Treatment of closed-angle glaucoma.
Diuretics
Muscarinic eye drops
Analgesia
Surgery (iridotomy)
Why are muscarinic eye drops used?
Causes pupillary constriction which helps open the irido-corneal angle.
What people are more at risk of closed-angle glaucoma?
Long-sighted middle aged or eldery people with shallow anterior chambers.
