Anatomy of the Eye Flashcards
Which bones make up the orbital cavity?
Floor: mainly maxilla, part of zygomatic bone
Roof: the frontal bone, part of sphenoid
Medial wall: Lacrimal, Ethmoid, part of maxilla
Lateral wall: mosty zygomatic bone, part of sphenoid
Which bones of the orbit are the weakest and therefore the most vulnerable to fracture in orbital trauma?
The floor and the medial walls of the orbit are the weakest
What is the main arterial blood supply and the venous drainage of the orbital cavity?
Artery: opthalmic artery (branch of internal carotid)
Vein: superior + inferior opthalmic veins which drain to the cavernous sinus, pterygoid plexus and facial vein
Which nerves give general sensory, special sensory and motor innervation to the eye?
General sensory: Opthalmic branch of Trigeminal (Va)
Special sensory: Optic
Motor Nerves: Occulomotor, Abducens and Trochlear
What is an orbital blow out fracture?
Something that causes a sudden increase in intra-orbital pressure fracturing the floor of the maxilla
Orbital contents can proplapse and bleed into the maxillary sinus
What can happen to the eye in an orbital blow out fracture and why?
Fractured site can trap structures e.g. soft tissue and extra occular muscles in the floor of the orbit, tethering the eye.
When asked to look up, the patient can’t
Why do you get numbness over the cheek, lower eyelid and upper lip on the affected side of an orbital blow out fracture?
The infraorbital nerve (branch of maxillary) runs through the floor of the orbital cavity. If damaged will affect sensation from Vb nerve dermatome
Which muscles make up the eyelid and what are their respective actions?
- Orbicularis oculi - closes the eyelid
- Levator palpebrae superioris - retracts eyelid
What components make up the eyelid
- Skin
- Subcutaneous tissue
- Tarsal plate (connective tissue gives shape to eye)
- Muscles
- Glands
Which glands are in the eyelid and what do they secrete?
Meibomian glands - inside tarsal plate, secrete oily component of tear fluid
Sebaceous glands - associated with lash follicle
What pathology can occur with the glands of the eyelid?
Stye - blockage of sebaceous glands located near the eyelash follicle, self limiting
Meibomian Cysts- blockage of meibomian gland, deeper in the eyelid, tends to be painless
What is blepharitis?
Inflammation of the eyelids
Including: skin, lashes, and Mebomian glands
What is the orbital septum?
A thin sheet of fibrous tissue from the orbital rim periosteum, that blends with the tarsal plates
What is the function of the orbital septum?
Acts as a barrier against superficial infections spreading from pre-septal to post- septal space
What is periorbital (pre-septal) cellulitis?
An infection of eyelid tissue superficial to the septum
- secondary to superficial infection (bites, wounds)
- Confined to tissue superficial to orbital septum
- Ocular function remains unaffected as protected by septum
What is an orbital (post- septal) cellulitis?
An infection within the orbit, posterior to the orbital septum
- Causes proptosis (eyeball pushed forward), expothalmus
- Reduced, painful eye movement
- Reduced visual acuity (may involve optic nerve)
Why is orbital ceullitis so dangerous?
Orbital veins drain to the cavernous sinus, pterygoid venous plexus and facial veins giving a potential route for infection to spread intracranially. Potential to cause:
- Cavernous sinus thrombosis
- Meningitis
What is the lacrimal apparatus?
Structures involved in tear film production and drainage
- lacrimal gland
- lacrimal sac
- lacrimal ducts (tear drainage)
Where do tears drain to?
The nasal cavity
What is epiphora?
Any obstruction in the drainage of tears that leads to the overflow of tears over the lower eyelid
What are the 3 layers of the eye and what is found in each?
Outer Fibrous tunic: Sclera & Cornea
Middle Vascular tunic: Choroid, Ciliary body, Iris
Inner: Retina
What 3 things maintain the eyeball position within the orbital cavity?
- suspensory ligaments
- extra-ocular muscles
- orbital fat
What is the conjunctiva of the eyeball?
A transparant mucus membrane that prodcues a component of tear film, covers the sclera but does not over the cornea
Highly vascular with small blood vessels within the membrane
What is conjunctivitis?
Inflammation or infection of the conjuctiva
Causes the eye to become very red
What is a subconjunctival haemorrhage?
Haemorrhage from blood vessles in the conjuctiva
Which point in the eye is light focused onto?
The Macula
Which structures refract light in the eye? Which is the main one?
- Cornea and it’s assoicated tear film → main refractor
- Lens
- Aqueous humour
- Vitreous humour
How can the shape of the eye affect refraction?
- Myopia = short sightedness. Eyeball is too long bringing point of focus infront of the retina
- Hypermetropia = long sightedness. Eyeball is too short lighy focused after the retina
What is the accomodation reflex?
Focusing near objects requires greater refraction of light
Eye accommodates by:
- pupil constriction to limit amount of light coming through
- Eyes converging ensures image focused at the same point in both eyes
- Lens becomes more biconcave (fatter) by contraction of ciliary muscle
What is presbyopia?
Natural stiffening of the lens with age, means the lens is less able to change shape so focusing near objects becomes harder
What are the 2 types of photoreceptor found in the retina? What does each do and how do they differ in location?
Rods: active at low light levels, do not see colour.
- Abundant in peripheral parts of the retina
Cones: see in high definition and colour
- concentrated in the macula of the reina
- The fovea (middle point) of macula is only cones
How do rods and cones respond to light?
- Generate action potentials in response to light passing through retinal gandlion cells
- RGC axons collect in an area of the optic disc forming the optic nerve (no photoreceptors here → blindspot)
- Action potentia propogate along a visual pathway to the occipital lobe for interpretation
How do you can you get blurred vision?
Pathology affecting
- transparancy of lens e.g cateracts
- ability to refract light
- irregular corenal surface
- ability of lens to change shape
- shape of eyeball
- Retinal detachment
- Macular degeneration
- Optic neuritis
What are the different chambers of the eye and the fluid in them?
Anterior and Posterior chambers: contain Aqeuous Humour
Vitrous Chamber contains Vitrous humour (more firm and jelly like)
What is the function of fluid in the eye?
Helps maintain shape and keeps retina pushed against all the other layers
Describe the production and drainage of aqueous humour
- Produced by cilliary processes of ciliary bodies
- Flow from posterior → through pupil → into anterior chamber
- Nourishes lens and cornea
- Drains through iridocorneal angle (between irish and cornea) via the trabecular meshwork into the canal of Schlemm
What is Glaucoma? What are the two types?
Optic Nerve Damage secondary to raised intracocular pressure due to a blockage in the drainage of aqeous humour from the eye
Acute = closed angle glaucoma (less common)
Chronic= open angle glaucoma (most common)
Explain what happens in open angle glaucoma?
- Blockage of the trabecular network which detriorates with age
- Aqeous humour unable to drain
- Increases intra-ocular pressure causing optic disc cupping
- Causes gradual loss of peripheral vision
- Iridocorneal angle is not affected
Explain what happens in closed angle glaucoma?
- Narrowing of the iridocorneal angle
- Iris is pushed forward closing the angle
- Aqueous humour cannot drain
- Raises intra ocular pressure more rapidly
- Opthalological emergency
How does a patient with acute closed angle glaucoma usually present?
- Older patients 55 yrs +
- Acutely painful red eye
- Irregular oval shaped pupil
- Blurring of vision
- Halo’s around lights due to corenal oedema
- Nausea and Vomiting
How do you treat closed angle glaucoma?
Medical drugs to reduce IOP then surgical treatment
- diueretics - reduces aqeous humour production
- muscarinic eye drops- pupillary constriction opens the irido-corneal angle
- strong analgesia
What is uvetitis?
Inflammation of the choroid layer in the eye
Associated with autoimmune conditions e.g. anklyosing spondyltitis and IBD
Explain what happens in colour blindness?
There are 3 types of cones: red, green and blue senstivie
Absence/ Dysfunction of any of these can lead to colour blindness
A patient is diagnosed with Grave’s Disease. On examinatio they have bilateral eye retraction with a wide eye appearance. How would you explain the clinical findings?
- Increased levels of circulating thyroid hormone increases the expression of adrenergic receptors of sympathetic tissues
- In this case, increased noradrenaline receptors on the superior tarsal muscle causes excessive retration of the lid and lid lag
