L20: Anatomy of the orbit and eye Flashcards
What is the shape of the orbital cavity and what is found within it?
Pyramidal shape with apex pointing posteriorly
(lateral and medial wall, roof and floor)
-eye, extra-occular eye muscles, lacrimal apparatus, fat, nerves, blood vessels
What openings are found in the orbital cavity?
-superior orbital fissure
-inferior orbital fissure
-optic canal
(lacrimal fossa- where the lacrimal sac sits)
(infra-orbital foramen beneath the orbital cavity- infra-orabital nerve of the maxillary division of the trigeminal nerve)
What bones make up the floor/medial aspect of the orbital cavity?
Maxilla
Ethmoid bone
Lacrimal bone
(site of fracture)
What structures run through the openings in the orbital cavity?
- opthalmic artery runs along with the optic nerve
- the opthalmic artery gives off a branch called the central retinal artery (optic canal)
- superior/inferior opthalmic veins which drain venous blood into the cavernous sinus, pterygoid plexus and facial vein: these exit via the superior/inferior fissures
- special sensory vision from retina via the optic nerve
- general sensory from the eye via the opthalmic division of the trigeminal nerve
- motor nerves to muscle CN3/4/6
What is found within the maxillary/ethmoid bone?
Maxillary sinus (air filled)
Series of air filled cavities
These are collectively known as paranasal air sinuses
Are the orbit and nasal cavity linked?
Nasolacrimal duct- connection via a bony channel from medial angle of the orbit to the nasal cavity
What are some implications due to the air sinuses and the nasolacrimal duct?
Implications for orbital trauma and spread of infection
What are the weakest parts of the orbital cavity?
Medial wall and floor of the orbit
What is an orbital blow out fracture?
- sudden increase in intra-orbital pressure, from retropulsion of the eye ball, which fractures the floor of the orbital cavity
- orbital contents can prolapse and bleed into the maxillary sinus
- fracture site can trap structures (soft tissue/extra-occular muscle)
- prevents upward gaze of affected side due to muscles being trapped
What are the signs and symptoms of a blow out fracture?
- history of trauma to the eye/orbit
- periorbital swelling/pain
- double vision (worse on vertical gaze)
- numbness over cheek, lower eyelid and upper lip/teeth/gums on affected side (due to infraorbital nerve lesion- cutaneous branch)
What does the eyelid consist of?
-skin
-subcutaneous tissue
-tarsal plate (connective tissue structure helping to give shape to the eyelid)
Muscles
-orbicularis oculi-palpebral part: supplied by facial nerve and through constriction it closes the eyelid
-levator palpebrae superioris (becomes slightly apeneurotic), which contracts and elevates the eyelid and is innervated by oculomotor nerve
-superior tarsal muscle (sympathetic innervation which elevates the lid)
Glands
-meibomian glands (found within the tarsal plate)
-sebaceous glands (associated with eyelash follicle)
What is a sty?
Superficial lump that has a white centre
- painful (can affect lower eyelid as well)
- blockage of the sebaceous glands
- often due to staphylococcus infection
What is a chalazian?
Meibomian gland cyst (blockage)
- painless
- more deep
- gradually get bigger
- not caused by infection
What is blepharitis?
Blockage of sebaceous glands along the whole eyelid
- doesn’t present as a single lump
- inflammation of the lids
What is the function of the meibomian cyst?
Oily (lipid rich) substance onto lid edges, prevents evaporation of tear film and tear spillage
What is the orbital septum?
Thin sheet of fibrous tissue that originates from the orbital rim periosteum and blends with the tarsal plates
- has a fissure (opening of the eye)
- orbital septum and tarsal plates separate subcutaneous tissue of eyelid and orbicularis oculi muscle from intra-orbital contents
What is the functon of the orbital septum?
Acts as a barrier: prevent infection in skin (superficial infection) from travelling deeper
What is periorbital (pre-septal) cellulitis?
Infection occuring within the eyelid tissue, superficial to the orbital septum
- inflammation of skin
- secondary to superficial infections (bites/wounds)
- more common in children
- ocular function remains unaffected
- can be difficult to differentiate b/w peri-orbital and more severe orbital cellulitis but if in any doubt, urgently refer and give high dose IV antibiotics and surgical drainage
What is orbital (post-septal) cellulitis?
Infection within the orbit, posterior to the orbital septum
- can arise from pre-septal cellulitis
- more commonly arised from infection spread from the paranasal air sinuses (sinusitis)
- damages orbital contents
What are the signs and symptoms of orbital cellulitis?
- reduced visual acuity
- reduced +/- painful eye movements
- proptosis (eye pushed forward): makes it hard to differentiate b/w periorbital as it is similar to the inflammation seen there (due to increased pressure)
Where do the superior and inferior optic veins drain into?
Cavernous sinus (so infection can spread to intracranial structures e.g. cavernous sinus thrombosis/meningitis)
What are the contents of the orbital cavity?
- nerves and blood vessels
- lots of fat
- lacrimal apparatus
- eyeball
- extra-ocular muscles
What is the tear film and lacrimal apparatus?
All the structures involved in tear film production and drainage
- tear film ensures the anterior surface of the eye is kept lubricated and protects
- blinking distributes tear film, rinsing and lubricating conjuctivae and cornea (from lateral to medial)
- once it reaches the medial aspect, it drains out via the lacrimal canaliculus
- the canaliculus drains into the lacrimal sac, then through the nasolacrimal duct, into the nasal cavity
What does the tear film consists of?
3 layers: oily, water, mucus
- meibomian glands
- lacrimal gland
- goblet cells in conjunctiva (conjunctiva is a transparent membrane found over the surface of the anterior eye)
What is epiphora?
Overflow of tears over the lower eyelid due to obstruction of the lacrimal apparatus
What is the anatomy of the eyeball?
Doesn't touch the orbital edges Eyeball is maintained in position by: -suspensory ligament (runs from lateral to medial, suspending the eyeball underneath like a sling) -extra-ocular muscles -orbital fat++
What are the 3 layers of the globe of the eye?
- Outer: sclera (usually white, except when it comes anterior, it becomes transparent and is known as the cornea)
- Middle: choroid (vascular to help supply the retina, continuous with the ciliary body: consists of ciliary muscles and processes-alters thickness of lens, it is also continuous with the iris)
- Inner: retina (photosensitive, contain retinal ganglion cells which convey AP’s out of back of eye via optic nerve)
What is the iris?
Muscular diaphragm with a central apperture which is the pupil
-the sphincter and dilator pupillae lie here
What is the uveal tract?
Chorois, ciliary body and iris
- can get get inflammtion of this: uveitis (anything that moves the iris- response to light intensity- causes pain)
- iritis (anterior uveitis) is a painful red eye which is photosensitive
What is the macula?
Area of back of eye which is responsible for highest acuity vision, colour
- high proportion of cones (colour photoreceptors)
- at tip of macula there is a central depression called the fovea- where the light will hopefully hit, and it this thinnest part of the retina so light doesn’t have to travel a far distance before reaching the cones
What are the different layers of the retina?
- the outermost layer that sits near choroid layer are pigment epithelial cells so don’t generate AP’s, but absorb excess light, to prevent scattering of light that may interfere with our ability to generate clear images
- photoreceptors are the next layer and convery AP’s to:
- retinal ganglion cells
- these axons converge to form the optic nerve
What is the space between the cornea and the iris?
Anterior chamber (there is also a posterior chamber which is between the iris in front and the lens behind, ciliary processes produce aqueous humor which circulates within the posterior chamber and passes through the pupil to enter the anterior chamber)
What is the role of the aqueous humor secreted by the ciliary processes?
Provides nutrients and oxygen to the cornea and lens, which are 2 structures of the eye which are avascular
What is the substance between the back of the lens and the retina?
Vitreous humour: firm, clear, gel-like fluid which fills the large portion of the globe of the eye
Where does the conjunctival membrane lie?
Covers the white of the eyes, and extends to where the sclera continues into the cornea
-it does not extend over the surface of the cornea
-it doesn’t full extend around the back of the eye, it reflects back onto the eyelid
(it is transparent and vascular)
Why can the eye appear red?
Conjunctivitis: inflammation of the conjunctival membrane, due to viral infection, eye not painful, gritty, watery
Sub-conjuctival haemorrhage: blood vessel bursts, no pain
What is the blood supply to the eye?
- branches of the opthalmic artery (branch of the internal carotid artery)
- enters with optic nerve through the optic canal
Central retinal artery: runs inside the optic nerve itself to enter the orbit and supply the retina (the retina is also draws supply from the underlying choroid layer)
Ciliary arteries: feed into extensive capillary bed in the choroid layer
How can the retinas function be disrupted?
Disruption of retinal function even if only one of its blood supply is occluded
What is central retinal artery occlusion?
When the central retinal artery is occluded:
- starving blood supply to superficial layers of the retina, but ciliary arteries are spared, so choroid blood supply is perfused still
- ischaemia of retina= pale
- see a cherry red spot: in area of macula, arises due to the rest of the retina appearing pale
How can temporal arteritis affect the eye?
Can also affect posterior ciliary arteries which supplythe poptic nerve as well, so you can get optic nerve ischaemia if you have vasculitis here
What factors allow light to be focused onto the macula densa?
- transparency (cornea, tear film, pupil, lens, fluid)
- regulating light entry (pupil)
- refraction to bring into focus (strongest part of refraction is the cornea- scattered light rays will be bent and focused through the pupil), lens refracts too
- shape of the eyeball: too long-short sighted, too short-long sighted
What are other names for long and short sighted?
Long: hypermetropic
Short: myopic
What is the accomodation reflex?
Focusing near objects requires greater refraction of light, as light from near objects is more divergent
- pupil constricts (limits amount of light entering the eye)
- eyes converge: move towards the midline (ensure image remains focused on same point of retina in both eyes)
- Lens becomes more biconvex (fatter) by contraction of the ciliary muscles
What happens to the lens with age?
Lens becomes stiffer with age and less able to change shape= presbyopia (age related inability to focua near-objects)
What is the ciliary muscle and how does it work?
Circumferential muscle that runs around the lens
-attached to lens by suspensory ligaments
When relaxed ring of muscle moves outwards: increase tension on lens and make it thinner= for far distance
When constrict the ciliary muscle: remove tension from suspensory ligaments, lens is thicker= for near objects
-ciliary muscle contracts under innervation of parasympathetic fibres of the occulomotor
What is phototransduction?
Light hits the retina, it is then converted into AP’s via the photoreceptors
Rods: active at low light levels, do not mediate colour vision, abundant in the peripheries of the retina
Cones: high definition, colour vision-active at high light levels, found concentrated within the macula of the retina
At the fovea we only have cones
- light passes through cells before it reaches the photoreceptors which convert light signals into action potentials
- AP’s propagated via retinal ganglion cells
- RGC axons collect in area of optic disc forming the optic nerve
- AP’s propagated along visual pathway to occipital lobe for interpretation
What is the optic disc?
Area where there is no photoreceptors
=blind spot
Why do we get decreased visual acuity?
-transparency of structures anterior to the retina e.g. opacity in lens: cataract
-refractive ability of structures anterior to retina: irregularity of corneal surface (astigmatism), ability of lens to change shape (presbyopia), shape of eyeball
(can correct refractive problem with glasses)
-retina or optic nerve: retinal detachment, age-related macular degeneration, optic neuritis
How can you tell if decreased visual acuity is caused by a refractive error or a non-refractive error?
Snellen chart- letters read at a distance of 6m
- errors of refraction : corrected with pin hole testing: removing all the light the eye has to refract, only light entering is directly perpendicular to the cornea which doesn’t require refraction
- not corrected with pin hole test: pathology involving retina/optic nerve
What is age related macula degeneration?
Leading cause of blindness
- thinning and atrophy of the macula
- affects central vision
What happens if there is a condition that affects aqueous draining of humor?
- aqueous humor inside the eye exerts intraocular pressure
- condition affects the drainage of aqueous humor, pressure rises, and this can cause damage to the optic nerve: glaucoma
How is aqueous humour produced and drained?
It is being constantly produced so requires drainage
- aqueous humor is secreted by the ciliary processes within the ciliary body
- flows from posterior chamber to anterior chamber through the pupil
- nourishes lens and cornea
- drains through iridocorneal angle (between iris and cornea)
- via trabecular meshwork into a canal of Schlemm which drains into the venous circulation
What is glaucoma?
Optic nerve damage secondary to raised introcular pressure
-can develop chronically/acutely
Chronic (most common)
- open-angle glaucoma (angle is normal)
- trabecular meshwork deteriorates as you age, so drainage is impeded
- increased IOP> optic disc cupping
- gradual loss of peripheral vision
Acute (less common)
-close angle glaucoma
-narrowing of iridocorneal angle (iris tips forwards slightly, then the liquid gets behind it and pushes is even further forward, narrowing it even more)
-ophthalmological emergency
-high risk of damage to optic nerve
=you see an irregular oval shaped pupil, in older patients, painful red eye, blurring of vision, nausea and vomiting
Require medical (reduce IOP) then surgical treatment
Why do we have binocular vision?
Gives a wider field of vision and depth perception (3D vision: stereoscopic vision)
- visual axis of both eyes need to be aligned (hit same spot on back of both retina- 2 images that reach the cortex are fused so perceived as one)
- eyes need to coordinate and move together (conjugate eye movement)
What causes diplopia?
Misalignment of 2 visual axes image focuses on different area of each retina so brain is unable to fuse: see two separate images
(these images can be displaced horizantally/vertically/diagonally)
What are the extraocular muscles?
4 recti muscles (superior, inferior, medial, lateral)
2 obliques (superior and inferior)
-overall 6 muscles to move the eyeball
-all insert into the sclera
-5 originate at apex of the orbit (expect IO which arises from the floor of the orbital cavity anteriorly)
-the recti muscles arise from common tendinous ring
Each muscle will have a certain pull and action on eye movement
What are the extraocular muscles innervated by?
CN3 (occulomotor)
(some are innervated by the CN4-trochlear/CN6-abducens)
-lateral recti innervated by CN6
-superior oblique innervated by CN4 (this muscle passes through the trochlear:pulley, so it inserts in a posterolateral position on superior surface of the eyeball)
What are the different axis of the orbit?
-visual axis (axis of the eyeball)
-axis of the orbit (different to the visual axis)
Extraocular muscles run in line with the axis of the orbit so they attach at an oblique angle
Where does the inferior oblique muscle run from?
Originates at floor of the orbit and inserts into the back of the posterolateral aspect of the eyeball
Are the extraocular muscles in use in the primary resting gaze?
Yes there is an equal and opposite pull of all extraocular muscles
- actions are balanced
- each muscle has an antagonist of its movement
How do you change the position of gaze?
Exert a greater pull through action of certain extraocular muscles, while the antagonists relax
-muscles moving both eyes must be highly coordinated and move stimultaneously
What are the terms of direction for the eye?
Adduction: towards nose (medial) Abduction: moving laterally Elevation: moving superiorly Depression: moving inferiorly Internal rotation: intorsion External rotation: extorsion
Which muscle moves the eye in only an adduction/abduction movement?
Adduction: Medial rectus muscle
Abduction: Lateral rectus muscle
What are the actions of the superior rectus muscle?
From primary resting gaze: -contracts to elevate the eyeball But due to it oblique nature of insertion -slightly adduct eyeball -slightly intorts the eyeball
When the gaze is more lateral:
-it becomes a much more powerful elevator of the eye
What are the actions of the inferior rectus muscle?
From primary resting gaze:
- depresses the eyeball if contracts
- slightly adducts the eye
- slightly extorts the eye
Eye positioned laterally
-more powerful depressor
What are the actions of the superior oblique muscle?
Primary resting gaze
- intorts the eyeball
- depresses
- slightly abducts it
Eye positioned medially
-more powerful depressor
What are the actions of the inferior oblique muscle?
From primary resting gaze
- extorts
- elevate
- slightly abduct the eye
Eye positioned medially
-more powerful elevator
What is strabismus?
Resting position of the eyeball may deviate due to actions of remaining working muscles (if a muscle is weakened)
How do you clinicallly examine eye movements?
Move your hand in a H shape
-we need to isolate an action of each muscle to test them
Lateral and medial rectus: abduction and adduction (easy to test as they only perform one action)
Move starting position of the eye:
Abducting start position laterally (R), and medially adducting (L): Superior and inferior rectus muscles are the main elevator and depressor of the eye when the eyeball is starting from the lateral position. Oblique muscles are the main elevator and depressor of the eye when the eyeball is starting from the medial position
What 2 muscles elevate the eye?
Superior rectus
Inferior oblique
What 2 muscles depress the eye?
Inferior rectus
Superior oblique
Why does stabismus occur?
Children: common, exact cause not always known, congenital or develops in infancy
Adults: acquired due to pathology or disease involving a number of structures
-cranial nerves can be affected (vasculopathic- microvascular ischaemia/physical compression/raised intracranial pressure)
What happens to the eye in CN3 palsie?
CN3 innervates all the muscles except lateral rectus and superior oblique
- eye held in abducted position, and depressed (down and out)
- also innervates the muscles of the eyelip (LPS) and the sphincter pupillae muscle
What happens to the eye in CN4 palsie?
This innervated the superior oblique muscle only
- eye is held adducted, extorted and elevated due to unapposed actions of the working muscles
- extortion of the eyeball is compensated by head tilt
Worsening diplopia when looking downwards and inwards (walking down the stairs, reading), due to the superior oblique being the main depressor of the eyeball when adducted
What happens to the eye in CN6 palsie?
Innervates the lateral rectus
=unapposed pull of the medial rectus muscle
-unable to abduct the eye on affected side
-horizantal dipolopia, worsening of diplopia when gaze is directed to the eye that is affected
