Session 8 Flashcards
Describe the orbital cavity
- 4 bony walls
- Pyramid shaped
- Base of pyramid faces outwards; apex deep inside orbital cavity
- Tough orbital rim
Which are 2 important bones involved in the orbital cavity?
- Ethmoid bone contributes to medial wall
- Maxillary bone contributes to floor
- Contain lots of air cavities
- More susceptible to fracture
- Weakest parts of orbital cavity
Outline some key anatomical relations of the orbit
- Anterior cranial fossa sits just superior
- Contains frontal lobe - can be damaged in penetrating eye trauma
- Ethmoid air sinuses near medial wall
- Maxillary air sinus forms part of floor
- Connected to nasal cavity via nasolacrimal duct
What are the implications for the anatomical relations of the orbit?
- Orbital surgery
- Spread of infection into or out of orbit e.g. acute sinusitis affecting ethmoid sinus can spread to orbit
- Orbital trauma
How does orbital blowout fracture occur?
- Trauma to eye/orbit e.g. eye is hit by fist or ball
- Eyeball propelled back into orbit
- Pressure in orbital cavity suddenly increases
- Pushes against walls of cavity
- Weakest wall (floor of orbit) fractures
What does orbital blowout fracture result in?
- Orbital contents prolapse
- Bleeding into maxillary sinus
- Soft tissue, blood and muscles near orbital floor can trap in fracture site
- Entrapment prevents upwards gaze (other eye movements may also be restricted)
What might be seen on an X-ray/CT scan in someone who has suffered an orbital blowout fracture?
- Opacity filling the maxillary sinus beneath the affected eye
What is classic presentation of an orbital blowout fracture?
- Painful eye
- Periorbital swelling and bruising
- Double vision, worse in vertical gaze
- Numbness over cheek, lower eyelid and upper lip on affected side
How do we manage orbital blowout fracture?
- CT Orbit and refer to ophthalmology
- Prophylactic antibiotics
- Avoid nose blowing, Valsalva manoeuvres and driving (until diplopia resolves)
- 1 week follow up - symptoms may resolve on their own
- Surgical repair 1-2 weeks post injury if symptoms persisit
What does the optic canal transmit?
- Optic nerve
- Ophthalmic artery (has several branches including central retinal artery)
What does the superior orbital fissure transmit?
- Branches of ophthalmic nerve (Va)
- Oculomotor nerve
- Trochlear nerve
- Abducens nerve
- Superior ophthalmic vein (communicates with cavernous sinus)
What does the inferior orbital fissure transmit?
- Infraorbital nerve (branch Vb)
- Inferior ophthalmic vein (communicates with pterygoid venous plexus
Which nerve is responsible for carrying sensation to the eye?
- Ophthalmic division of trigeminal nerve
What provides the main arterial supply to the orbit and the eye?
- Ophthalmic artery (branch of ICA) and its branches
- Incl central retinal artery
Outline the pathway taken by the central retinal artery
- Runs inside optic nerve
- This allows artery to get inside eyeball and give branches that supply retina
What provides the main venous drainage of the orbit and eye?
- Ophthalmic veins (superior and inferior)
- Provide connections with cavernous sinus, pterygoid plexus and facial vein
Outline the blood supply to the retina
- Supplied by central retinal vein
- Also draws supply from underlying choroid layer
- Ciliary arteries feed extensive capillary bed within choroid layer (choriocapillaris)
- Retina requires both circulations to function properly
What do the eyelids consist of?
- Skin
- Subcutaneous tissue
- Muscles
- Tarsal plate
What is the function of the tarsal plates?
- Connective tissue
- Give eyelid firmness and shape
What are the key muscles that run within the eyelid?
- Orbicularis oculi (palpebral part)
- Levator palpebrae superioris
What is the action of orbicularis oculi (palpebral part)?
- Runs through eyelid itself
- Closes eyelid
- Supplied by facial nerve
What is the action of levator palpebrae superioris?
- Retracts eye lid
- 2 components
1. Skeletal muscle supplied by oculomotor nerve
2. Superior tarsal muscle (smooth muscle) innervated by sympathetics
Which glands are found within the eyelids?
1.Meibomian glands
- found within tarsal plate
- modified sebaceous
- provide lipid layer of tear film
- prevent tear evaporation and spillage over lid
2. Glands associated with lash follicle
- oily substance
What is a stye?
- Due to blockage in gland associated with a hair follicle
- Outer part of lid affected
- Painful
- Red with a white punctum
- Infected (staphylococcus)
- Treat with warm compresses and may need abx
What is a Meibomian cyst?
- Due to blockage in Meibomian glands
- Deeper within lid
- Painless
- Firm lump palpable
- Enlarges gradually
- Blocked duct - non-infective
- 1/3 resolve spontaneously
- Surgical incision if persists
What is blepharitis?
- Inflammation of eyelid margin
- Multifactorial causes e.g. staphylococcus, Meibomian gland dysfunction
- Crusting, dry eyelids, swollen, red
- Not serious
- Treat with warm compress and lid hygiene
What is the orbital septum?
- Thin fibrous sheet originating from orbital rim
- Separates intra-orbital contents from muscle and subcutaneous tissue of eyelid
- Blends with tarsal plates
What is the role of the orbital septum?
- Barrier against infection spreading from superficial eyelid region (pre-septal) into the orbital cavity (post-septal)
What is infection involving the superficial tissues of the eye called?
- Pre-septal (periorbital) cellulitis
What is infection involving tissues within the orbit of the eye called?
- Post-septal cellulitis
- Very concerning
What causes pre-septal cellulitis?
- Secondary to superficial infections e.g. from bites, wounds
Where does periorbital cellulitis affect the eye?
- Confined to tissues superficial to orbital septum and tarsal plates
- Painful
- Eye movements and vision remain unaffected
What should you do if a patient comes in with pre-septal cellulitis?
- If you can’t tell whether it’s peri-orbital or orbital cellulitis, urgently refer
What is orbital (post-septal) cellulitis?
- Infection within the orbit
- Spread of infection from paranasal air sinus
- Proptosis/exophthalmos
- Reduced and painful eye movements
- Reduced visual acuity
Why is orbital cellulitis so concerning?
- Orbital veins drain to cavernous sinus and pterygoid venous plexus
- Potential route for infection to spread intracranially
- Can lead to cavernous sinus thrombosis and meningitis
- Damage to optic nerve can cause permanent blindness
What are the contents of the orbital cavity?
- Nerves
- Blood vessels
- Lots of fat
- Lacrimal apparatus
- Eyeball (globe)
- Extra-ocular muscles
What does tear film consist of?
- Three layers
1. Oily (Meibomian glands)
2. Water (lacrimal gland)
3. Mucus (goblet cells in conjunctiva)
What is the function of blinking?
- Distributes tear film across surface of eye
- Rinses and lubricates conjunctiva and cornea
How do we drain our tears?
- Lacrimal gland produces tears
- Blinking sweeps tears across surface of eye to medial corner of eye
- Lacrimal punctum
- Lacrimal canaliculus
- Lacrimal sac
- Nasolacrimal duct
- Drains to inferior meatus of nasal cavity
What happens if drainage of tears becomes obstructed?
- Epiphora (overflow of tears over lower eyelid)
What can block the drainage of tears?
- Infection
- Injury
- Stenosis
What maintains the eyeball in position?
- Suspensory ligament (sits underneath it like a sling)
- Extra-ocular muscles
- Lots of orbital fat
Describe the conjunctival membrane
- Transparent
- Mucous membrane
- Reflects onto inner surface of upper and lower lid
- Does not run over cornea
- Vascular
Outline conjunctivitis
- Uncomfortable, gritty
- Watery +/- discharge
- Infectious (typically viral)
- Very contagious
- Self-limiting
Outline sub-conjunctival haemorrhages
- Burst blood vessel in conjunctiva
- Painless
- No other symptoms
- Often spontaneous (without cause)
What are the 3 layers of the eye from outermost to innermost?
- Sclera (continues as cornea anteriorly)
- Choroid (vascular; continuous with ciliary body and iris)
- Retina (photosensitive layer
Where does the optic nerve start?
- At the optic disc of the retina
- Might lead to a blind spot because there are no photoreceptors at this site
What are the points of highest acuity vision?
- Macula
- Fovea
- Thinnest points of retina so light doesn’t have to travel through so many cell layers to reach photoreceptors
- Contain lots of cones
- Known as central vision
What are the symptoms of a central retinal artery occlusion?
- Sudden painless loss of sight in one eye, developing over seconds
What is a central retinal artery occlusion?
- Blockage stopping blood getting to vessels supplying front of retina
- E.g. due to an embolus
- Choroidal layer remains perfused with blood
What is seen on examination of an eye with central retinal artery occlusion?
- Pale, ischaemic retina
- Cherry red spot = macula
- Macula is thinnest part of retina so underlying choroid is accentuated in this area
- Appears red due to pallor of ischaemic retina
What is the globe of the eye filled with?
- Aqueous humour (water) that bathes lens and cornea to provide them with energy
- Vitreous humour (firm and jelly-like)
What are the chambers of the eye?
- Anterior chamber between cornea and iris
- Posterior chamber between iris and lens/ciliary body
What is the ciliary body of the eye?
- Contains ciliary muscle and ciliary processes that secrete aqueous humour
Outline the production and drainage of aqueous humour
- Secreted by ciliary processes in ciliary body
- Flows from posterior chamber to anterior chamber via pupil
- Nourishes lens and cornea
- Drains through iridocorneal angle
- Via trabecular meshwork into canal of Schlemm
- Ends up in venous circulation
What is glaucoma?
- Optic nerve damage secondary to raised intraocular pressure
- Sight threatening
What can cause a rise in intra-ocular pressure?
- Blockage in drainage of aqueous humour from anterior chamber
Outline open-angle glaucoma
- Chronic
- Most common type
- Many asymptomatic
- Increased intra-ocular pressure leads to increased optic disc cupping
- Gradual loss of peripheral vision
Outline closed-angle glaucoma
- Acute
- Less common
- Narrowing of iridocorneal angle so aqueous humour cannot drain
- Ophthalmological emergency
What is a typical presentation of acute closed-angle glaucoma?
- Older patients aged 55+ (most common 70s-80s)
- Acutely painful red eye
- Irregular oval-shaped pupil
- Pupil is fixed (doesn’t constrict or dilate)
- Blurring of vision
- Halos around light
- Nausea and vomiting
How is acute closed-angle glaucoma treated?
- Drugs to reduce IOP
- Surgical treatment
How does light enter the eyes so that we can see?
- Light needs to reach and be focused onto macula
- Pupil regulates light entry via action of muscles in iris
- Tear film, cornea and lens refract light to bring it into focus
- Shape of eyeball also important
What happens if the eyeball isn’t the correct length?
- Too long = myopic/short-sighted
- Too short = hypermetropic/long-sighted
What is the accommodation reflex?
- Focusing near objects requires greater refraction of light to focus onto retina
- This is beyond capability of cornea (fixed shape)
Outline the accommodation reflex
- Pupil constricts to limit light coming through
- Eyes converge to ensure image remains focused on same point of retina in both eyes
- Lens becomes more biconvex (fatter) by contraction of ciliary muscle
What is presbyopia?
- Age-related inability to focus near objects
- Lens becomes stiffer with age and less able to change shape
What is the function of the cones?
- Colour vision
- High definition
- Active at high light levels
- Lots found in macula and fovea
What is the function of the rods?
- Non-colour vision
- Low acuity
- Active at low light levels
- Lots found in peripheral retina
What do the photoreceptors do?
- Convert light signals into action potentials
- These are then propagated via retinal ganglion cells
Outline the process of phototransduction
- Action potentials propagated via retinal ganglion cells
- These collect at optic disc to form optic nerve
- Action potentials propagated along visual pathway
- Reach occipital lobe for interpretation
What is visual acuity?
- Ability of eye to discern shapes and details of what we see
How is visual acuity measured?
- Snellen chart
- Read set of letters of increasingly smaller size
- One eye at a time
- 6m distance
- Normal vision = 6/6
-Top number = 6m distance (constant) - Bottom number = pt’s score
What can cause decreased visual acuity?
- Lack of transparency of structures anterior to retina e.g. cataracts
- Changed refractive ability of structures anterior to retina e.g. astigmatism, presbyopia, change in eyeball shape
- Problems with retina or optic nerve e.g. retinal detachment, age-related macular degeneration, optic neuritis
How do we discern the cause of decreased visual acuity?
- Check for red reflex using ophthalmoscope
- Absence suggests light prevented from reaching retina and reflecting back
- Red colour is due to colour of retina
What are cataracts?
- Clouding of lens due to protein degradation
- Light scatters so isn’t focused onto retina
What are the different reasons for decreased visual acuity?
- Refractive - due to changes in cornea, lens or eyeball size
- Non-refractive due to retina or optic nerve problem
How do we determine whether decreased visual acuity is refractive or non-refractive?
- Repeat Snellen chart with pinhole
- This allows light to enter directly perpendicular to cornea and lens
- Light does not need to be refracted to focus on macula
- If acuity improves with pinhole = refractive
- If acuity does not improve with pinhole = non-refractive
Why do we have binocular vision?
- Allows for wider field of vision and depth perception
- Enables 3D vision
-Visual axis of both eyes needs to be aligned so that light hits retina in same spot in each eye - Also need conjugate eye movement
- Allows brain to fuse information from each eye and create a single image
What causes diplopia?
- Misalignment of two visual axes
- Image focuses on different areas of each retina
- Brain unable to fuse information
- 2 images seen, can be displaced horizontally, vertically and/or diagonally
What are the extra-ocular muscles?
- Superior rectus
- Inferior rectus
- Medial rectus
- Lateral rectus
- Superior oblique
- Inferior oblique
What is the attachment of the extra- ocular muscles?
- Sclera
What is the origin of the extra-ocular muscles?
- Apex of orbit
- Except inferior oblique - this originates from floor of orbital cavity anteriorly
- 4 recti arise from a common tendinous ring
What is the innervation of the extra-ocular muscles?
- Most are innervated by oculomotor nerve (CN III)
- Lateral rectus innervated by abducens nerve (CN VI)
- Superior oblique innervated by trochlear nerve (CN IV)
What direction do the extra-ocular muscles travel in?
- Run in line with axis of orbit
- Therefore some muscles attach at an oblique angle
- Confers several actions of movement on globe
Describe the action of the extra-ocular muscles when our gaze is resting
- Even at rest, constancy of activity in all extra-ocular muscles on eyeball
- During resting gaze their actions are balanced allowing for forward gaze
- Each muscle has antagonist of its movement
How do the extra-ocular muscles work to allow us to change the position of our gaze?
- Antagonists relax while certain extraocular muscles exert greater pull
- Muscles moving both eyes must be highly coordinated and move simultaneously
- Visual axes must remain aligned for conjugate gaze
What are the different directions the eyeball can move in?
- Elevation/depression
- Abduction/adduction (towards nose)
- Intorsion/extorsion
What is the action of medial rectus?
- Adduction of eyeball
- Inserts into medial aspect sclera
What is the action of lateral rectus?
- Abduction of eyeball
- Inserts into lateral aspect of sclera
Where do superior and inferior recti arise from?
- Apex of orbit
Where does superior rectus insert?
- Obliquely into superior anterolateral surface of globe
What is the action of superior rectus?
- Elevates eyeball
- Slightly adducts
- Slightly intorts
- More powerful elevator when eye is positioned laterally
Where does inferior rectus insert?
- Obliquely into anteroinferior surface of globe
What is the action of inferior rectus?
- Depresses eyeball
- Slightly adducts
- Slightly extorts
- More powerful depressor when eye is positioned laterally
What is the origin of the superior oblique muscle?
- Arises from apex of orbit
- Passes through trochlea
Where does superior oblique muscle insert?
- Superior-posterior aspect of globe
What is the action of superior oblique?
- Intorts eyeball
- Depresses
- Slightly abducts
- More powerful depressor when eye is positioned medially
Where does inferior oblique arise?
- Anteromedial surface of floor of orbit
Where does inferior oblique insert?
- Infero-posterior surface of globe
What is the action of inferior oblique?
- Extorts eyeball
- Elevates
- Slightly abduct (pulls eye laterally)
- More powerful elevator when eye is positioned medially
Which extraocular muscles are stronger elevators and depressors of the eyeball when the eye is in the adducted position?
- Obliques
Which extraocular muscles are stronger elevators and depressors of the eyeball when the eye is in the abducted position?
- Recti
Outline how the anatomical actions of the extraocular muscles help make sense of abnormalities of gaze
- If a muscle/muscles is weakened, its influence is lost
- Other muscle actions are no longer antagonised (balanced out)
- Resting position of eyeball may deviate (strabismus) due to actions of remaining working muscles
- Difficulties with moving eye in certain directions of gaze
If the eye is in an adducted position, which muscle may be damaged?
- Lateral rectus
- Abduction is no longer occurring
- Could be due to CN VI lesion
If eye is in an elevated, adducted position, which muscle may be damaged?
- Superior oblique
- Depression and abduction no longer occurring
- Could also be due to CN IV lesion
How do we clinically examine eye movements?
- Pt needs to keep head still and follow finger movement
- Draw an H shape with finger
- Test abduction and adduction of eyes (LR and MR)
- When eye position starts abducted, up and down movements is mostly controlled by rectus muscles
- When eye position starts adducted, dominant muscles are obliques for up and down movements
What does CN III innervate?
- Superior rectus, inferior rectus, medial rectus, inferior oblique
- Levator palpebrae superioris
- Sphincter pupillae
What are the acquired causes of CN III cranial nerve palsies?
- Vasculopathic (microvascular) lesions e.g. due to diabtes/hypertension
- Pupil is spared
- Compressive lesions e.g. due to raised ICP, tumour, posterior communicating artery aneurysm
- Pupil is involved
What is ocular misalignment (strabismus)?
- Common in children (congenital or develops in infancy)
- Exact cause not always known
- Treat by cover 1 eye, specific lens prescription, surgery
- In adults it is acquired due to pathology or disease involving neuromuscular junctions or nerves supplying extraocular muscles
What kind of damage to nerves can lead to strabismus?
- Vasculopathic - microvascular ischaemia secindary to diabetes or hypertension
- Physical compression e.g. from tumour or aneurysm
- Raised ICP
What happens to the eye when CN IV is affected by palsies?
- Loss of intorsion, depression and abduction of eyeball
- Because superior oblique muscle is no longer innervated
- Eye is extorted, slightly elevated and adducted
- Extorsion of eyeball compensated by head tilt
- Worsening diplopia on downward/medial gaze
What happens to the eye when CN VI is affected by palsies?
- Innervates lateral rectus
- Unopposed pull of medial rectus muscle
- Unable to abduct eye on affected side
- Diplopia that worsens on horizontal gaze
What is the most likely cause for CN III, IV and VI lesions?
- Vasculopathic
- Patients will be otherwise asymptomatic
- Lesions usually self-resolve within a few months
In the case of a CN III, IV or VI lesion, what history is concerning?
- Headache +/- vomiting could suggest raised ICP (secondary to tumour or haemorrhage)
- Recent head injury
- Presence of pupil involvement in CN III lesions and eye pain/headache
