Lecture 16 - Anatomy of the eye Flashcards
Orbital cavity borders
roof : anterior cranial fossa
floor : maxilla
Medial wall : ethmoid and lacrimal bone
Lateral wall : maxillary
Orbit
Protects and contains the eyeball, muscles, nerves and vessels + most lacrimal apparatus
Holes in the orbit
Superior orbital fissure - CN III, IV, Va, VI and superior opthalmic vein
Optic canal - optic nerve and opthalmic artery
Inferior orbital fissue - Vb branch and inferior opthalmic vein
Lacrimal fossa
Blood supply of orbit
Artery: opthalmic artery branches (from ICA)
Vein: Inferior and superior opthalmic veins which drain into the cavernous and pterygoid plexus and facial vein
Innervation to orbit
General sensory: Va (including cornea and conjunctiva)
Motor of eye: CN III, IV, VI
Special sensory: CN II
Weakest parts of orbit
The inferior and medial walls are the weakest (maxillary and ethmoid bone)
Most prone to fracture as thin due to sinuses
Orbital blow out fracture
Sudden increase in intra-orbital pressure fractures the floor of the orbit
Maxillary sinus fills with blood
Fracture site can trap tissues, fat and muscles located near the orbital floor
Prevents upward gaze on affected side as tethered
Presentation of orbital blow out fracture
- History of trauma to orbit
- Periorbital swelling
- Pain
- Diplopia
- Numbness over cheek, lower eyelid and upper lip, teeth and gums (due to infraorbital nerve and superior alveolar nerve lesion Vb)
Eyelid
Consists of:
- Skin
- tarsal plate
- subcutaneous tissue
- muscles - levator palpebrae superioris and obicularis oculi
- Levator aponeurosis attaches to tarsal plate
- Melbomian glands
- Sebaceous glands
Orbicularis oculi
Closes eyelid
Innervated by facial nerve
If CN VII lesion: Lagophthalmos
Levator palpebrae superioris
Opens eyelid
Innervated by occulomotor nerve
Lesion causes complete ptosis
[Superior tarsal muscle innervated by sympathetics can cause partial ptosis if disrupted]
Meibomian glands
Secrete oily, lipid rich tear fluid on to the edges of the eyelid.
Prevents evaporation of tear film and tear spillage
Stye
Blockage or infection (staph) of sebaceous gland or hair follicle on outer part of the eyelid where follicles are found
Painful
Meibomian cyst
Block meibomian cyst
Oily secretions build up
Deeper
Blepharitis
Inflammation of eye lids, including the skin, lashes and meibomian glands.
Can crust around edges
Foreign body sensation
Orbital septum
Thin sheet of fibrous tissue originating from the orbital rim periosteum blending into the tarsal plate
Separates subcutaneous tissue and the orbicularis oculi from intra-orbital contents.
Tarsal plate
Dense bands of connective tissue
Strengthens and gives shape to the eyelid
Contains meibomian glands
Periorbital cellulitis
Infection occuring within eyelid tissue, superficial to the orbital septum
Secondary to superficial infections e.g. via bites or wounds or bacterial sinusitis (fronto-ethmoidal sinuses in children)
Ocular function unaffected
Orbital cellulitis
Infection within the orbit deep to the orbital septum
More serious as can spread intracranially via the cavernous or pterygoid plexus causing meningitis ad venous thrombosis
Hard to distinguish between peri-orbital cellulitis
Presentation:
- proptosis
- exopthalmos
- reduced eye movements
- reduced visual acuity
- +/- pain eye movements
Treat with IV antibiotics and drainage
Lacrimal apparatus
Lacrimal gland - tear production
Lacrimal sac
Lacrimal ducts and nasolacrimal duct - drains tears into nasal cavity
Epiphora
Overflow of tears due to duct obstruction
Blinking
Due to orbital part of orbicularis oculi innervated by the facial nerve
Covers eye in tear film lubricating fluid which rinses the eye, conjuctiva and cornea and sweeps dust
Layers of the eyball
Outer - Sclera (white) and cornea anteriorly (transparent)
Middle (uvea): Iris, choroid and ciliary body
Inner - retina containing rod and cone cells
How is the eyeball position maintained?
By suspensory ligaments
Extra-ocular muscles
Fat
Lacrimal gland
Secretes lacrimal fluid (tears)
Under parasympathetic innervation via the facial nerve greater pertrosal
Lies in fossa on the superolateral orbit
Lacrimal ducts
Allows tears to be drained into the conjuctival sacs via the lacrimal lakes medially
Drained into the lacrimal sacs via lacrimal caniculi and then to the nasolacrimal duct to the nasal cavity
Lacrimal gland to nasal cabity
- Lacrimal gland secreted lacrimal fluid
- Lacrimal ducts
- Conjunctival sac
- Lacrimal lake
- Lacrimal caniculi
- Lacrimal sac
- Nasolacrimal duct
- Nasal cavity
Conjunctiva
Thin, transparent mucous membrane
The anterior surface of the eye is covered in a conjunctival membrane covering the sclera and lines inside of the eyelid
Forms a conjunctival sac
Secretes mucous component of the lacrimal fluid
Highly vascular with small blood vessels within its membrane
Limbus
Junction between the conjunctiva and cornea as the cornea has its own epithelial lining
Conjunctivitis
Inflammed conjunctiva causes small blood vessels to dilate and eye looks red
- Viral aetiology
- Feels gritty
- Teary
- Contagious
Subconjunctival heamorrhage
Small blood vessel rupture in the conjunctiva
Not dangerous or site threatening
Spontaneous
Painless
Red eye
Conjunctivitis or subconjunctival haemorrhage
Uveitis
Inflammation of the choroid
- Red, painful eye
- Worse when focusing or looking at bright light
- Associated with autoimmune conditions such as ankylosing spondylitis and inflammatory bowel disease
Sclera
Opaque white covering
Gives eyeball shape
Attachment site for extraocular muscles
Continues with dural sheath covering the optic nerve
Conjunctivitis in neonate
Infective organism - chlamydia from mother’s vaginal mucosa
Treat with: erythromycin
Structures that refract ligh to focal point
Cornea and tear film
Lens
Aqueous and vitrous humour
Myopia
Short sighted
Hypermetropia
Long sighted
Accomodation reflex
- Pupil constriction - limits amount of light entering pupil which ensures light from near objects pass through centre to fovea via the iris (PS stimulation)
- Eyes converge - ensures image remains focused on the same point of retina in both eyes
- Lens becomes more biconvex (fatter) by contraction of circular ciliary muscles (PS innervation) and loosening of the suspensory ligaments
Why do we need the accomodation reflex
Light from nearer objects diverges more which surpasses the refraction power of the cornea
Presbyopia
Ageing causing the lens to become stiffer therefore cannot refract as much light onto retina
Can’t focus on near objects as well
Iris
Provides colour to eye
Thin, contractile diaphragm with the pupil being the central aperture
Anterior to lens
Muscles: control size of pupil
Sphincter pupilae
Dilator pupilae
Lens
Transparent
Focuses light on back of eye
Posterior to iris
Biconvex
Enclosed in capsule
Avascular and no nerves as transparent - receives nutrients from the aqueous humour
Edges of the lens is attaches to the ciliary body via the suspensory ligaments
Cataracts
Ageing causes gradual degradation of proteins
Therefore lens becomes cloudy
Rod cells
Active at low light intensity
No colour vision
Located in the periphery of the retina
20 x more rod cells
Cone cells
Active at high light intensity Colour vision High definition Located centrally (macula of retina) In fovea, only cone cells are present
Fovea
Centre of macula
Macula
Dark region in retina lateral to the optical disc where vision is centred
Tyoes of cone cells
Blue sensitive
Red sensitive
Green sensitive
Optic disc
Blind spot as no photoreceptors
Area where there is accumulation of retinal axons that leave the eye at the optic nerve
Layers of retina
Neurosensory - senses light
Pigmented - contains melanin
Pigmented layer
Absorbs scattered light
Reduces reflection
Focuses images onto the retina
How we see
- APs generated in response to light via rod and cone cells pass via the retinal ganglion cells
- The retinal ganglion cell axons accumulate at the optic dics to form the optic nerve
- APs propagate along the visual patheay wo the primary visual cortex in the occipital lobe for interpretation
Astigmatism
Irregularity of corneal surface, decreasing the ability to refract light
Optic nueritis
Inflammation of the optic disc causes blurring of vision
Pin - hole testing
Refraction of light does not affect light passing into the pupil perpendicularly to the lens as does not need to be refracted.
Therefore, if make small hole and look through it where only perpendicular light passes through the pupil, vision acuity should improve if there is a refractive error
If does not improve:
Problem with optic nerve or retina
Chambers of eye
Anterior - between cornea and iris
Posterior - between iris and lens
Vitreous
Aqueous humour
Secreted by the ciliary proccessus inside the ciliary body in the posterior chamber
Fills anterior and posterior chamber
Flows into the anterior chamber via the pupil
Provides nutrients to lens and cornea
Contributes to shape of eyeball with vitreous humour
Drains into the irido-corneal angle via the trabecular meshwork into the canal of Schlemm back to venous circulation
Aqeous humour in elderly
Drainage of aq humour can become obstructed
Increases intraocular pressure
Can cause glaucoma if untreates
Glaucoma
Irreversible damage and death of optic nerve fibres leading to impairment of vision and blindness
Chronic open angle glaucoma
Most common
Painless
Insidious
The trabecular meshwork deteriorates due to age decreasing aq humour drainage.
Increased intra-ocular pressure causes disc cupping and loss of peripheral vision gradually.
Acute closed angle glaucoma
Irido-corneal angle decreases
Trabecular meshwork is blocked leading to increased intra-ocular pressure
Medical emergency as sight threatening in hours
Acutely painful red eye
Seen in 55+ yr old patients [common = 70 - 80 yr olds]
Irregular oval shaped pupil - sluggish and dilated
Blurry vision
Halos around objects due to corneal oedema
Nausea and vomiting
Eye is hard and tender to palpate through upper eyelid
How to treat acute closed angle glaucoma
Diuretics - reduce aq humour production
Muscarinic eye drops - pupillary constriction which opens the irdo-corneal angle
Strong analgesia
Laser eye surgery - iridotomy
Risk factor for closed angle glaucoma
Long sited
Middle aged
Shallow chambers
