Eyes Flashcards
from which embryonic structure are eyes derived?
from the diencephalic part of neural tube
what are the three main layers of the eye?
fibrous layer
vascular layer
sensory layer
what are the components of the fibrous layer of the eye? how much of the outer layer do each of them make up?
sclera (posterior 5/6)
cornea (anterior 1/6)
what are the components of the vascular layer of the eye?
choroid
ciliary body
iris
what is the sensory layer of the eye made up of?
retina
what is the cut off point of the retina called, in the anterior segment of the eye?
ora serrata
what are the two functions of the ciliary body?
accomodation - changing shape of crystalline lens
secretion of acqueous humour
what is vitreous humour, where is it found and what is its function?
it’s a gel found in the posterior segment of the eye, its function is to provide cushioning for the retina
what is the normal intraocular pressure?
21mmHg
what structures attach the crystalline lens of the eye to the ciliary body?
zonules
what muscle allows the ciliary body of the eye to contract?
ciliaris muscle
how is the ciliaris muscle innervated?
through oculomotor nerve (CNIII)
what are the five layers of the cornea of the eye?
- non-keratinised stratified squamous epithelium
- base membrane: Bowman’s membrane
- stroma
- Descemet’s membrane
- single endothelial layer
what is the clinical significance of Bowman’s membrane in the cornea of the eye?
if cornea is scratched/damaged superficially with Bowman’s membrane left intact = no lasting damage, epithelium will heal
if cornea is scratched more deeply with a disruption of Bowman’s membrane = scarring and opacity
what is the clinical significance of the endothelial layer of the cornea?
if its cell count decreases (from 2500cells/mm2), there aren’t enough pumps to keep out acqueous humour. result: acqueous humour in cornea, cornea becomes opaque
what is the main pathology behind corneal opacity of the eyes?
acqueous humour in cornea, caused by a weakening of the corneal endothelium
where is acqueous humour reabsorbed in the eye?
reabsorbed by trabecular meshwork in the angle of anterior chamber and shunted into Schlemm’s canal, before joining episcleral veins
what makes corneal transplants easier compared to other organ transplants, and why?
there is no risk of organ rejection, since the cornea is avascular
what makes the cornea transparent?
very neatly arranged collagen fibres in the corneal stroma;
lack of blood vessels;
endothelial pump keeping out acqueous humour
what is the normal amount of epithelial cells in the endothelial layer of the cornea in the eye?
2500 cells/mm2
what is Schlemm’s canal in the eye?
a tube going all the way round the cornea beyond the angle of the anterior chamber, draining away acqueous fluid
how is the eye subdivided in regions, and using which landmarks?
lens: anterior and posterior segments
anterior segment further subdivided by iris - anterior and posterior chamber
where does the lacrimal gland sit and why?
above eye on temporal side of orbit. if it was on the nasal side, tears would drain into the lacrimal sac straightaway without going over the cornea
where is the lacrimal sac and where does it drain?
it sits between the orbit and the nasal cavity
drains into the inferior meatus
which nerve supplies parasympathetic fibres to the lacrimary gland above the eye?
facial nerve (CNVII)
where is the fovea centralis found in the retinal layer of the eye?
directly at the back opposite the lens on horizontal plane
what is the structure and function of the fovea centralis in vision?
it’s a depression in the retina with increased exposure to rods/cones, so allows for more sensory input into the optic nerve axons = better focus
which type of photosensory cells are packed close together at the fovea centralis in the eye?
cones
which photosensory cells are more sensitive to low lighting?
rods
how many layers make up the retina?
10 layers
which are the most important layers of the retina involved with vision?
- nerve cell layer (ganglion cells) - receives sensory information and relays it to brain as CNII
- rods/cones layer (photosensory cell layer) - picks up sensory information
- pigment epithelial layer - support for rods/cones layer)
what is the crystalline lens of the eye made up of?
neatly arranged collagen fibres which allow for transparency
what is the blood supply for the retina of the eye?
outer layers - supplied by choroid vasculature
inner layers - supplied by central retinal artery
what is the shape of the crystalline lens in the eye?
biconvex
what are the muscles of the eye involved with accommodation? what are their actions?
constrictor pupillae (circular) - constricts pupil dilator pupillae (radial) - dilates pupil ciliaris - changes lens shape
what three actions take place in the eye during accommodation?
- lens changes shape
- pupil constricts
- eyes converge
what is the main pathology behind cataracts?
disruption of collagen fibres of lens in the long term, without the ability of the tissue to repair itself as it’s avascular
what is an important factor in cataract development?
lens absorption of UV rays
what is the tarsal plate in the eye, and what is its function?
it’s a rigid plate found in eyelids, which keep the LPS from scrunching up the eyelids
what is the meibomian gland in the eyelid, and what is its purpose?
gland in inner eyelid secreting oil, which covers tear film on top of conjunctiva and keeps it moist (stops it from evaporating
where are the meibomian glands found?
just inside the conjunctival mucous membrane
what is the purpose of the conjunctiva?
secreting oil for moistening the eye (meibomian gland)
contributing to tear film
providing immune surveillance
stopping pathogens reaching eyeball
what are the layers of the tear film on top of the conjunctiva of the eye?
oily layer
acqueous layer
mucous layer
what is the purpose of the tear film on top of the conjunctiva of the eye?
remove debris
maintain cornea moist
immune surveillance
keep smooth surface for light refraction
what prompts blinking the eyes?
the acqueous layer of the tear film evaporating, causing the mucous and oily layers to touch and break up the acqueous layer
what is the cut off time for normal and abnormal tear film break up time during blinking?
less than 10 seconds - normal
more than 10 seconds - abnormal
what is the concept of refraction when talking about vision?
it’s the ability of light waves to bend when they reach the cornea and lens of the eye
which structures in the eye allow for refraction, and which one is more efficient at bending light?
cornea and lens
cornea more powerful
what are the ways light can be refracted ? through what shapes do they need to pass in order to bend that way?
- light rays converge when they go through a biconvex shape
- light rays diverge when they go through a biconcave shape
what is the concept of accommodation when talking about vision?
it’s the eye’s ability to focus on from far away to up close, by adapting the angle at which light is refracted by the cornea and lens
when does accommodation occur in vision?
accommodation occurs when the eye needs to focus on something close by
what is the result of accommodation on the light rays entering the eye?
they are bent at a higher angle
why does seeing things close up require accommodation?
because at close range the light rays are divergent and don’t enter the lens directly, so in order to focus the image on the retina they have to be bent further
why is accommodation not normally needed when looking at things in the distance?
because light rays from objects at a distance hit the lens directly (horizontally), so there is no need to bend them further to create an image on the retina
what is the pathology behind myopia?
eyeball slightly too long or cornea too thick
light rays converging fall short of the retina and don’t make a clear image on the retina
what is the pathology behind hyperopia?
eyeball slightly too long or cornea too flat
light rays converge too far behind the retina and individual accommodates in the distance, so has no accommodation power left when things are at close range
what type of lenses are needed for myopia and hyperopia?
myopia - biconcave lenses
hyperopia - biconvex lenses
what are the components of the visual pathway from the point of light waves reaching the eye?
- convergence of light waves on retina
- rods/cones info relayed to nerve fibres
- optic nerve fibres enter optic canal
- right and left optic canals meet at optic chiasm
- nasal fibres crossover and join the temporal fibres on opposite side
- radiation of fibres synapse at thalamus
- fibres from thalamus reach the primary visual cortex in the occipital lobe
what are the muscles that move the eye?
Recti muscles (lateral, medial, superior, inferior) Oblique muscles (superior and inferior)
what is the nerve supply to the lateral rectus?
cranial nerve VI (abducens)
what is the nerve supply to the superior oblique muscle in the eye?
cranial nerve IV (trochlear)
what are the actions of the medial rectus on the eye?
Adduction
what are the actions of the lateral rectus muscle on the eye?
Abduction
what is the common action of all recti muscles on the eye except one? what is the exception?
All recti muscles are adductors, except the lateral rectus which is an abductor
what is the common action of the oblique muscles in the eye?
abduction
which eye muscles cause elevation of the eye?
superior rectus
inferior oblique
which eye muscles cause depression of the eye?
inferior rectus
superior oblique
what are the actions of the superior rectus muscle on the eye?
elevation
adduction
intortion
what are the actions of the inferior rectus muscle on the eye?
depression
adduction
extortion
what are the actions of the superior oblique muscle on the eye?
depression
abduction
intortion
what are the action of the inferior oblique muscle on the eye?
elevation
extortion
abduction
which eye muscles achieve abduction of the eye?
lateral rectus
superior oblique
inferior oblique
what does the mnemonic RADSIN mean?
Recti Adduct (lateral rectus exception = abducts) Superiors Intort
what is the effect of the ciliaris muscle contracting, with regards to lens shape in the eye?
contraction of ciliaris muscle causes zonules to grow lax - lens not taut, will become rounder and thicker
what type of squint can myopia cause?
divergent squint
what type of squint can hyperopia cause?
convergent squint
what is the pathology of astigmatism?
the cornea curving on more than one meridian - causes light to converge in different meridians = several blurry images
what are toric lenses?
they are cilindrical lenses for astigmatism, which are weighted so that they always sit the same - on the same meridian
what type of lens is needed to correct astigmatism?
cylindric lens
what is the pathology of presbiopia?
crystalline lens of the eye becomes less stretchy over the years, (damage, UV absorption, loss of collagen elasticity) - less able to change shape to see up close
what is an emmetrope?
an individual with perfect vision
what is the usual treatment for acute anterior uveitis?
topical steroids
pupil dilating drops
what is used to treat infective conjunctivitis?
topical antibiotics
what is used to treat allergic conjunctivitis?
topical antihistamines
remove allergen
when are systemic steroids necessary to treat acute red eye instead of topical steroids?
when the whole eye is involved - scleritis
when are systemic antibiotics necessary to treat acute red eye instead of topical antibiotics? why is that?
orbital cellulitis - because the whole orbit is involved, including other tissues
what are some of the common symptoms of acute red eye?
redness pain discharge grittiness photophobia loss of vision
what are important elements of eye examination that should be carried out if someone presents with acute red eye?
best corrected visual acuity visual fields fundoscopy slit lamp fluoroscine staining
what is the management of corneal abrasion?
topical antibiotics
analgesia
what are some conditions which cause acute red eye?
conjunctivitis (infective or allergic) corneal abrasion anterior uveitis acute angle closure glaucoma scleritis orbital cellulitis
why is anterior uveitis characterised by an abnormal shaped pupil?
because inflammation causes parts of the iris to get stuck to the cornea
what are the parts of the eye involved in anterior uveitis?
iris
ciliary body
cornea
what is a symptom often associated with anterior uveitis?
hypopyon formation in bottom of anterior chamber
what are two risk factors for developing acute closed angle glaucoma? why can they contribute to the condition?
hypermetrope (short eye and narrow angle)
elderly (thicker, less mobile lens)
what is the difference between closed angle and open angle glaucoma?
open angle - caused by abnormality in trabecular meshwork
closed angle - caused by impaired drainage of the acqueous humour into the anterior chamber
where does the acqueous humour distribute in open angle and closed angle glaucoma?
open angle - in the anterior chamber of the anterior segment (in front of iris)
closed angle - in the posterior chamber of the anterior segment (behind iris)
what are the most common eye conditions associated with gradual vision loss?
cataracts
glaucoma (open and closed angle, chronic or acute)
age-related macular degeneration (wet or dry)
diabetic retinopathy
what characterises dry AMD?
development of drusen between the choroid and the pigment epithelium
what characterises wet AMD?
development of vascular membranes between the pigment epithelium and the photoreceptor layer in the retina
which type of AMD is curable with anti-VEGF injections?
wet AMD
what is the purpose of anti-VEGF injections in AMD?
stop the formation and bleeding of vascular membranes which form between the pigment epithelium and the photoreceptor layer
what are the eye conditions which can be treated with anti-VEGF injections?
wet AMD
diabetic retinopathy
central retinal vein occlusion
what triggers the release of VEGF for the formation of new blood vessels?
hypoxia in nearby tissues
how is vision affected by glaucoma?
peripheral vision lost first, eventually will become tunnel vision
how is vision affected by AMD?
central vision distorted, peripheral vision unaffected
what are the most common causes of sudden vision loss?
- central retinal artery occlusion
- central retinal vein occlusion
- anterior ischaemic optic neuropathy (arteritic and non-arteritic)
what is a likely cause for central retinal vein occlusion and why?
atheroma in central retinal artery can press on vein and occlude it, as they both travel in the same sheath
what are the differences in presentation between central retinal artery and vein occlusion?
CRA - loss of afferent function, efferent function unaffected
CRV - oedema and blood leaking into retina
what are the types of anterior ischaemic optic neuropathy?
arteritic - giant cell arteritis
non-arteritic - atheroma
why can giant cell arteritis affect vision?
because it affects middle caliber blood vessels, which includes the posterior short ciliary arteries that supply the optic nerve
which blood vessels supply blood to the optic nerve?
short posterior ciliary arteries
how can giant cell arteritis be picked up on a blood test?
presence of raised ESR and CRP
what other symptoms can present in giant cell arteritis, on top of visual loss?
jaw pain when chewing
pain in scalp
headaches
loss of appetite
what is the treatment for cataracts?
lens removal with phacoemulsification and replacement with artificial lens
which visual chart is useful in detecting whether someone as age related macular degeneration? what does it show?
Amsler grid. shows whether there is distortion of central vision
what is the most common general pathology behind sudden visual loss?
vascular related abnormality
what is the general pathology behind cataracts?
gradual opacification of the crystalline lens
what is the main risk factor for cataracts?
age
what is the most significant consequence of glaucoma?
raised intracranial pressure
what is normal intraocular pressure?
21mmHg
what are the three main clinical signs of chronic glaucoma?
- visual field loss
- optic disc damage
- raised intraocular pressure
what are the treatment options for chronic glaucoma?
- medical: topical drops to reduce secretion of acqueous humour
- surgical: trabeculectomy to allow an alternative drainage route for the acqueous humour
what are the ligaments called that stop the eye abducting or adducting too much?
medial and lateral check ligaments
what is the suspensory ligament of the eye?
a sling of fibrous tissue that suspends the eyeball and prevents it from downward displacement
