visual system Flashcards
What is the anatomy of the external eye?
-Pupil in centre, iris around it, sclera (white), lateral and median canthus, caruncle medially, upper/lower eyelid, palpebral fissure above. Limbus (border between cornea & sclera)
What is the lacrimal system? How does it work?
-Tear produced by lacrimal gland over eye, drains medially via 2 puncta to open to medial lid margin, flow via superior & inferior canaliculi to reach tear sac where they exit via duct to nasal cavity
What is the basis of tears? What are the afferent, efferent and neurotransmitter involved?
- Tears are basal, reflex and emotional.
- Afferent is cornea, cranial nerve V1 (trigeminal ophthalmic).
- Efferent is parasympathetic with acetylcholine neurotransmitter
What are the functions of the tear film?
- Tear film is most superficial part of eye.
- Maintains smooth cornea air surface & provides oxygen to cornea (avascular), removal of debris & releases bactericide
What is the structure of the tear film and their respective functions?
-3 layers: superficial lipid layer (reduces tear film evaporation), water layer (thickest - tear gland), mucinous layer (keeps tear film close to eye, maintains surface wetting)
What is conjunctiva? Where is it? What is it nourished by? When do these become visible?
- Conjuctiva is the thin transparent tissue covering outer surface of eye.
- Begins at outer edge of cornea, covers visible part of eye & lines inside of eyelids.
- Nourished by tiny blood vessels that aren’t visible except in conjuctivitis
What are the 3 layers of the coat of the eye from superficial to deep + what are they?
Sclera (hard and opaque), choroid (pigmented & vascular), retina (neurosensory tissue)
What is the sclera? Its structure? Its function?
- White part of the eye, tough and opaque.
- When it comes to front of eye seamlessly changes into cornea.
- Function is outer protective coat with high water content
What is the cornea? Its function?
- Cornea is transparent dome shaped window covering front of eye.
- protection of eye, contributes to refractive power, clear window to look through (focuses light on retina)
What is the structure of the cornea (layers) + their functions?
5 layers:
- epithelium
- bowman’s membrane
- stroma (collagen fibres for transparency)
- descemet’s membrane
- endothelium (pumps fluid out of cornea preventing corneal oedema)
Does the cornea have blood vessels? How does cornea get oxygen & glucose?
- Cornea has no blood vessels.
- Oxygen from air and glucose produced by fluid sitting between iris & cornea and is absorbed by endothelium
What happens when you hydrate the cornea?
becomes white
What is the uvea? What happens in disease of one part?
- Uvea is the vascular coat of eyeball that lies between sclera and retina.
- Has 3 parts: iris, ciliary body and choroid.
- These are connected so disease of one affects other portions too ( to different degrees)
Where is the choroid and what is it composed of?
- Lies between retina and sclera.
- Made of layers of blood vessels that nourish the back of the eye
What does the iris do and how?
Iris controls light levels inside the eye, has tiny muscles that dilate and constrict pupil size
What is the structure of the lens? What are its functions?
- Has outer acellular capsule and regular inner elongated cell fibres.
- Function is to accommodate for long and near distance (focus), transparency, focusing power, refractive power, elasticity.
What is the retina? What is its function?
- Retina is very thin layer in inner part of eye.
- Captures light rays, which are then sent to brain for processing via optic nerve
Where is optic nerve? What is its function? Why is it blind spot of eye?
- Nerve that transmits electrical impulses from retina to brain.
- Connects to back of eye near macula, with visible part being optic disc.
- Blind spot of eye because where it meets retina there are no light sensitive cells
Where is the macula? What is its function?
- Macula is in centre of retina, temporal to optic nerve.
- Small & sensitive part of retina responsible for detailed central vision.
- To appreciate detail & perform tasks like reading requiring central vision
What is in the centre of the macula? What is its structure and what does it do?
-Fovea is centre of macula, most sensitive part with highest concentration of cones (to perceive in detail) and low concentration of rods
What is central vision for? What is it assessed by? What is another name for it & what does loss of this vision lead to?
- central vision for detailed day vision, colour vision, reading, face recognition.
- foveal vision
- Assessed by visual acuity test.
- Foveal vision loss leads to poor visual acuity
What is peripheral vision? What is it assessed by? If we lose this vision what happens?
- Peripheral vision for shape, movement, night vision, navigation.
- Assessed by visual field assessment.
- Loss of visual field inability to navigate (may need stick)
What is the structure of the retina (layers) and their functions?
- Retinal outer layer has photoreceptors (1st order neurone to detect light).
- Middle layer has bipolar cells (2nd order neurone) for local signal processing to improve contrast sensitivity & regulate sensitivity.
- Inner layers has retinal ganglion cells (3rd order neurone) for transmission of signal from eye to brain (via optic nerve)
What are the 2 types of photoreceptors?
rods & cones
What are rods, features and what are they responsible for?
- Rods have longer outer segment with photosensitive pigment, more sensitive to light, slow response to light, responsible for night vision (scotopic).
- More numerous
What are cones, features and what are they responsible for?
- Cones are less sensitive to light but faster response, needed for day light vision and colour vision.
- Less numerous
What is concentration of photoreceptors in fovea, periphery and blindspot respectively?
- Fovea: highest concentration of cones.
- Periphery highest concentration of rods.
- Blindspot no photoreceptors
What is rod (scotopic) vision?
Peripheral & night vision, more pigment, higher spatial & temporal summation, recognises motion
What is cone (photopic vision)?
Central & day vision, recognises colour & detail
What are different cones sensitive to? What about rods?
Different cones sensitive to one wavelength of light.
S cones (blue), M cones (green) L cones (red).
-Rods not really sensitive to any colour (night vision)
What is deuteranomaly?
Not completely colour blind but cant perceive red
What is achromatopsia?
Total colour blindness
What is the colour blindness test and what control is used?
Ishihara test. Number 25 is control seen by everyone
What is refraction? What happens when light goes from one medium to another?
- Light passing from one medium to another.
- As light moves from one medium to another velocity changes and thus path/direction
What is the refractive index? What value do we get?
Refractive index = speed of light in vacuum/ speed of light in medium.
-Always value of 1 or more because speed of light in vacuum is fastest
What is angle of reflection? Angle of refraction?
Angle of reflection = angle of incidence.
Angle of refraction is either smaller or bigger than angle of incidence depending on direction of light
What is a convex lens? Use?
Convex lens (converging lens) takes light rays and brings them to focal point at even distance to central plane of lens. -Eye is convex lens and so is camera.
What is a concave lens?
Concave lens takes light rays and spreads them outwards
What is emmetropia?
Perfect eye - adequate correlation between axial length & refractive power.
-Parallel light rays fall on retina without need for accomodation
What is ametropia?
Mismatch between axial length & refractive power. Parallel light doesn’t fall on retina with no accomodation
What is myopia? What happens to the rays? What are potential causes? What are symptoms?
- Parallel rays converge at focal point anterior to retina.
- Causes: excessive long globe (axial myopia) or excess refractive power (refractive myopia).
- Symptoms: blurred distance vision, squinting to improve, headache
What is treatment for myopia?
- correction with diverging lens (negative lens to push focus to back of eye)
- contact lens
- correction by removing lens to reduce refractive power of eye
What is hyperopia? What happens to the rays? What are potential causes?
- Parallel rays converge at focal point posterior to retina.
- Causes: excessive short globe (axial hyperopia) or insufficient refractive power (refractive hyperopia)
What are symptoms of hyperopia? What is amblyopia?
- Symptoms: visual acuity at near is blurry, asthenopic symptoms –> eye pain, headache in front, burning in eyes, blepharoconjuctivitis.
- Amblyopia is uncorrected hyperopia >5D in one eye (treat quickly in kids to prevent learning difficulties)
What is treatment for hyperopia?
- correction with converging lens (positive lens)
- correction with positive lens + cataract extraction
- contact lens
- intraocular lens
What is astigmatism? What are symptoms ?
- Parallel rays focus in 2 focal lines so 2 focal points instead of one.
- Cause: refractive media not spherical so refract differently on different meridians causing more than 2 focal points (cornea has different shape), image slightly unfocused (blurry).
- Symptoms: headache, eye pain, blurred vision, distorted vision, head tilting + turning
What is treatment for astigmatism?
- For regular astigmatism cylinder lenses with or without spherical lenses (convex or concave), surgery.
- Irregular astigmatism rigid cylinder lenses, surgery
What is presbyopia?
Naturally occurring loss of accommodation for near objects with age, but distant vision intact
What is treatment for presbyopia?
Convex lenses in near vision (reading glasses, bifocal glasses, trifocal glasses, progressive power glasses)
What is correction for presbyopia
- spectical lenses (monofocal - spherical cylindrical) or multifocal lenses
- contact lenses (create higher quality of optical images and less influence on size of retinal image than spectacle lenses
What are different types of optical correction?
contact lens, intraocular lens, surgical correction
What are contact lenses and their disavantages?
Need to be clean & disinfect, expensive, complications include infectious keratitis, giant papillary conjuctivitis, corneal vascularisation, severe chronic conjuctivitis
What are intraocular lenses? Advantages? Disadvantages?
- Artificial lens implanted in eye, replacement of cataract lens.
- Gives best optical correction for aphakia, avoid magnification & distortion of spectacle lenses.
- Problem is monofocal lenses so when cattaract surgery we lose near vision
What is cataract?
Cloudy area in lens of eye leading to less vision
What are options for surgical correction?
- Keratorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty.
- intraocular surgery - clear lens extraction, phakik IOL
What is process of surgical lasik surgery? Purpose?
Cutting corneal flap, flipping corneal flap, photorefractive treatment with laser, corneal stroma reshaped, corneal flap put back
What is process of cataract extraction?
-Remove natural lens with ultrasound + insert artificial intraocular lens in its place (clear lens extraction + artificial intraocular lens)
What is ICL (implantable contact lens) process and why? (staar intra-collamer lens)
- To correct for myopia & astigmatism.
- Add special ICL lens on top of natural lens.
- If cataract natural lens will become opaque so will have to remove them
What is the near response triad (adaptation for near vision)?
- Adaptation for near vision.
1. pupillary miosis (sphincter pupillae) to increase depth of vision
2. convergence (medial recti both eyes) to align both eyes towards near object
3. accomodation (circular cilliary muscle) to increase refractive power of lens for near vision
From the eye how does information go to the primary visual cortex?
Optic nerve (ganglion nerve fibres) - optic chiasm (some cross) - optic tract (ganglion fibres exit as optic tract) - lateral geniculate nucleus in thalamus (synapse) -> optic radiation (4th order neuron) - primary visual cortex or striate cortex
What are 1st, 2nd 3rd order neurones and where do they send signals?
- 1st order neurones: rod & cone retinal photoreceptors.
- 2nd order neurones: retinal bipolar cells.
- 3rd order neurones (retinal ganglion cells - partial decussation at chiasm)
What fibres cross at the optic chiasm?
- Fibres coming from nasal retina (responsible for temporal visual field).
- Uncrossed fibres come from temporal retina (responsible for nasal visual field)
What do lesions anterior to chiasm affect? What do lesions posterior to chiasm affect?
- Lesions anterior to chiasm affect visual field of one eye only.
- Lesions posterior to chiasm affect visual field in both eyes
What does a lesion at the optic chiasm cause and why?
Damages crossed fibres from nasal retina of both eyes so lose temporal visual field - bitemporal hemianopia
What would a right sided lesion posterior to the optic chiasm cause and why? What would a left sided lesion posterior to optic chiasm cause and why?
- Right sided lesion posterior to optic chiasm would cause left homonymous hemianopia in both eyes (in right part of eyes).
- Left sided lesion would cause right homonymous hemianopia.
What would dissection of optic nerve cause?
Monocular blindness (one eye completely blind)
what would lesion crossing optic chiasm cause?
bitemporal hemianopia
What would damage to right non-crossing fibres of eye lead to?
Right nasal hemianopia (temporal part of eye)
What would damage to optic tract lead to? If on right or left?
- Homonymous hemianopia.
- If right left homonymous hemianopia
When do you get homonymous hemianopia with macular sparing and why? Differentials?
- Damage to primary visual cortex usually due to stroke leads to contralateral homonymous hemainopia with macular sparing because area representing macula receives dual blood supply from posterior cerebral arteries from both sides so is spared.
- If it doesn’t cross vertical midline usually neurological, if it doesn’t cross horizontal midline usually glaucoma
What is usually cause of homonymous hemianopia?
stroke
What is usual cause of bitemporal hemianopia?
Pituitary gland enlargement due to tumour
When does pupillary constriction happen? Why? What is it mediated by?
- When too much light.
- Pupillary constriction decreases spherical aberrations & glare, increases depth of field, reduces bleaching of photo-pigment, protect eye from damage.
- Mediated by parasympathetic nerve within oculomotor nerve causing circular muscles to contract
When does pupillary dilation happen? Why? What is it mediated by?
In dark. Increases light sensitivity allowing more light in.
-sympathetic nerve causing radial muscles to contract
What is the afferent pathway of pupillary reflex?
- Rod & cone photoreceptors synapse on bipolar cells and then retinal ganglion cells.
- Fibres cross at chiasm to go both sides. Exit at optic tract before entering geniculate nucleus.
- Each afferent pathway synapses on edinger-westphal nuclei on both sides of brainstem.
What is efferent pathway of pupillary reflex?
From edinger westphal nucleus go with oculomotor nerve which synapses at ciliary ganglion and with short posterior ciliary nerve to pupillary sphincter.
-Efferent common for both eyes
What is direct light reflex? What is consensual light reflex + basis?
Direct is constriction of pupil in stimulated eye. Consensual is constriction of pupil on not stimulated eye ( because afferent pathway on either side stimulates efferent pathway on both sides)
What happens if have right afferent defect (eg damage to right optic nerve)?
- No pupil constriction in either eye when right pupil stimulated.
- Constriction in both eyes when left stimulated
What happens if have right efferent defect (eg damage to right oculomotor nerve)?
No constriction in right eye on either case.
-Left pupil constricts always
What test is done to test for pupillary reflex on both sides?
swinging torch test
Why is voluntary/involuntary movement of the eye necessary?
tracking stimuli
How many extraocular + cranial nerves innervating them?
6 extraocular muscles by 3 cranial nerves (oculomotor, trochlear, abducens)
What is duction, version, vergence, convergence?
- Duction - one eye moves.
- Version - both eyes move to same direction.
- Vergence: both eyes move to different directions.
- Convergence: both eyes move simultaneously to adduct (inward movemenet) when both eyes viewing near object
What Is saccade & when is it used? What is smooth pursuit and when is it used?
- Saccade is short fast burst of movement.
- Used in reflexive, scanning, predictive, memory guided movement.
- Smooth pursuit is slower movement driven by motion of moving target across retina
What are the extraocular muscles of the eye and their actions?
Lateral rectus moves eye laterally.
Medial rectus moves eye medially.
Superior rectus moves eye up and medially.
Inferior rectus moves eye down and medially.
Superior oblique moves eye down & out.
Inferior oblique moves eye up and out.
What does the oculomotor nerve innervate?
- Superior branch innervates superior rectus & levator palpebrae superioris (raises eyelid).
- Inferior branch innervates inferior rectus, medial rectus, inferior oblique, parasympathetic to constrict pupil
What does the trochlear nerve innervate?
Superior oblique
What does the abducens nerve innervate?
lateral rectus
How do you test the extraocular muscles?
- Isolate muscle to be tested by maximising its action versus action of other muscles.
- For lateral rectus look laterally.
- For medial rectus look medially.
- For superior rectus look up and out.
- For inferior rectus look down and out.
- For superior oblique look down and in.
- for inferior oblique look up and in
What is action of moving eye up and down, right and left, and rotating eye around?
Eye up: supraduction or supraversion.
Eye down: infraduction, infraversion.
Right - dextroversion, left levoversion.
Rotate - tortion
What is 3rd nerve palsy and what do you see and why?
-Affected eye is down and out (unopposed action of superior oblique and lateral rectus) + droopy eyelid (no levator palpebrae superioris)
What is 6th nerve palsy and what do you see and why?
- Affected eye cannot abduct so deviated inward because of loss of lateral rectus.
- Double vision worsens on gazing to side of affected eye (abudencs nerve)
What is optokinetic nystaglus reflex? When is it useful?
- Nystagmus is oscillatory eye movement.
- Its for smooth pursuit & fast phase reset saccade.
- Useful in testing visual acuity in pre-verbal children by observing presence of nystagmus in response to moving grating patterns of various spacial frequencies.
- Presence of nystagmus here shows that they have sufficient level of visual acuity to perceive grating pattern
What does right oculomotor nerve (3rd nerve palsy) present with?
-Right eye deviated down & out (unopposed superior oblique & lateral rectus), cant open right eye (ptosis due to failure of levator palpebrae superioris), cannot elevate or adduct right eye (damaged medial and superior rectus) -Right pupil dilated comparison to left but normal direct & consensual light response (under-action of pns carried by oculomotor nerve)
What can cause oculomotor nerve damage? when is pupil spared and when not?
- medical lesions: those affecting vasculature to nerve usually don’t affect pupil (pupil sparing) because pns fibres are in outer portion of nerve (microvascular disease due to hypertension, diabetes mellitus).
- surgical lesions - posterior communicating artery aneurysm that compresses outer portion of nerve fibres so compresses pns too. (pupil affected)
Treatment for oculomotor damage?
If cause is posterior communicating artery aneurysm assess aneurism (may involve clipping).
If microvascular disease need better metabolic control of condition.
What does adies syndrome present with?
- Tonically dilated pupil in one eye (damage in pns ciliary ganglion involved in efferent reflex. This is where oculomotor nerve synapses before going to iris).
- Pupil slow to constrict compared to left in direct reflex.
- Pilocarpine drops cause rapid constriction of right pupil (muscarinic agonist on M3 receptors in iris sphincter muscle results in contraction and constriction of pupil, independent of pns).
- Absent knee jerk reflex & impaired sweating (damage in dorsal root ganglia of spinal cord).
- Get more meiosis with near accomodation than with light because light-near dissociation due to abberant re-innervation, due to damage of ciliary ganglion, causing upregulation of post-synaptic receptors causing fibres directed to ciliary body to target iris instead)
