CASE 3 - visual pathways and defects, eyes, disability Flashcards
what controls the shape of the lens and is involved in aqueous humour production?
ciliary body
describe/draw the visual pathway
nerves from what side of the eye dessucate at the chiasm?
nasal aspect of eye
lesion at 1
left anopia
lesion at 2
bitemporal hemianopia
lesion at 3
right homonymous hemianopia
lesion at 4 (if just one division of radiation)
homonymous quadrantopia
lesion at 5
homonymous hemianopia with macula sparing
upper vs lower quadrantopia
upper - if radiations for temporal lobe
lower - if parietal lobe
PITS
what is seen if macula is damaged?
loss of central vision
what is seen in central scomata / macular degeneration?
what is seen in glaucoma and retinis pigmentosa?
what do the 2 optic nerves converge to form at the base of the brain?
optic chiasm
where do the optic tracts terminate?
lateral geniculate nucleus
a relatively small number of fibres leave the optic nerve before reaching the lateral geniculate nucleus, to terminate where? what are they involved in?
- terminate in the pretectal area and the superior colliculus
- involved in the mediation of the pupillary light reflex
from the lateral geniculate nucleus, where do third-order thalamocortical neurones project through?
the retrolenticular part of the internal capsule and form the optic radiation
where does the optic radiation terminate?
primary visual cortex of the occipital lobe
where is the primary visual cortex?
on the medial surface of the occipital lobe — the calcarine sulcus
what is the rest of the occipital lobe?
he visual association cortex — it is concerned with interpretation of visual images, recognition, depth perception and colour vision.
the lateral geniculate nucleus and visual cortex receives information relating to what?
the contralateral half of the visual field
what is accomodation?
= the prcoess in which the eyes see objects at different distances and maintain clear images of the objects by convergence and divergence of light
-ve vs +ve accomodation
-ve accomodation:
- maximises ability to see objects at a distance clearly
- the ciliary muscles are relaxed
- lens is stretched out
- fibres around the eye are tight
+ve accomodation
- maximises ability to see objects close to you in focus
- ciliary muscles tighten
- lens becomes rounder in shape
- fibres around the lens are relaxed
what is it called when the point of focus falls short of the retina and what problems does it cause?
myopia — hard to focus on distant objects
what is it called when the point of focus is behind the retina and what problems does it cause?
hypermetropia — hard to focus on nearby objects
concave vs convex glasses
concave — myopia
convex — hypermetropia
the cornea bends the light before is passes through what?
the aqueous humour
when is the light bent?
- by the cornea
- by the lens
bent twice therefore image upside down
what does the light pass through to get to the retina?
vitreous body
where is most of the light focused onto?
macula
what is myopia?
nearsighted — close objects clear, far objects blurry
when does myopia occur?
when the eyeball is too long or the cornea is too curved — light converges before retina
what kind of prescription is seen in myopia?
-ve
what is hypermetropia?
farsighted — close objects blurry, distant objects clear
what causes hypermetropia?
- eyeball shorter than usual — retina closer to pupil, causing light to travel past the retina
- cornea may be flat (the cornea should be curved to direct light onto the retina)
what kind of prescription is seen in hypermetropia?
+ve
what are zonular fibres and what affects them?
= fibres that suspend the lens in position during accommodation
- affected by ciliary muscle
- enable changes in lens shape for light focusing
what happens when the ciliary muscle contracts?
- releases tension on the lens caused by the zonular fibres
- causes the lens to become more SPHERICAL, adapting to short range focus
what happens when the ciliary muscles relax?
the zonular fibers become taut, flattening the lens, increasing long range focus.
label
the primary visual cortex receives afferents from where?
the lateral geniculate nucleus of the thalamus
damage to primary visual cortex vs visual association area
primary = blindness
association = deficits in visual interpretation and recognition
transduction signal from photons to reduction in glutamate release
- photon absorption — retinaldehyde (retinal)
- 11-cis-retinal converted to all-trans-retinal by light
- opsin activated
- G protein (transducin) dissociation
- alpha subunit interacts with cGMP phosphodiesterase
- hydrolyses cGMP to reduce local concentration of cGMP
- cGMP cation channels close
- reduced cation inflow
- hyperpolarisation
- reduction in glutamate release
what determines which wavelengths retinal absorbs?
opsin
is more glutamate released in dark or light?
dark
myopia vs hyperopia
> myopia = nearsighted. eyeball too long or curved therefore blurry image far away. concave lens needed. -ve prescription
> hyperopia = farsighted. eyeball shorter than usual or too flat. close image blurry. convex lens needed. +ve prescription
what is LASIK surgery?
- laser-assisted in situ keratomileusis
- this is done with 2 lasers, one to open up a thin flap in the surface of the cornea, and another to reshape the cornea underneath. cornea is flattened and the protective flap is then smoothed back over and stays in place without stitches
- faster recovery time than lasek - suitable for majority of people
what is LASEK surgery?
- laser-assisted subepithelial keratectomy
- surgeon creates an ultra thin flap only in the cornea’s outer protective cover (epithelium). they then use a laser to reshape the cornea, flattening its curve, and then replaces the epithelium
- more effective option if you have higher levels of myopia
- can correct the vision of people with thinner corneas or pre-existing medical condtions
- slightly smaller risk of dry eyes
what is PRK surgery?
- photorefractive keratectomy
- laser used to reshape cornea — laser used on the surface of the cornea, not underneath a corneal flap like in LASIK (LASIK has a shorter recovery period and less pain after
procedure)
what is strabismus?”
= a disorder in which both eyes do not line up in the same direction
= the 2 eyes do not align correctly, and the brain suppresses the image received from one of the eyes to avoid a perception of diplopia (‘double vision’)
what is esotropia?
inward turning
what is exotropia?
outward turning
what is hypertropia?
upward turning
what is hypotropia?
inward turning
what are some causes of strabismus?
- very bad hyperopia
- family history
- medical conditions eg. Down’s syndrome, stroke sufferer
> nerve damage
> muscles around eye don’t work properly
what are the treatment options for stabismus?
> patches or eye drops — these obscure the vision of the ‘good’ eye and force the brain to process the image from the affected eye
- drops used are dilating drops (usually atropine) — the dilation of the pupil causes the vision to become very blurry and as such the input from that eye is ignored by the brain
- treatment should be only used for 4-6 hours a day so as not to impact the development of the good eye
what can overpatching the eye cause?
reverse amblyopia
what is amblyopia?
= a condition whereby there is a reduction in visual acuity due to a problem with focussing in early childhood
- results in reduced acuity as the brain is not stimulated to develop correctly
- not necessarily an inherent problem with the eye itself
- affects about 1-5% of the population
- most commonly caused by strabismus (‘lazy eye’)
- also caused by refractive defects (eg. different shape eyes) and congenital cataracts
- it can be treated if detected early enough
- if it is detected late, then the neurological pathways required for the correct development of vision cannot be formed and there is a lifelong reduction of visual acuity
treatment for amblyopia
> strabismus — same as before (patches or eye drops)
refractive error — glasses
congenital cataracts — surgically. account for less than 3% of the total number of cases of amblyopia
- lens is removed and replaced with an artificial plastic lens
- although the artificial lens cannot adjust its focus like the normal lens, it provides a clear image, and glasses can be used for near and far vision
what is cataracts?
= leading cause of blindness in the world
- usually a condition of the elderly (65y<)
- nearly all patients older than 50 demonstrate some degree of degenerative lens changes
- in most cases, there is normal ageing changes in which progressive yellowing of the lens nucleus (nucleus sclerosis) and hydration of the lens cortex are seen
- prolonged exposure to uv light is cataractogenic
what is glaucoma? describe using aqueous humour passage and function
= a progressive loss of vision associated with elevated intraocular pressure
- aqueous humour is produced in the ciliary body and passes from the posterior chamber through the pupil to the anterior chamber
- pressure in the aqueous humour plays a crucial role in maintaining the shape of the eye
- as this pressure increases (due to INCREASED PRODUCTION or DECREASED DRAINAGE of aqueous humour), it compresses the retinal blood vessels, causing degeneration of the optic nerve
- the optic nerve axons are compressed, and vision is gradually lost from the periphery inward
- early detection and treatment with medications (eye drops) or surgery (trabeculectomy) to reduce intraocular pressure are essential
what is presbyopia?
= the gradual loss of your eye’s ability to focus on nearby ages, associated with ageing
what are the 2 types of pituitary tumour?
- hormone secreting — normally other symptoms patients seek help for (due to excessive secretion if hormones) before visual field defects
- non-hormone secreting — vision loss will occur prior to any other systematic symptoms
how can pituitary tumours affect vision?
- the pituitary gland is some distance from the optic chiasm
- therefore the tumour needs to be quite large before it can interfere with the chiasm
what is a relative afferent pupillary defect (aka. Marcus Gunn pupil)?
the pupil remains dilated despite exposure to bright lights, and the individual’s 2 pupils may become different sizes
what is a relative afferent pupillary defect indicative of?
a unilateral dysfunction in the optic nerve or dysfunction in the retina
what are some causes of a relative afferent pupillary defect?
> disease of retina — retinal detachment, retinal ischaemia
optic nerve disease, specially before the optic chiasm — optic neuritis, inflammation from MS
severe glaucoma — increased pressure causes damage to optic nerve
trauma
what are symptoms of a relative afferent pupillary defect?
> pupil will not constrict as much as unaffected eye in presence of bright light
retinal detachment — floaters, flashes of light, shadow in vision field
optic neuritis — pain with eye movement, loss of vision
glaucoma — headaches, nausea/vomiting, eye redness, blurred vision
treatment of relative afferent pupillary defect
> dependent on underling cause
optic neuritis — may not need treatment, oral steroids
glaucoma — eye drops, laser treatment, or surgery
retinal detachment — surgery
tumour — removal surgery, radiation, chemotherapy
describe the snellen test
- measures visual acuity
- tests our ability to discriminate letters and numbers at a viewing distance of 6 metres (20 feet)
- normal vision = 6/6
- first number = distance at which chart is viewed — 6 metres/20 feet
- second number = the distance at which a person with ideal vision can see a letter clearly
eg. 6/5 — you can see a line of letters from 6 metres, which someone with normal vision can only see when they are 5 metres away
describe the confrontation test
- involves having the patient looking directly at your eye or nose and testing each quadrant of their visual field by having them count the number of fingers that your are showing
- testing peripheral vision
describe automated perimetry
- uses a computer program to test an individual’s visual field
- patient sits and looks into a dome-shaped instrument
- patient looks at an object in the middle of the dome throughout the test
- there will be small flashes of light on the dome — patient presses a button when they see a flash of light
- the computer program then provides the doctor with a map of your visual field
describe the pupillary reflex test
does the pupil constrict in the presence of light
what is the optic nerve surrounded in?
oligodendrocytes NOT schwann cells
what forms the 3 stages of Piaget’s adaptation process?
assimilation, accommodation, equilibrium
what is the order of layers of the retina from outer to inner?
- retinal pigment epithelium
- receptor layer
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- retinal ganglion cell layer
- optic nerve layer
what type of photoreceptor have longer outer segments, more outer segment discs, and consequently have more photo pigment?
rods
what type of photoreceptor are less sensitive to light and require high (daylight) illumination levels?
cones
in transduction, what does opsin activate?
transducin
in transduction, what does the alpha subunit-GTP complex activate?
PDE
in transduction, the Na channels close due to what?
decreased concs of cGMP
the axons of the optic tract terminate in all of the following except what?
- lateral geniculate nucleus
- superior colliculus
- pretectum
- lentiform nucleus
- suprachiasmatic nucleus
lentiform nucleus
how many layers of cells are there in the lateral geniculate nucleus?
6
