Week 11: Visual Field Defects Flashcards
What is vitreous humour?
viscous jelly like substance that lies between the lens and the retina; it keeps the eyes spherical
What is refraction and when does it occur?
- bending of light ways
- occurs when light passes from one transparent medium e.g air to another e.g cornea
- light is slowed down as it moves from one media to another
- both the cornea and the lens perform refraction
What does accommodation mean?
lens changing shape to see closer images
How does the lens change shape?
Due to contraction of ciliary muscles which relieves tension on the zonule fibres, allowing the lens to become rounder
What does emmetropic mean?
- emmetropic eye = normal
- focuses parallel light rays on the retina without the need for accomodation
What is hyperopia?
- far sightedness
- when eyeball is too short
- light rays are focused behind the retina
- accommodation is needed for distant objects, and near objects cannot be brought into focus
In hyperopia, which lens do you need to correct vision?
convex
What is myopia?
- nearsightedness
- when eyeball is too long
- light rays converge before retina
In myopia, which lens do you need to correct vision?
concave
How does corrective laser surgery, or photorefractive keratectomy, correct vision?
uses a laser to reshape the cornea and increase or decrease the amount of refraction possible
What is the function of the pigmented epithelium in the retina?
- provides nutrients which allows photoreceptors to operate
- contains melanin which absorbs any light not absorbed by the retina
- improves resolution by stopping stray rays bouncing around in the eye
Which excitatory neurotransmitter do bipolar neurones release onto ganglion cells?
glutamate
Explain phototransduction?
- conversion of light energy into electrical signals
- in the dark photoreceptors are depolarised and continually release glutamate
- light causes depolarising channels to close, hyperpolarising the membrane potential and reducing glutamate release
In rods, what are photoreceptors called?
rhodopsin
In cones, what are photoreceptors called?
- one of 3 different colour sensitive opsins
- red cones longest wavelength, then green then blue
What is retinal?
- resides inside opsin proteins
- retinal is a form of VitA that is essential for vision
- when retinal is hit by light, it changes shape and activates the opsin molecule
- this allows phototransduction
What are the two types of retinal bipolar cells?
- ON bipolar cells depolarise in response to light
2. OFF bipolar cells hyperpolarise in response to light
Which receptors are expressed by ON bipolar cells?
mGluR6 & TRPM1
Which receptors are expressed by OFF bipolar cells?
AMPA & kainate receptors
What happens in the ON/OFF pathway if we have medium intensity light that suddenly gets brighter?
- cone is hyperpolarised
- on and off bipolar cells are attached to on and off ganglion cells
- so the off pathway decrease glutamate release onto the ganglion cell to decrease firing
What is the receptive field?
the area of the retina that causes any change in response of a neurone
Why do we have a surrounding receptive field?
due to lateral inhibition:
- comes from amacrine cells as they form inhibitory synapses with a ganglion cell
- this allows the retina to compare different things
What is Young-Helmholtz Trichromatic Theory
“any colour can be made of red, green or blue”
- at each point in the retina, there exists a cluster of 3 receptor types, each type being maximally sensitive to either red, green or blue
- when all cone types are equally active, we percieve white
- so within your eye are receptors that receive waves of light and translate them into one of three colours: blue, green, and red. These three colours can then be combined to create the entire visible spectrum of light as we see it.
What is colour opponency?
- colour is coded with an opponent process: two “colours” are compared with one colour reducing ganglion cell activity and the other increasing it
- this explains the colour wheel
What are the two opponent pathways?
- Red & Green
2. Blue & yellow
What is melanopsin and what is its function?
- 5th photopigment
- melanopsin is sensitive to light across the extent of the retina and directly activates ganglion cells
- encodes luminance
Where is melanopsin expressed?
ipRGCs
What are ipRCGs required for?
- for normal photoentrainment of the circadian clock
- pupillary light reflex
What are the 3 layers of the lateral geniculate nucleus and what do each contain?
- Parvocellular layer 3-6 –> contains small cell bodies
- Magnocellular layers 1+2 –> contains large cell bodies
- Koniocellular layers –> in between layers with very small cell bodies
What are the 3 types of retinal ganglion cells? (classified based on which layers they project to)
- Magnocellular (M-type)
- larger cell type
- 5% of population
- large receptive field
- important for detection of stimulus movement - Parvocellular (P-type)
- smaller cell type
- 90% of population
- sensitive to stimulus form and fine detail - Non-M non-P (K-type)
- medium cell type
How is input to the LGN from two eyes kept seperate?
layer 6 is ipsilateral
layer 5 contralateral
layer 4 ipsilateral
What is the LGN target?
- primary visual cortex
- also known as Brodmann’s area 17 or striate cortex
- located in the occipital lobe either side of calcarine sulcus
How is the primary visual cortex organised?
- orientation columns
- ocular dominance columns
- colour processing “blobs”
- arranged in 6 layers
What is an orientation column?
column of the cortex that is connected together and all selected to the same orientation of the stimulus
What is ocular dominance?
the inputs from the two eyes are largely seperate in V1
What occurs at the ‘blobs’ in V1?
higher, more detailed levels of Colour opponency
colour processing
What is meant by blurred vision?
- vision is out of focus/not sharp
- no distortion / nothing missing
- no shadows
- not bent
- result of a refractive or structural problem
What is glare?
- difficulty seeing in bright light
- corneal or lens problem, often due to cataract
Which conditions affect the retina?
- wet macular degeneration
- macular hole
- macular pucker
- retinal detachment
What could things looking pale be due to?
- optic nerve diseases - optic neuritis, compressive optic nerve disease, tumours along the visual pathway
- conditions affecting the retina
What is a ‘floater?
- smudge/ cobweb seen in vision
- typically moves
- vitreous haemorrhage/ posterior vitreous detachment
If a patient has a homonymous field defect, what is this a sign of?
defect of visual pathways
If a patient has a heteronymous field defect, what is this a sign of?
retinal, optic nerve defect
If you have a centre visual field defect, where is the lesion likely to be?
macular
What does a cecocentral field defect mean?
central defect joins in with blind spot
An arcuate visual field defect (an arc) is typical of what condition/
glaucoma
What is cataracts?
- opacity (cloudy) area of the lens
- common ageing change
- examine against red reflex or using slit lamp
What are some symptoms of cataracts?
- blurred vision
- glare
- change in refraction
- faded colours
- trouble with bright light
- trouble seeing at night
Explain the blood supply to a healthy retina
- choroid supplies outer 2/3rds of retina, vascular supplies inner 1/3rd
- requires diffusion from choroid
What is the function of retinal pigment epithelium (RPE)
- maintains environments of photoreceptors
- removes waste product from cones and rods
- reduce function leads to drusen
What is ARMD?
Dry age-related macular degeneration:
build up of waste productions from rods and cones
What is Drusen?
small white/yellow deposits
What are the signs of Dry ARMD?
- drusen
- RPE pigmentation
- RPE atrophy
- gradual deterioration
- particularly affects reading vision
What does a photoextraction with lens implant entail?
- cataract surgery
- usually local anaesthetic
- takes 10 mins
- eye may feel swollen but not painful
- topical steroid for 2 weeks
- risk of vision loss (1-2%)
What is CRAO?
Central retinal artery occlusion
What are 3 potential cases of CRAO?
- Embolic e.g carotid artery disease
- Giant cell arteritis/ temporal arteritis
- Carotid artery disease
What is metamorphosia?
- liner objects look round or curved
- symptom of distortion of the retina
- distorted vision/ ‘change in form’
What is Wet ARMD?
- layer between choroid and RPE thins so vessel moves under retina causing exudate and bleeding
- rapid loss of vision
How do we treat wet ARMD?
- treatment of choroidal neovascular membranes with intravitreal injection of antiVEGF
- vascular endothelial growth factor (VEGF) stimulates growth of CNM
- vision loss can be revered
