Week 11: Visual Field Defects

Question 1

What is vitreous humour?

Answer 1

viscous jelly like substance that lies between the lens and the retina; it keeps the eyes spherical

Question 2

What is refraction and when does it occur?

Answer 2

• 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

Question 3

What does accommodation mean?

Answer 3

lens changing shape to see closer images

Question 4

How does the lens change shape?

Answer 4

Due to contraction of ciliary muscles which relieves tension on the zonule fibres, allowing the lens to become rounder

Question 5

What does emmetropic mean?

Answer 5

• emmetropic eye = normal

- focuses parallel light rays on the retina without the need for accomodation

Question 6

What is hyperopia?

Answer 6

• 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

Question 7

In hyperopia, which lens do you need to correct vision?

Answer 7

convex

Question 8

What is myopia?

Answer 8

• nearsightedness
• when eyeball is too long
• light rays converge before retina

Question 9

In myopia, which lens do you need to correct vision?

Answer 9

concave

Question 10

How does corrective laser surgery, or photorefractive keratectomy, correct vision?

Answer 10

uses a laser to reshape the cornea and increase or decrease the amount of refraction possible

Question 11

What is the function of the pigmented epithelium in the retina?

Answer 11

• 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

Question 12

Which excitatory neurotransmitter do bipolar neurones release onto ganglion cells?

Answer 12

glutamate

Question 13

Explain phototransduction?

Answer 13

• 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

Question 14

In rods, what are photoreceptors called?

Answer 14

rhodopsin

Question 15

In cones, what are photoreceptors called?

Answer 15

• one of 3 different colour sensitive opsins

- red cones longest wavelength, then green then blue

Question 16

What is retinal?

Answer 16

• 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

Question 17

What are the two types of retinal bipolar cells?

Answer 17

1. ON bipolar cells depolarise in response to light

2. OFF bipolar cells hyperpolarise in response to light

Question 18

Which receptors are expressed by ON bipolar cells?

Answer 18

mGluR6 & TRPM1

Question 19

Which receptors are expressed by OFF bipolar cells?

Answer 19

AMPA & kainate receptors

Question 20

What happens in the ON/OFF pathway if we have medium intensity light that suddenly gets brighter?

Answer 20

• 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

Question 21

What is the receptive field?

Answer 21

the area of the retina that causes any change in response of a neurone

Question 22

Why do we have a surrounding receptive field?

Answer 22

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

Question 23

What is Young-Helmholtz Trichromatic Theory

Answer 23

“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.

Question 24

What is colour opponency?

Answer 24

• 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

Question 25

What are the two opponent pathways?

Answer 25

1. Red & Green | 2. Blue & yellow

Question 26

What is melanopsin and what is its function?

Answer 26

- 5th photopigment - melanopsin is sensitive to light across the extent of the retina and directly activates ganglion cells - encodes luminance

Question 27

Where is melanopsin expressed?

Answer 27

ipRGCs

Question 28

What are ipRCGs required for?

Answer 28

- for normal photoentrainment of the circadian clock | - pupillary light reflex

Question 29

What are the 3 layers of the lateral geniculate nucleus and what do each contain?

Answer 29

1. Parvocellular layer 3-6 --> contains small cell bodies 2. Magnocellular layers 1+2 --> contains large cell bodies 3. Koniocellular layers --> in between layers with very small cell bodies

Question 30

What are the 3 types of retinal ganglion cells? (classified based on which layers they project to)

Answer 30

1. Magnocellular (M-type) - larger cell type - 5% of population - large receptive field - important for detection of stimulus movement 2. Parvocellular (P-type) - smaller cell type - 90% of population - sensitive to stimulus form and fine detail 3. Non-M non-P (K-type) - medium cell type

Question 31

How is input to the LGN from two eyes kept seperate?

Answer 31

layer 6 is ipsilateral layer 5 contralateral layer 4 ipsilateral

Question 32

What is the LGN target?

Answer 32

- primary visual cortex - also known as Brodmann's area 17 or striate cortex - located in the occipital lobe either side of calcarine sulcus

Question 33

How is the primary visual cortex organised?

Answer 33

- orientation columns - ocular dominance columns - colour processing "blobs" - arranged in 6 layers

Question 34

What is an orientation column?

Answer 34

column of the cortex that is connected together and all selected to the same orientation of the stimulus

Question 35

What is ocular dominance?

Answer 35

the inputs from the two eyes are largely seperate in V1

Question 36

What occurs at the 'blobs' in V1?

Answer 36

higher, more detailed levels of Colour opponency | colour processing

Question 37

What is meant by blurred vision?

Answer 37

- vision is out of focus/not sharp - no distortion / nothing missing - no shadows - not bent - result of a refractive or structural problem

Question 38

What is glare?

Answer 38

- difficulty seeing in bright light | - corneal or lens problem, often due to cataract

Question 39

Which conditions affect the retina?

Answer 39

- wet macular degeneration - macular hole - macular pucker - retinal detachment

Question 40

What could things looking pale be due to?

Answer 40

1. optic nerve diseases - optic neuritis, compressive optic nerve disease, tumours along the visual pathway 2. conditions affecting the retina

Question 41

What is a 'floater?

Answer 41

- smudge/ cobweb seen in vision - typically moves - vitreous haemorrhage/ posterior vitreous detachment

Question 42

If a patient has a homonymous field defect, what is this a sign of?

Answer 42

defect of visual pathways

Question 43

If a patient has a heteronymous field defect, what is this a sign of?

Answer 43

retinal, optic nerve defect

Question 44

If you have a centre visual field defect, where is the lesion likely to be?

Answer 44

macular

Question 45

What does a cecocentral field defect mean?

Answer 45

central defect joins in with blind spot

Question 46

An arcuate visual field defect (an arc) is typical of what condition/

Answer 46

glaucoma

Question 47

What is cataracts?

Answer 47

- opacity (cloudy) area of the lens - common ageing change - examine against red reflex or using slit lamp

Question 48

What are some symptoms of cataracts?

Answer 48

- blurred vision - glare - change in refraction - faded colours - trouble with bright light - trouble seeing at night

Question 49

Explain the blood supply to a healthy retina

Answer 49

- choroid supplies outer 2/3rds of retina, vascular supplies inner 1/3rd - requires diffusion from choroid

Question 50

What is the function of retinal pigment epithelium (RPE)

Answer 50

- maintains environments of photoreceptors - removes waste product from cones and rods - reduce function leads to drusen

Question 51

What is ARMD?

Answer 51

Dry age-related macular degeneration: | build up of waste productions from rods and cones

Question 52

What is Drusen?

Answer 52

small white/yellow deposits

Question 53

What are the signs of Dry ARMD?

Answer 53

- drusen - RPE pigmentation - RPE atrophy - gradual deterioration - particularly affects reading vision

Question 54

What does a photoextraction with lens implant entail?

Answer 54

- 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%)

Question 55

What is CRAO?

Answer 55

Central retinal artery occlusion

Question 56

What are 3 potential cases of CRAO?

Answer 56

1. Embolic e.g carotid artery disease 2. Giant cell arteritis/ temporal arteritis 3. Carotid artery disease

Question 57

What is metamorphosia?

Answer 57

- liner objects look round or curved - symptom of distortion of the retina - distorted vision/ 'change in form'

Question 58

What is Wet ARMD?

Answer 58

- layer between choroid and RPE thins so vessel moves under retina causing exudate and bleeding - rapid loss of vision

Question 59

How do we treat wet ARMD?

Answer 59

- treatment of choroidal neovascular membranes with intravitreal injection of antiVEGF - vascular endothelial growth factor (VEGF) stimulates growth of CNM - vision loss can be revered