Visual Fields

Question 1

what is the definition of visual fields

Answer 1

extent to which you can perceive around a central point of fixation

Question 2

what degrees of vision do humans have in the superior, inferior, nasal and temporal fields in each eye

Answer 2

60 degrees superior
60-70 degrees inferior
50 degrees nasal
90 degrees temporal

Question 3

why is lateral vision the widest field of view

Answer 3

less bone obstruction - orbital bone lies behind the plane of the cornea

Question 4

how many axons does the optic disc have

Answer 4

1.2M axons

Question 5

how many layers does the retina have

Answer 5

9 layers

Question 6

what are the inner 3 layers of the retina

Answer 6

ganglion cell layer
elastic layer
ABH (amacrine, bipolar, horizonal cells)

Question 7

what does the ABH layer in the retina do

Answer 7

amplify neuronal signals and converts them from chemical energy into electrical energy

Question 8

does the optic disc lie medially or laterally in the retina

Answer 8

medially

Question 9

what happens to the optic tract fibres as they exit the eye

Answer 9

generally, the fibres are split into inferior and superior portions, which rotate 90 degrees anticlockwise (if you’re facing the patient) as they travel to the LGN.

as the inferior (now lateral) fibres exit the LGN, they encounter the temporal horns of the lateral ventricles so they separate and travel around the gyrus

Question 10

what is the ‘visual backup’

Answer 10

macular fibres synapse with the medial lip of the occipital cortex which is perfused by the PCA and the occipital branch of the MCA meaning it has blood supply from 2 great arteries

these macula fibres are the highest quality images

Question 11

what is above and below the optic chiasm

Answer 11

above: rathkes pouch
below: pituitary

Question 12

what kind of lesion causes bitemporal hemianopia

Answer 12

full optic chiasm lesion - this is less common nowadays

Question 13

what kind of visual defect would a pituitary lesion partially compressing the optic tract cause

Answer 13

bitemporal superior quadrantinopia - as it compresses the inferior fibres of the optic chiasm which account for the superior aspect of vision

Question 14

what kind of visual defect would a lesion from Rathkes pouch partially compressing the optic tract cause

Answer 14

bitemporal inferior quadrantinopia - as it compresses the superior fibres of the optic chiasm which account for the inferior aspect of vision

Question 15

what are common complaints of partial compression of the optic chiasm, from pituitary causes and rathkes

Answer 15

pituitary: can’t see traffic lights when in car

Rathkes: trip and fall a lot/bumps into thing, has headaches - may be younger child

Question 16

what kind of lesion causes incongruous left hemianopia

Answer 16

a right postchiasmal lesion

Question 17

what kind of lesion causes homonymous hemianopia

Answer 17

post-LGN lesion of full thickness - usually CVA

Question 18

how do homonymous hemianopia and incongruous heminaopia differ

Answer 18

homonymous hemianopia is usually complete, due to a CVA or TIA but incongruous hemianopia stems from SOL compression or demyelination which tends to be more varying and patchy in its effect

Question 19

what part of the optic tract is affected in left homonymous superior quadrantinopia

Answer 19

lateral 1/2 of the right post LGN fibres (temporal branch)

Question 20

which part of the optic tract is affected in left inferior quadrantinopia

Answer 20

medial 1/2 of the right post LGN fibres

Question 21

what is the rule for any chiasm lesion

Answer 21

causes bitemporal loss

Question 22

what is the rule for any retrochiasmal lesion

Answer 22

always contralateral homonymous hemianopia (e.g. R fibres = both L fields obscured)