Visual Fields Flashcards
what is the definition of visual fields
extent to which you can perceive around a central point of fixation
what degrees of vision do humans have in the superior, inferior, nasal and temporal fields in each eye
60 degrees superior
60-70 degrees inferior
50 degrees nasal
90 degrees temporal
why is lateral vision the widest field of view
less bone obstruction - orbital bone lies behind the plane of the cornea
how many axons does the optic disc have
1.2M axons
how many layers does the retina have
9 layers
what are the inner 3 layers of the retina
ganglion cell layer
elastic layer
ABH (amacrine, bipolar, horizonal cells)
what does the ABH layer in the retina do
amplify neuronal signals and converts them from chemical energy into electrical energy
does the optic disc lie medially or laterally in the retina
medially
what happens to the optic tract fibres as they exit the eye
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
what is the ‘visual backup’
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
what is above and below the optic chiasm
above: rathkes pouch
below: pituitary
what kind of lesion causes bitemporal hemianopia
full optic chiasm lesion - this is less common nowadays
what kind of visual defect would a pituitary lesion partially compressing the optic tract cause
bitemporal superior quadrantinopia - as it compresses the inferior fibres of the optic chiasm which account for the superior aspect of vision
what kind of visual defect would a lesion from Rathkes pouch partially compressing the optic tract cause
bitemporal inferior quadrantinopia - as it compresses the superior fibres of the optic chiasm which account for the inferior aspect of vision
what are common complaints of partial compression of the optic chiasm, from pituitary causes and rathkes
pituitary: can’t see traffic lights when in car
Rathkes: trip and fall a lot/bumps into thing, has headaches - may be younger child
what kind of lesion causes incongruous left hemianopia
a right postchiasmal lesion
what kind of lesion causes homonymous hemianopia
post-LGN lesion of full thickness - usually CVA
how do homonymous hemianopia and incongruous heminaopia differ
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
what part of the optic tract is affected in left homonymous superior quadrantinopia
lateral 1/2 of the right post LGN fibres (temporal branch)
which part of the optic tract is affected in left inferior quadrantinopia
medial 1/2 of the right post LGN fibres
what is the rule for any chiasm lesion
causes bitemporal loss
what is the rule for any retrochiasmal lesion
always contralateral homonymous hemianopia (e.g. R fibres = both L fields obscured)
