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Best description of visual defects I have seen on RU-vid thus far. I'm currently studying for STEP 1 USMLE- so this was very crucial for my understanding/learning. Thank you so much!
Wondering if macular sparing should also occur from an MCA stroke affecting Meyer's loop at hairpin turn since there is still blood supply to macula from PCA? Or does it not work like that since hairpin turn is further downstream, so MCA lesion would affect both macula and the quadrant? Thanks!
Very good explanation! i have one doubt though, for lesion G (contralateral homonymous hemianopia with macular sparing), i think that because the macula has more cortical representation on the visual cortex, so even if there is a lesion on the cortex, some healthy cortex may not be affected by the ischemia. Therefore it can still function and macular vision is spared. am i right or wrong? please reply thanks
Thanks. Remove the doubt. The mechanism for macular sparing is currently still controversial. There are 2 hypotheses and the most widely accepted (and tested) one is the one presented in this video. Basically, the macula has a dual blood supply from the MCA and PCA and the occlusion of 1 of these arteries, but not the other, may result in macular sparing due to collateral circulation. The other hypothesis, presented by Smith and Richardson (www.sciencedirect.com/science/article/pii/0002939466904752), is that incomplete damage to the visual cortex might explain macular sparing because they found a variety of patterns by which the 4 branches of the PCA supply the visual cortex. Memorize the first hypothesis if you don't plan on a career in neuroscience.
here is 1 that no one has nebtioned yet, completely blind on the right side of both eyes, no lesions and with a water cyst behind the left eye filling a small space where the front lower lope of the brain is suppose to be but isn't there it is missing, also left eye twitches 24/7
I think you are describing a Contralateral Homomyous Hemianopsia caused by a stroke/infarction of the PCA. See No. 6 of "Visual Field Defects" pg. 511 in First Aid 2017. (If this is happening to you, you should seek medical help immediately.
Do the terms Ipsilateral and Contralateral refer to the eyes or the visual fields? Why is 'D' contralateral while 'A' is ipsilateral?' D' affects both eyes, but it affects opposite half of the visual field, so there contralateral refers to visual field and not the eye? 'A' affects only one eye, but affects both halfs of visual fields of that eye, so there, ipsilateral refers to the eye, not the visual field. Similarly in 'B', it refers to the eye and not the visual field.
I think its bc once the nerve fibers split from the left LGB into the Meyers and Non-Meyers loops- the Meyer's loop is running both laterally and superiorly in the brain, taking fibers from both the left and right eye. (It would be like splitting string cheese first in half (left and right) and then splitting both halves into a top and bottom. Gathering the Top parts of both the left and right would be the Meyer's Loop of the eye. This is where the lesion is. ... if that makes sense visually.) The top parts of the If the Meyer's loop contains fibers relaying BOTH Left temporal AND Right nasal visual fields, it would make sense that a lesion in 1 Meyer's loop would affect both eyes.
