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Time for another #neuroanatomy #tweetorial #medstudent twitter!
By popular request, this one is On the VISUAL PATHWAY and localizing VISUAL FIELD DEFICITS!
cc @DxRxEdu @CPSolvers @CrystalYeoMDPhD @Tracey1milligan
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But first, a moment of silence for #GeorgeFloyd #AhmaudArbery #BreonaTaylor
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I’m grateful for all the ❤️ these #EndNeurophobia #tweetorials have received, but hope you’ll check out the rest of my feed too, which seeks to amplify the voices of those who I’m learning from in the path to being an antiracist ally.
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Want to learn more? Follow @InequityKills @LashNolen @uche_blackstock @DrOniBee @michellemorse @gradydoctor
Read #HowToBeAnAntiracist by @DrIbram
Watch 13th
Listen to, learn from, and amplify voices here that seek to end racism
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Because why are we learning all of these medical facts anyway? To serve patients and communities, to stand up for what is right and just.
Remember Virchow who discovered the cell? He also said “Medicine is a social science; politics nothing but medicine on a large scale.”
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OK the visual pathway!
2 eyes
Each has an optic nerve
The optic nerves (ON) join at the optic chiasm
Then the pathway diverges into left and right again:
Optic tracts
LGN of thalamus
Optic radiations (sup and inf on each side)
Visual cortex of posterior occipital lobe
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Meyer's loop is name for part of the inf radiation in the temporal lobe before it proceeds posteriorly, not the whole inferior radiation, as i wrongly thought for awhile :)
Divide these structures generally into
- eye/ON
- chiasm
- retrochismal (tract/LGN/radiations/cortex)
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Remember the left side of the brain controls the right side of the body and right side of the brain controls the left side of the body.
So makes sense that the left brain needs to see the right side of the world, right brain needs to see the left side of the world
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Anatomy would’ve been easy if the left brain saw out of the right eye and the right brain out of the left eye, but the system is way cooler.
EACH eye can see BOTH the left and right sides of the world.
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So the brain wants to sort information from each eye into left and right SIDES OF THE WORLD=visual fields. So how does the system do this?
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Each optic nerve (ON) carries all info from its respective eye.
Let’s follow what’s happening in the left ON: it’s carrying information from the left (tan) AND right (blue) sides of the world.
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The info from the RIGHT side of the world is already on the correct side of the system (the LEFT). It does NOT cross at the chiasm but proceeds back on the left: tract, LGN, radiations, LEFT occipital lobe.
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Now what about the LEFT field (tan) in the left eye. It’s on the WRONG side–needs to cross over to the right. It crosses AT THE CHIASM.
So the lateral (temporal) field of the left eye crosses at the chiasm to arrive in the RIGHT tract,LGN,radiations, RIGHT occipital lobe.
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For the right eye same deal: LEFT visual field information (tan) in that eye is on the correct side (right), so it does NOT cross at the chiasm, but proceeds back on the right: tract, LGN, radiations, RIGHT occipital lobe, along with the crossed LEFT info from the LEFT eye/ON
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But RIGHT field information in the right eye (blue) is on the WRONG side, it needs to cross to join RIGHT field info from the left eye: it crosses at the chiasm to join the RIGHT field info from the left eye we discussed above: LEFT tract, LGN, radiations, LEFT occipital lobe.

SO: once we are POSTERIOR to the chiasm (tract, LGN, radiations, occipital lobes), we are divided into FIELDS: left brain sees right visual field (in BOTH eyes), and right brain sees left visual field in BOTH eyes).
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SO now we can understand lesions:
A lesion of the eye or ON (@ #1) causes visual deficit in THAT EYE alone…the other eye still sees both sides of the world.
We’ll have to discuss the DDx of optic neuropathy in another tweetorial–DDx depends on time course and +/- pain
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A RETRO-chiasmal lesion (tract, LGN, radiations, occipital lobe) causes a CONTRALATERAL field defect in BOTH eyes, e.g., left occipital lobe: right homonymous hemianopia.
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To understand the radiations, note that just as L->R and R->L, up goes down, and down goes up! The inferior radiation carries the superior quadrant and the superior radiation carries the inferior quadrant.
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So a LEFT INFERIOR radiation lesion causes a RIGHT SUPERIOR quadrantanopia.
A LEFT SUPERIOR radiation lesion causes RIGHT INFERIOR quadrantanopia
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So when localizing a homonymous (same in both eyes) deficit, the lesion must be RETRO-chiasmal=in the BRAIN
Then just go opposite of everything: right sided field deficit =left side of brain; lower quadrant only= superior radiation; superior quadrant only=inferior radiation.
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So in sum so far:
Unilateral visual problem in ONE EYE ONLY=eye or ON
Bilateral visual field problem, SAME in both eyes = retrochiasmal (BRAIN): tract/LGN/radiation/cortex--DDx=use time course: stroke v tumor v infection v inflammatory
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if hemianopia=most commonly occipital cortex (tract/LGN lesion = rare)
if quadrant = radiation (use law of opposites!):
-upper quadrant = contralateral inf. radiation
- lower quadrant = contralateral sup. radiation
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Now what about THE CHIASM!?
Remember what crosses there: info from each
LATERAL/TEMPORAL field from EACH eye. So a lesion of the chiasm causes BITEMPORAL hemianopia: loss of the lateral / temporal field in both eyes!
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This is important to recognize because the chiasm sits atop the pituitary, so bitemporal hemianopia = look for a sellar lesion (usually pituitary adenoma, but other possibilities there too).
Pt may present with car accident from being unaware of loss of periph vision!
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Are you seeing this all more clearly now?
Hope so!
#EndNeurophobia

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