General Principles

• Visual Pathway: Retina → Optic Nerve → Optic Chiasm → Optic Tract → Lateral Geniculate Nucleus (LGN) of Thalamus → Optic Radiations → Primary Visual Cortex (Occipital Lobe).
• Rule of Thumb: Pre-chiasmal lesions cause ipsilateral, monocular defects. Post-chiasmal (retrochiasmal) lesions cause contralateral, homonymous (affecting the same side of the visual field in both eyes) defects.
• Information from the right visual field projects to the left hemisphere, and vice-versa.
• Information from the superior visual field projects to the inferior retina and then to the inferior optic radiations (Meyer's loop) and inferior calcarine sulcus.
• Information from the inferior visual field projects to the superior retina and then to the superior optic radiations (Baum's loop/parietal) and superior calcarine sulcus.

Lesion Locations & Corresponding Defects

1. Optic Nerve

   • Defect: Ipsilateral monocular blindness (anopia) or scotoma (localized blind spot).
   • Etiology: Optic neuritis (e.g., multiple sclerosis), glaucoma, retinal artery/vein occlusion, trauma, tumor.
   • Key Finding: Relative Afferent Pupillary Defect (RAPD) or Marcus Gunn pupil on swinging-flashlight test.

2. Optic Chiasm

   • Defect (Central Lesion): Bitemporal hemianopia. Loss of the outer (temporal) visual fields in both eyes.
   • Etiology: Pituitary adenoma (most common), craniopharyngioma, suprasellar meningioma, aneurysm of the anterior communicating artery.
   • Defect (Lateral/Junctional Lesion):
     • Junctional Scotoma of Traquair: Ipsilateral temporal hemianopic defect due to compression of ipsilateral nasal crossing fibers at the nerve-chiasm junction.
     • Junctional Scotoma: Ipsilateral optic neuropathy (e.g., central scotoma) plus a contralateral superior temporal defect ("pie in the sky") due to compression of inferonasal fibers from the opposite eye (Wilbrand's Knee) crossing into the optic nerve before decussating.

3. Optic Tract

   • Defect: Contralateral homonymous hemianopia.
   • Etiology: Stroke (anterior choroidal artery), tumor, trauma.
   • Note: Defects are often incongruous (asymmetric between the two eyes) because fibers from corresponding retinal points are not yet perfectly aligned.

4. Optic Radiations (Geniculocalcarine Tract)

   • Meyer's Loop (Temporal Lobe):
     • Defect: Contralateral superior homonymous quadrantanopia ("pie in the sky").
     • Etiology: Stroke (MCA territory), temporal lobe tumors, anterior temporal lobe resection for epilepsy.
   • Baum's Loop (Parietal Lobe):
     • Defect: Contralateral inferior homonymous quadrantanopia ("pie on the floor").
     • Etiology: Stroke (MCA territory), parietal lobe tumors.
     • Associated Signs: If in the dominant (usually left) parietal lobe, may see Gerstmann syndrome (agraphia, acalculia, finger agnosia, right-left disorientation). If non-dominant, may see contralateral hemineglect.

5. Visual Cortex (Occipital Lobe)

   • Defect: Contralateral homonymous hemianopia with macular sparing.
   • Etiology: Posterior Cerebral Artery (PCA) stroke is the classic cause.
   • Macular Sparing: The macula (central vision) has a dual blood supply from both the MCA and PCA, so a PCA occlusion may spare the occipital pole where macular fibers terminate.
   • Temporal Crescent Defect: A lesion to the most anterior part of the visual cortex can cause a monocular visual field loss in the contralateral eye's far temporal periphery.