Overview
This lecture discusses lesions of the visual pathway, how they affect visual fields, related terminology, and the clinical features of different lesion locations.
Visual Field-Retina Relationship
- The superior visual field projects onto the inferior retina, and vice versa (inverted relationship).
- Temporal visual field projects to nasal retina, nasal field projects to temporal retina (reversed relationship).
Key Terminology
- "Homonymous" defects affect the same side of the visual field in both eyes.
- "Congruous" defects look the same in both eyes; "incongruous" defects are different in each eye.
Lesions at Different Sites
Optic Nerve
- Lesion causes total blindness in the affected eye.
- Ipsilateral direct light reflex and contralateral consensual reflex are lost.
Junctional Scotoma
- Lesion near optic chiasm affects optic nerve and Wilbrand’s knee (lower nasal fibers).
- Causes total blindness in one eye plus contralateral upper temporal field defect.
Optic Chiasm
- Central chiasmal lesions affect crossing nasal fibers, causing bitemporal hemianopia.
- Partial optic atrophy and lost temporal pupillary reflexes are seen.
- Lateral chiasmal lesions affect uncrossed temporal fibers, causing binasal hemianopia, partial atrophy, and nasal pupillary reflex loss.
Optic Tract
- Lesion causes contralateral incongruous homonymous hemianopia.
- Hemianopic pupillary response (Wernicke’s pupil) and possible optic atrophy.
- May be associated with 3rd nerve palsy or hemiplegia.
Lateral Geniculate Body
- Lesion also causes contralateral incongruous homonymous hemianopia.
- Pupillary reflex is spared, partial optic atrophy possible.
- Advanced effects: sectoranopia/quadruple sectoranopia (not explained here).
Optic Radiations
- Temporal (Meyer's loop/inferior) lesion: contralateral superior quadrantanopia ("pie in the sky").
- Parietal (superior) lesion: contralateral inferior quadrantanopia ("pie on the floor").
- Full optic radiation lesion: contralateral incongruous homonymous hemianopia, no pupillary or RAPD changes.
Visual Cortex
- Supplied by posterior and middle cerebral arteries; macular area has dual supply.
- Posterior cerebral artery stroke: contralateral congruous homonymous hemianopia with macular sparing.
- Occipital tip lesions: contralateral congruous homonymous macular defect.
- No optic atrophy or pupillary reflex changes.
Key Terms & Definitions
- Homonymous Hemianopia — Visual loss is the same side in both eyes.
- Congruous/Incongruous — Congruous: identical field defects in both eyes; incongruous: defects differ.
- Junctional Scotoma — Blindness in one eye plus contralateral upper temporal field defect.
- Bitemporal Hemianopia — Loss of temporal visual fields in both eyes.
- Binasal Hemianopia — Loss of nasal visual fields in both eyes.
- Meyer's Loop — Inferior optic radiation fibers in the temporal lobe.
- Baum’s Loop — Superior optic radiation fibers in the parietal lobe.
- Wernicke's Pupil — Hemianopic pupillary response due to optic tract lesion.
- Macular Sparing — Preservation of central vision due to dual blood supply.
Action Items / Next Steps
- Review anatomical relations of optic chiasm and pathways.
- Understand clinical localization of visual field defects.
- Prepare for potential questions on defect congruity and associated reflex changes.