Visual Field Defect Quiz
The visual pathway is the most reliable localizer in all of neurology. Damage at each point along it produces a field defect found nowhere else, so the field chart alone tells you where the lesion is. This quiz gives you the defect and asks for the site.
Reading the pathway from front to back
Damage to one optic nerve blinds that eye alone, because the fibres have not yet crossed. At the optic chiasm the nasal fibres cross, so a lesion there — most often a pituitary tumour pressing from below — knocks out both temporal fields, producing bitemporal hemianopia.
Behind the chiasm every lesion produces a defect on the opposite side of both eyes. An optic tract lesion produces a complete contralateral homonymous hemianopia. Further back, the pathway splits, and the split is what makes quadrantanopias possible.
Why quadrants matter
Fibres carrying the upper visual field loop forward through the temporal lobe as Meyer's loop before turning back to the occipital cortex. A temporal lesion therefore takes out the upper quadrant on the opposite side — the pie in the sky defect. Fibres carrying the lower field run straight back through the parietal lobe, so a parietal lesion takes out the lower quadrant, the pie on the floor.
This is why a quadrantanopia is more informative than a hemianopia. A hemianopia says the lesion is behind the chiasm. A quadrantanopia says which lobe.
Macular sparing
An occipital lesion often spares the very centre of vision, because the occipital pole where the macula is represented receives blood from both the posterior and middle cerebral arteries. Macular sparing in a homonymous hemianopia is therefore a strong hint that the lesion is occipital and vascular rather than in the tract.
Common questions
What causes bitemporal hemianopia?
A lesion at the optic chiasm, most commonly a pituitary adenoma compressing it from below. The crossing nasal fibres carry the temporal visual fields of both eyes, so damage at the crossing point removes peripheral vision on both sides.
What is Meyer's loop?
The bundle of optic radiation fibres that loops forward into the temporal lobe before travelling back to the occipital cortex. It carries the upper visual field, so temporal lobe lesions produce a contralateral upper quadrantanopia.
How do you tell a temporal from a parietal lesion on a field chart?
By which quadrant is lost. Temporal lobe lesions affect Meyer's loop and remove the upper contralateral quadrant. Parietal lesions affect the dorsal radiations and remove the lower contralateral quadrant.