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Which visual field defect is most characteristically caused by compression of the optic chiasm?

A Homonymous hemianopia due to damage of the optic tract on one side
B Quadrantanopia due to damage of the optic radiation in the temporal or parietal lobe
C Central scotoma due to damage of the macula or optic nerve
D Bitemporal hemianopia due to damage of decussating nasal retinal fibres ✓
E Monocular blindness due to damage of the optic nerve before the chiasm

The answer is D: Bitemporal hemianopia due to damage of decussating nasal retinal fibres

Explanation

Why D is correct

See The Visual Pathway. Option D is correct because the optic chiasm (the X-shaped crossing point where the two optic nerves meet, sitting just above the bony hollow that holds the pituitary gland) is where fibres swap sides. Light from the outer, temporal, half of each visual field lands on the inner half of the retina, the nasal half (the half nearer the nose). The nasal retinal fibres decussate, meaning they cross to the opposite side, in the chiasm, while the temporal retinal fibres stay on their own side. A mass pressing on the middle of the chiasm, classically a pituitary tumour growing upwards, squeezes exactly these crossing fibres. Both temporal fields are lost together, which is called bitemporal hemianopia (hemianopia means loss of half of the visual field). Patients often notice they bump into things at the sides, as though wearing horse blinkers. A useful memory aid: the chiasm is the only place where the fibres cross, so a lesion there is the only one that knocks out both outer fields.

Site: the optic chiasm, just above the pituitary fossa.
Crossing fibres: nasal retinal fibres decussate here, and they carry the temporal visual fields.
Lesion: pressure on the chiasm, such as a pituitary tumour, causes bitemporal hemianopia.

Why the other options are incorrect

A. Homonymous hemianopia due to damage of the optic tract on one side: A homonymous defect is the expected pattern for damage to the optic tract, which lies behind the chiasm after the nasal and temporal fibres have been combined by visual field side. The stem specifically asks about optic chiasm compression, so this is an off-target mechanism rather than the characteristic chiasmal defect.
B. Quadrantanopia due to damage of the optic radiation in the temporal or parietal lobe: Quadrantanopia is caused by damage to the optic radiations running through the temporal or parietal lobe, which is behind the chiasm. A chiasmal lesion is too anterior to cause a quarter field loss, so this is an off-target location.
C. Central scotoma due to damage of the macula or optic nerve: Central scotoma is a patch of missing central vision caused by disease at the macula or in the optic nerve, not by compression across the crossing nasal fibres in the chiasm. It would be the answer if the stem described loss of central vision, but both outer fields are lost here.
E. Monocular blindness due to damage of the optic nerve before the chiasm: Monocular blindness occurs when one optic nerve is damaged before the chiasm, so only the affected eye loses vision. Chiasmal compression instead damages the crossing nasal fibres from both eyes and gives a bilateral temporal field defect.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which statement best describes the visual field defect caused by a lesion of the left optic tract?
2
Light from an object in the temporal visual field of the right eye falls on which part of that eye's retina?

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