A 52-year-old man attends the emergency department with sudden difficulty noticing objects on one side while reading and walking. Formal perimetry shows a right-sided homonymous hemianopia, and the defect is asymmetric between the two eyes.
Where is the lesion most likely located?
A. Left lateral geniculate body
B. Left optic radiation
C. Left optic tract
D. Right occipital cortex
E. Optic chiasm
Answer
Left optic tract
Detailed discussion for MRCP
To localise visual field defects accurately, you must first decide whether the defect is monocular or binocular, then whether it is homonymous or heteronymous, and finally assess congruity.
A homonymous hemianopia means loss of the same side of the visual field in both eyes (right or left). This automatically places the lesion posterior to the optic chiasm, because fibres have already crossed.
The optic tract contains fibres from:
- Ipsilateral temporal retina
- Contralateral nasal retina
Together, these fibres represent the contralateral visual field. Therefore:
- Left optic tract lesion → right homonymous hemianopia
The key discriminator here is incongruity.
- In the optic tract, fibres are still relatively dispersed, so damage produces incongruous homonymous hemianopia (the two eyes are affected unequally).
- As fibres move posteriorly (optic radiation → occipital cortex), they become more precisely organised, producing increasingly congruous defects.
Why the other options are wrong:
- Left optic radiation: also causes right homonymous hemianopia, but typically congruous, often quadrantic rather than complete.
- Right occipital cortex: causes left homonymous hemianopia and often shows macular sparing due to dual PCA/MCA supply.
- Optic chiasm: produces bitemporal hemianopia, not homonymous.
- Lateral geniculate body lesions are rare and often produce sectoral defects rather than classic hemianopia.
High-yield MRCP pearls:
- The further posterior the lesion, the more congruous the defect.
- Macular sparing = occipital cortex (classic exam favourite).
- Quadrantanopias localise to optic radiations, but in real life many are occipital—exam answers still favour radiations.
Cheat sheet (exam-oriented)
- Pre-chiasmal → monocular visual loss
- Chiasmal → bitemporal hemianopia
- Post-chiasmal → homonymous hemianopia
- Optic tract → incongruous homonymous hemianopia
- Optic radiation / occipital cortex → congruous homonymous hemianopia
- Temporal lobe (Meyer’s loop) → superior quadrantanopia
- Parietal lobe → inferior quadrantanopia
- Occipital cortex → macular sparing
Mnemonic: PITS
- Parietal → Inferior
- Temporal → Superior
Flash cards
- Q: What does a homonymous hemianopia indicate?
A: A post-chiasmal lesion.
Explanation: Both eyes lose the same side of the visual field. - Q: Which structure causes incongruous homonymous hemianopia?
A: Optic tract.
Explanation: Fibres are still widely distributed. - Q: Why do occipital lesions spare the macula?
A: Dual blood supply (PCA + MCA). - Q: Which lesion causes bitemporal hemianopia?
A: Optic chiasm. - Q: Upper quadrantanopia localises to which lobe?
A: Temporal lobe (Meyer’s loop). - Q: Inferior quadrantanopia localises to which lobe?
A: Parietal lobe. - Q: Congruity increases as lesions move in which direction?
A: Anterior → posterior. - Q: Left optic tract lesion causes which defect?
A: Right homonymous hemianopia. - Q: Which lesion classically shows macular sparing?
A: Occipital cortex. - Q: Are optic radiation defects usually complete hemianopias?
A: No, often quadrantic. - Q: Which vascular territory supplies the occipital lobe?
A: Posterior cerebral artery. - Q: Can optic tract lesions affect pupillary reflexes?
A: Yes, due to involvement of afferent fibres. - Q: What does incongruous mean in visual fields?
A: Asymmetric defect between eyes. - Q: What does congruous mean?
A: Identical defect in both eyes. - Q: Which lesion causes monocular blindness?
A: Optic nerve. - Q: Most common real-world cause of quadrantanopia?
A: Occipital lobe infarction. - Q: Exam-preferred cause of quadrantanopia?
A: Optic radiation lesion. - Q: Inferior chiasmal compression causes which defect first?
A: Superior temporal quadrants. - Q: Pituitary tumours compress which part of the chiasm?
A: Inferior aspect. - Q: Craniopharyngiomas compress which part of the chiasm?
A: Superior aspect.
MCQs to test yourself
- A right incongruous homonymous hemianopia most strongly suggests a lesion in:
A. Right optic nerve
B. Left optic radiation
C. Left optic tract
D. Optic chiasm
E. Right occipital cortex
Answer: C – Optic tract lesions cause incongruous homonymous defects. - Which feature best localises a lesion to the occipital cortex?
A. Incongruity
B. Pupillary abnormality
C. Quadrantanopia
D. Macular sparing
E. Monocular blindness
Answer: D – Dual blood supply preserves macular vision. - A superior homonymous quadrantanopia points to damage in the:
A. Parietal lobe
B. Optic chiasm
C. Temporal lobe
D. Optic nerve
E. Retina
Answer: C – Meyer’s loop carries inferior retinal fibres. - Bitemporal hemianopia most commonly results from:
A. MCA infarction
B. Occipital tumour
C. Pituitary adenoma
D. Temporal lobe epilepsy
E. Optic tract infarct
Answer: C – Compression of crossing nasal fibres. - Increasing congruity of a homonymous hemianopia suggests a lesion that is:
A. More anterior
B. Pre-chiasmal
C. More posterior
D. Retinal
E. Peripheral
Answer: C – Fibre organisation increases posteriorly.
- Lesions before the chiasm → monocular visual loss
- Chiasmal lesions → bitemporal hemianopia
- Post-chiasmal lesions → homonymous defects
- More posterior lesions = more congruous defects
- Optic tract lesions → incongruous homonymous hemianopia
6. A 60-year-old man presents with sudden complete loss of vision in his left eye. The most likely site of pathology is:
A. Left optic tract
B. Optic chiasm
C. Left optic nerve
D. Right occipital cortex
E. Left optic radiation
Answer: C – Optic nerve lesions cause monocular blindness.
7. A woman has bitemporal hemianopia worse in the upper temporal quadrants. The lesion most likely involves:
A. Superior optic chiasm
B. Optic tract
C. Occipital cortex
D. Inferior optic chiasm
E. Temporal lobe
Answer: D – Inferior chiasmal compression (e.g. pituitary adenoma).
8. A patient develops a left superior homonymous quadrantanopia after a seizure. Where is the lesion?
A. Right parietal lobe
B. Left temporal lobe
C. Right temporal lobe
D. Left occipital cortex
E. Optic chiasm
Answer: C – Temporal lobe (Meyer’s loop) → contralateral superior quadrantanopia.
9. Which visual field defect is most likely to be highly congruous?
A. Optic tract lesion
B. Early optic radiation lesion
C. Retinal detachment
D. Occipital cortex infarct
E. Optic nerve neuritis
Answer: D – Occipital cortex lesions are the most congruous.
10. A right homonymous hemianopia with macular sparing suggests a lesion in the:
A. Left optic tract
B. Left optic radiation
C. Left occipital cortex
D. Optic chiasm
E. Retina
Answer: C – Macular sparing is classic for occipital cortex lesions.
11. A patient has an inferior homonymous quadrantanopia affecting both eyes. The lesion is most likely in the:
A. Temporal lobe
B. Parietal lobe
C. Optic nerve
D. Optic chiasm
E. Retina
Answer: B – Parietal lobe lesions cause inferior quadrantanopia.
12. Which lesion would most likely affect pupillary light reflexes in addition to visual fields?
A. Occipital cortex
B. Optic radiation
C. Optic tract
D. Parietal lobe
E. Temporal lobe
Answer: C – Optic tract involvement may affect afferent pupillary fibres.
13. A patient with a left optic tract lesion will lose vision from which retinal fibres?
A. Left nasal and left temporal
B. Right nasal and right temporal
C. Left nasal and right temporal
D. Left temporal and right nasal
E. Only macular fibres
Answer: D – Left temporal + right nasal retina → right visual field.
14. A congruous right homonymous hemianopia without macular sparing most likely indicates a lesion in the:
A. Left optic tract
B. Left optic radiation
C. Right optic nerve
D. Optic chiasm
E. Retina
Answer: B – Optic radiation lesions produce congruous defects.
15. Which statement about congruity of visual field defects is correct?
A. Incongruity increases posteriorly
B. Congruity is highest at the optic nerve
C. Congruity increases from optic tract to occipital cortex
D. Congruity is irrelevant for localisation
E. Only chiasmal lesions are congruous
Answer: C – Posterior lesions are more congruous.
16. A man develops a left homonymous hemianopia after a right posterior cerebral artery infarct. The lesion is in the:
A. Right occipital cortex
B. Left occipital cortex
C. Right optic tract
D. Left optic radiation
E. Optic chiasm
Answer: A – PCA supplies the occipital cortex.
17. Which defect is most consistent with a craniopharyngioma?
A. Upper temporal bitemporal hemianopia
B. Lower temporal bitemporal hemianopia
C. Homonymous hemianopia
D. Monocular blindness
E. Central scotoma
Answer: B – Superior chiasmal compression affects lower temporal fields.
18. A lesion producing a homonymous defect but sparing pupillary reflexes is least likely to involve the:
A. Optic nerve
B. Optic tract
C. Optic radiation
D. Occipital cortex
E. Visual association cortex
Answer: A – Optic nerve lesions affect pupillary reflexes.
19. Which structure is immediately posterior to the optic chiasm?
A. Optic radiation
B. Occipital cortex
C. Lateral geniculate body
D. Optic nerve
E. Visual association cortex
Answer: C – Fibres synapse at the lateral geniculate body.
20. In MRCP exams, quadrantanopia is most often attributed to lesions of the:
A. Occipital cortex
B. Retina
C. Optic nerve
D. Optic radiation
E. Optic tract
Answer: D – This is the classic exam answer despite real-world data.
Summary for quick exam revision
Homonymous hemianopia always localises the lesion to a post-chiasmal site, and the side of visual loss is opposite to the lesion. Incongruous defects point towards optic tract involvement, whereas congruous defects suggest optic radiation or occipital cortex lesions. Temporal lobe damage causes superior quadrantanopia and parietal lobe damage causes inferior quadrantanopia, remembered using the PITS mnemonic. Macular sparing is a hallmark of occipital cortex lesions due to dual vascular supply. Chiasmal lesions produce bitemporal hemianopia, most commonly from pituitary tumours. As a rule for MRCP, the more posterior the lesion, the more congruous the visual field defect.