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Visual pathway and localisation of field defects
The shape of the field defect reveals where the damage is: before the chiasm the defect is in one eye, at the chiasm it is bitemporal, and behind it it is homonymous — and the more posterior the lesion, the more congruous the defect between the two eyes.
Why it matters in the exam
Exams love localisation questions. Optic nerve: a defect in one eye — central, altitudinal or arcuate — with an RAPD. Junctional scotoma: a central defect in one eye with a superotemporal defect in the other — a lesion where the optic nerve joins the chiasm. Chiasm: bitemporal hemianopia; a pituitary adenoma presses from below so the defect starts in the superotemporal quadrants, and a craniopharyngioma presses from above and starts in the inferior quadrants. Pituitary apoplexy — sudden headache, reduced vision and cranial nerve palsies — is an emergency.
Behind the chiasm: optic tract — incongruous homonymous hemianopia with an RAPD in the eye contralateral to the lesion (the eye that lost the larger temporal field). Meyer loop in the temporal lobe — superior homonymous quadrantanopia ("pie in the sky"). Parietal lobe — inferior quadrantanopia ("pie on the floor"). Occipital cortex — highly congruous homonymous hemianopia, usually with macular sparing because of dual blood supply from the PCA and MCA; the usual cause is a PCA stroke. Bilateral occipital damage causes cortical blindness with normal pupils, and sometimes denial (Anton syndrome).
Tests: confrontation fields for screening, Humphrey 24-2 or 30-2 for the central field and 10-2 for defects near fixation, Goldmann for the peripheral field. Any new homonymous hemianopia requires brain imaging, and any bitemporal defect — pituitary MRI and hormonal tests. A significant homonymous hemianopia usually precludes driving. Remember that a patient with hemianopia often complains of "poor vision in one eye" — usually the eye that lost the temporal field — so both eyes are always tested separately.
Key points
- Before the chiasm — one eye; at the chiasm — bitemporal; behind — homonymous.
- The more posterior, the more congruous.
- Adenoma → superotemporal defect first.
- Temporal → superior; parietal → inferior.
- Occipital: macular sparing, PCA stroke.
Summary table
| Location | Defect | Typical cause |
|---|---|---|
| Optic nerve | One-eye defect, RAPD | Inflammation, ischaemia, compression |
| Optic nerve–chiasm junction | Junctional scotoma | Meningioma |
| Chiasm | Bitemporal hemianopia | Pituitary adenoma, craniopharyngioma |
| Optic tract | Incongruous hemianopia, contralateral RAPD | Tumour, demyelination |
| Meyer loop | Superior homonymous quadrantanopia | Temporal lobe stroke or tumour |
| Occipital cortex | Congruous hemianopia with macular sparing | PCA stroke |
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Sources
- AAO Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology, 2024-2025 — The patient with visual field loss: chiasmal and retrochiasmal disorders
- Walsh and Hoyt's Clinical Neuro-Ophthalmology: The Essentials, 3rd edition (2016) — Topical diagnosis of visual field defects
- Kanski's Clinical Ophthalmology, 9th edition (2020) — Neuro-ophthalmology
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.