# Visual Field Defects and Visual Pathway Lesions

> Visual field defects for NEET-PG Medicine: localising lesions from optic nerve to occipital cortex using hemianopia, quadrantanopia and macular sparing.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/visual-field-defects-and-visual-pathway-lesions
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Bradley's Neurology in Clinical Practice; Adams and Victor's Principles of Neurology; Harrison's Principles of Internal Medicine
- How to cite: "Visual Field Defects and Visual Pathway Lesions", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/visual-field-defects-and-visual-pathway-lesions

## Direct answer

Visual field defects localise lesions along the visual pathway, because each site produces a characteristic pattern of loss. A lesion of the optic nerve causes monocular loss, a chiasmal lesion causes bitemporal hemianopia, and an occipital cortical lesion causes a congruous homonymous hemianopia that often spares the macula.

## What you must remember

- **Monocular loss:** a lesion of the optic nerve, such as retrobulbar optic neuritis, causes visual loss in one eye, often with a relative afferent pupillary defect.
- **Bitemporal hemianopia:** the classic sign of chiasmal compression, most often from a pituitary adenoma or craniopharyngioma, and often beginning in the superior quadrants with pituitary adenoma.
- **Homonymous hemianopia:** loss of the same half of the field in both eyes, arising from the optic tract or any site behind it.
- **Congruity:** tract lesions give incongruous defects that differ between the two eyes, while radiation and occipital lesions give congruous defects that match closely.
- **Pie in the sky:** a superior quadrantanopia from a temporal lobe lesion damaging Meyer's loop, whereas a parietal lesion gives an inferior quadrantanopia called pie on the floor.
- **Macular sparing:** preservation of central vision in a homonymous hemianopia, which favours an occipital pole lesion because the macula has dual blood supply from the middle and posterior cerebral arteries.
- **Pupils:** the light reflex is preserved in occipital lesions because pupillary fibres leave the optic tract before the lateral geniculate body, so a relative afferent pupillary defect points to the optic nerve or anterior pathway instead.

## Localising the lesion, site by site

Localisation starts by asking which half of the field is lost and in which eye. The table links each level of the pathway to its typical defect and the clue that confirms it.

| Lesion site | Typical field defect | Key clue |
|---|---|---|
| Optic nerve | Monocular loss, often central | Relative afferent pupillary defect |
| Optic chiasm | Bitemporal hemianopia | Pituitary mass, superior quadrants first |
| Optic tract | Incongruous homonymous hemianopia | Unequal defects in the two eyes |
| Temporal optic radiation | Superior quadrantanopia | Pie in the sky |
| Parietal optic radiation | Inferior quadrantanopia | Pie on the floor |
| Occipital cortex | Congruous homonymous hemianopia | Macular sparing, normal pupils |

Congruity and the pupils do most of the work at the bedside. A congruous homonymous defect with a normal pupil points back to the occipital lobe, whereas an incongruous defect points forward to the tract.

Worked example: a patient has a right homonymous hemianopia with macular sparing and normal pupils. The defect is congruous, the pupils are normal and the field is spared centrally, so the lesion lies in the left occipital lobe, a typical territory of posterior cerebral artery infarction.

## Where the examiner tests the lesion site

Field questions usually give a short description or diagram and ask only for the site.

- **Trap:** Calling bitemporal loss an optic nerve problem. — **Correct:** Bitemporal hemianopia means the chiasm, because the crossing nasal fibres from both eyes are damaged; a single eye's loss means the nerve.
- **Trap:** Mixing up superior and inferior quadrantanopia. — **Correct:** Temporal lobe lesions give a superior quadrantanopia (pie in the sky), and parietal lesions give an inferior one (pie on the floor).
- **Trap:** Assuming macular sparing excludes a cortical lesion. — **Correct:** Sparing is a clue to an occipital cause, because the macula is supplied by both the middle and posterior cerebral arteries.

## Frequently asked questions

### Which visual field defect suggests a chiasmal lesion?

Bitemporal hemianopia suggests a chiasmal lesion, usually compression by a pituitary adenoma or craniopharyngioma. The defect often begins in the superior quadrants because the mass presses on the chiasm from below.

### What is macular sparing in homonymous hemianopia?

Macular sparing is the preservation of central vision in homonymous hemianopia, seen mainly with occipital lobe lesions. The macula has a dual blood supply from the middle and posterior cerebral arteries, so a posterior cerebral artery infarct often leaves the central field intact.

### Why does a temporal lobe lesion cause a superior quadrantanopia?

A temporal lobe lesion causes a superior quadrantanopia because it damages Meyer's loop, the part of the optic radiation carrying inferior retinal fibres. Those fibres serve the superior visual field, so that quadrant is lost on the opposite side. The classic description is pie in the sky.

### Does the pupillary reflex help localise an occipital lesion?

Yes, the pupillary light reflex is preserved in occipital lesions because pupillary fibres leave the optic tract before the lateral geniculate body. A relative afferent pupillary defect therefore points towards the optic nerve or anterior pathway rather than the occipital cortex.