Damage to which structure can cause monocular visual field loss described as an ipsilateral, oval or kidney-shaped defect?

• A. Optic nerve lesion
• B. Pituitary adenoma
• C. Pontine lesion
• D. Periventricular leukomalacia

Answer: (A)

When damage occurs in the optic nerve before the optic chiasm, the visual field loss is monocular, confined to the eye on that side. The nerve contains a mix of fiber bundles from different retinal regions, so injury can produce irregular, partial defects that look like an oval or kidney-shaped Scotoma within that eye’s field. This pattern is characteristic of pre-chiasmatic (optic nerve) pathology.

In contrast, a lesion compressing the optic chiasm (such as a pituitary adenoma) typically causes loss in the temporal fields of both eyes (bitemporal hemianopia), not a monocular defect. A pontine lesion affects brainstem pathways and usually yields contralateral or more complex field patterns, not a pure monocular defect. Periventricular leukomalacia tends to cause cortical visual impairment, often bilateral, rather than a single-eye, irregular field loss.