A 58-year-old man presents to the ophthalmology clinic with a 3-week history of progressive vision loss in his left eye. He reports difficulty reading and bumping into objects on his left side. On examination, visual acuity is 6/18 in the left eye. Confrontation visual fields reveal a complete loss of vision in the left upper and lower quadrants, sparing the central 10°. Fundoscopy shows a normal optic disc and macula. MRI brain with contrast shows a mass in the left temporal lobe compressing the optic radiations. What is the most likely visual field defect?

Explanation

## Visual Field Defect Classification **Key Point:** A left temporal lobe lesion compressing the optic radiations produces a **right homonymous hemianopia** — loss of the right half of the visual field in both eyes. The patient's symptoms (difficulty on the left side, loss of left upper and lower quadrants in the left eye) are consistent with a complete left-sided visual field loss, which in the context of a post-chiasmal lesion means a **left homonymous hemianopia** — but critically, the left temporal lobe carries fibers representing the **right** visual hemifield (from both eyes), so a left temporal lobe lesion causes **right homonymous hemianopia**. ### Anatomical Basis - The optic radiations carry visual information from the **contralateral** visual hemifield. - The **left** optic radiations (temporal and parietal lobes) carry information from the **right** visual hemifield of both eyes. - A mass in the **left temporal lobe** compressing the left optic radiations → loss of the **right** visual hemifield in both eyes = **right homonymous hemianopia**. - The stem states "complete loss of vision in the left upper and lower quadrants" — this describes a **homonymous hemianopia** pattern (both upper and lower quadrants of one hemifield lost), not a quadrantanopia (which is loss of only one quadrant). ### Differential Diagnosis of Visual Field Defects | Defect | Anatomy | Pattern | |--------|---------|---------| | **Homonymous hemianopia** | Optic tract, LGN, or entire optic radiations | Entire half of visual field, same side in both eyes | | **Superior quadrantanopia** | Meyer's loop (temporal lobe) | Upper quadrant only, contralateral | | **Inferior quadrantanopia** | Parietal optic radiations | Lower quadrant only, contralateral | | **Bitemporal hemianopia** | Optic chiasm | Bilateral temporal fields | | **Altitudinal hemianopia** | Optic nerve / retinal artery | Horizontal division (upper or lower half) | **High-Yield (KD Tripathi / Kanski):** A lesion of the **entire** optic radiation (temporal + parietal components) produces a **complete homonymous hemianopia**, often with macular sparing due to the dual blood supply of the occipital cortex. Temporal lobe lesions alone (Meyer's loop) classically cause a **contralateral superior quadrantanopia** ("pie in the sky"), but when the entire radiation is compressed, a full homonymous hemianopia results. **Clinical Pearl:** The key distinction — quadrantanopia involves loss of ONE quadrant; homonymous hemianopia involves loss of ALL quadrants on one side (both upper and lower). The stem explicitly states loss of BOTH upper and lower quadrants, making homonymous hemianopia the correct answer. ### Why This Is NOT Quadrantanopia Quadrantanopia involves loss of a single quadrant (either upper OR lower), not both. The stem describes complete loss of both upper and lower quadrants on the left side, which is the definition of a hemianopia, not a quadrantanopia.