A 58-year-old man from Delhi presents with a 3-week history of progressive blurring of vision in the left eye. He reports that he cannot see objects on the right side of his visual field in both eyes. On examination, visual acuity is 6/9 in both eyes. Pupils are equal and reactive. Fundoscopy reveals a pale optic disc on the left with temporal pallor. MRI brain shows a 2 cm mass in the left temporal lobe compressing the optic tract. What is the most likely visual field defect?

Explanation

## Clinical Diagnosis This patient has a **left temporal lobe mass compressing the optic tract**, resulting in a homonymous hemianopia (loss of the right visual field in both eyes). ### Anatomical Basis **Key Point:** The optic tract carries crossed nasal fibres (from the ipsilateral nasal retina, representing the contralateral visual field) and uncrossed temporal fibres (from the ipsilateral temporal retina, representing the ipsilateral visual field). Compression of the optic tract results in loss of the entire contralateral visual field in both eyes. ### Why Homonymous Hemianopia Without Macular Sparing? **High-Yield:** Macular sparing occurs only when the lesion is in the **occipital cortex** (posterior visual cortex), because the macular representation has a large cortical territory with dual blood supply and bilateral representation. Lesions anterior to the occipital cortex (optic tract, lateral geniculate nucleus, optic radiations in the temporal or parietal lobes) do NOT spare the macula. Since this mass is in the **left temporal lobe** compressing the optic tract (not the occipital cortex), the macula is NOT spared. The patient loses the entire right visual field, including the central macula. ### Visual Field Defect Pattern | Location of Lesion | Visual Field Defect | Macular Sparing? | |---|---|---| | Optic nerve | Monocular vision loss | N/A | | Optic chiasm | Bitemporal hemianopia | No | | Optic tract | Homonymous hemianopia | No | | Lateral geniculate nucleus | Homonymous hemianopia | No | | Optic radiations (temporal lobe) | Homonymous hemianopia | No | | Optic radiations (parietal lobe) | Homonymous hemianopia | No | | Occipital cortex | Homonymous hemianopia | Yes (usually) | **Clinical Pearl:** The temporal pallor of the optic disc on the left indicates chronic compression of the optic tract, as the crossed nasal fibres (which represent the temporal retina) are preferentially affected in optic tract lesions, leading to temporal optic atrophy. ### Mnemonic: TRACT **T**emporal lobe lesion → **R**etrochiasmal → **A**ffects **C**ontralateral field → **T**emporal pallor of disc.