A 72-year-old woman with hypertension and diabetes mellitus presents with progressive visual field loss over 3 months. She reports bumping into objects on her right side and difficulty navigating stairs. On examination, visual acuity is 6/6 in both eyes. Confrontation visual field testing shows a complete loss of vision in the right half of both visual fields. Pupillary light reflexes are normal and symmetric. MRI brain reveals a left occipital lobe infarct. What is the most likely visual field defect?

Explanation

## Diagnosis: Right Homonymous Hemianopia with Macular Sparing ### Clinical Presentation The patient demonstrates: - Progressive visual field loss over 3 months (subacute onset) - Loss of vision in the right half of both visual fields (homonymous) - Normal visual acuity (6/6) — preserved central vision - Normal pupillary responses - Left occipital lobe infarct on MRI ### Anatomical Basis **Key Point:** The **left occipital lobe** receives blood supply from the **posterior cerebral artery (PCA)**. Lesions in the occipital cortex produce **homonymous visual field defects on the contralateral side** (right in this case). ### Why Macular Sparing Occurs **High-Yield:** **Macular sparing** in occipital lobe infarction occurs because: 1. The **macula (central 5°–10° of vision) has dual blood supply** — both from the PCA (occipital lobe) AND from the middle cerebral artery (MCA) via the angular gyrus 2. When the PCA is occluded, the MCA territory may preserve macular function 3. Additionally, the **occipital pole (representing the macula) is located at the tip of the occipital lobe**, which may escape ischemia if the infarct does not extend to the very tip ### Macular Sparing vs. Macular Involvement | Feature | Macular Sparing | Macular Involvement | |---------|-----------------|--------------------| | **VA** | Preserved (6/6 or near) | Reduced (depends on extent) | | **Central field** | Intact | Defective | | **Cause** | Dual blood supply to macula; infarct spares occipital pole | Infarct extends to occipital pole or involves MCA territory | | **Prognosis** | Better functional vision | Worse functional impact | **Clinical Pearl:** Preserved visual acuity (6/6) in the setting of a complete homonymous hemianopia is a **red flag for macular sparing**. The patient can read and recognize faces because the macula is intact, even though peripheral fields are lost. ### Differential Considerations ```mermaid flowchart TD A[Homonymous visual field loss]:::outcome --> B{Macular involvement?}:::decision B -->|Macula spared| C[Occipital infarct with preserved dual blood supply]:::action B -->|Macula involved| D[Larger infarct or MCA involvement]:::action E[Left occipital lobe] -->|PCA territory| F[Right homonymous defect]:::outcome G{Bitemporal loss?}:::decision --> H[Pituitary/chiasmal lesion]:::outcome I{Superior quadrant loss?}:::decision --> J[Meyer's loop involvement in temporal lobe]:::outcome ``` ### Localization Rule - **Occipital lobe lesion** → homonymous hemianopia (congruent) - **Temporal lobe (Meyer's loop)** → superior quadrantanopia - **Parietal lobe** → inferior quadrantanopia - **Chiasm** → bitemporal hemianopia **Mnemonic:** **"POMS"** — **P**arietal (inferior), **O**ccipital (hemianopia), **M**eyer's loop (superior), **S**chiasm (bitemporal). ### Why VA is Preserved The patient's visual acuity of 6/6 is maintained because the macula (responsible for central, high-acuity vision) is spared due to dual blood supply from both PCA and MCA territories. This is a **hallmark feature distinguishing macular-sparing hemianopia from macular-involving hemianopia**.