A 52-year-old man with hypertension presents with sudden loss of vision in the upper half of the visual field of the left eye, sparing the macula. Which of the following is NOT a recognized cause of this altitudinal visual field defect?

Explanation

## Altitudinal Visual Field Defects: Definition & Causes **Key Point:** An altitudinal (altitudinal = "altitude" or height-based) defect is a visual field loss that respects the horizontal meridian, affecting either the upper or lower half of the visual field while sparing the macula. This pattern is characteristic of **vascular ischemia to the optic nerve head or retina**. ## Recognized Causes of Altitudinal Defects | Cause | Mechanism | Typical Presentation | |-------|-----------|---------------------| | Anterior ischemic optic neuropathy (AION) | Ischemia to optic nerve head (short posterior ciliary arteries) | Sudden upper or lower altitudinal loss; optic disc edema | | Posterior ischemic optic neuropathy (PION) | Ischemia to retrobulbar optic nerve | Altitudinal loss; optic disc initially normal | | Central retinal artery occlusion (CRAO) | Occlusion of CRA or branch | Sudden monocular vision loss; may be altitudinal if branch involved | | Branch retinal artery occlusion (BRAO) | Occlusion of CRA branch | Altitudinal or sectoral loss corresponding to vascular territory | **High-Yield:** Altitudinal defects = **monocular** vascular pathology (optic nerve head or retina). The horizontal nature reflects the vascular supply pattern of these structures. ## Why Homonymous Hemianopia Does NOT Cause Altitudinal Defects **Clinical Pearl:** Homonymous hemianopia from occipital lobe infarction causes loss of the **same side of the visual field in both eyes** (e.g., right homonymous hemianopia = loss of right visual field in both eyes). This is a **bilateral, symmetric** field defect that **does not respect the horizontal meridian** — it respects the **vertical meridian** (the nasal–temporal boundary). Therefore, it cannot produce an altitudinal (horizontal) defect. **Mnemonic:** **AION-PION-BRAO** = **monocular altitudinal**; **Occipital lesions** = **binocular homonymous** (respects vertical meridian, not horizontal). ## Anatomical Basis ```mermaid flowchart TD A["Monocular VF Loss"]:::outcome --> B{"Horizontal meridian\nrespected?"}:::decision B -->|Yes| C["Altitudinal Defect"]:::outcome C --> D["Optic nerve head or\nretina ischemia"]:::outcome D --> E["AION / PION / BRAO"]:::action B -->|No| F["Other monocular\npattern"]:::outcome G["Binocular VF Loss"]:::outcome --> H{"Vertical meridian\nrespected?"}:::decision H -->|Yes| I["Homonymous\nHemianopia"]:::outcome I --> J["Optic tract, LGN,\nor occipital cortex"]:::outcome ``` **Warning:** Do not confuse **altitudinal** (horizontal meridian) with **hemianopia** (vertical meridian). Altitudinal = monocular vascular; hemianopia = binocular retrochiasmal. [cite:Harrison 21e Ch 29]