A 58-year-old man with hypertension presents with sudden loss of the superior half of the visual field in his right eye. Which is the most common site of lesion in the optic pathway?

Explanation

## Superior Altitudinal Visual Field Defect — Optic Nerve Head Lesion ### Clinical Presentation Analysis **Key Point:** Loss of the **superior half of the visual field in one eye** (monocular superior altitudinal hemianopia) localizes the lesion to the **ipsilateral optic nerve or retina**, not to the chiasm, tract, or optic radiations (which produce binocular/homonymous defects). In a 58-year-old hypertensive patient with **sudden** monocular superior visual field loss, the most common diagnosis is **Anterior Ischemic Optic Neuropathy (AION)**, which affects the **optic nerve head**. ### Why the Optic Nerve Head? **High-Yield:** AION is the most common acute optic neuropathy in patients over 50 years of age. It results from ischemia of the short posterior ciliary arteries supplying the optic nerve head. 1. **Monocular defect:** Any lesion posterior to the chiasm produces binocular (homonymous) defects. A monocular defect mandates a pre-chiasmal lesion — optic nerve or retina. 2. **Altitudinal pattern:** The optic nerve head has a superior and inferior vascular supply. Ischemia preferentially affects the inferior portion of the nerve head, causing loss of the **superior** visual field (inferior nerve fibers subserve the superior visual field). 3. **Risk factors:** Hypertension, diabetes, hyperlipidemia, and small cup-to-disc ratio ("disc at risk") are classic risk factors for AION. 4. **Sudden onset:** Vascular events at the optic nerve head present acutely, consistent with the stem. ### Optic Pathway Lesions and Visual Field Defects | Lesion Site | Visual Field Defect | Laterality | |-------------|-------------------|------------| | **Optic nerve head (AION)** | **Altitudinal hemianopia (superior or inferior)** | **Monocular** | | Optic chiasm (inferior fibers) | Bitemporal superior quadrantanopia | Binocular | | Optic tract | Homonymous hemianopia | Binocular | | Meyer's loop (temporal lobe) | Superior homonymous quadrantanopia | Binocular | | Parietal lobe | Inferior homonymous quadrantanopia | Binocular | | Occipital cortex | Homonymous hemianopia ± macular sparing | Binocular | ### Why Not the Other Options? - **B (Optic chiasm — inferior nasal fibers):** Chiasmal lesions produce *binocular* field defects (bitemporal hemianopia or quadrantanopia), not monocular loss. - **C (Optic tract):** Produces contralateral homonymous hemianopia — binocular, not monocular. - **D (Meyer's loop):** Produces *superior homonymous quadrantanopia* — a binocular defect affecting the same quadrant in both eyes. The stem specifies loss in **one eye only**. ### Clinical Pearl **AION mnemonic — "AION = Altitude + One eye + Older + hypertension":** Sudden altitudinal (superior or inferior) monocular visual field loss in an older hypertensive patient = AION at the optic nerve head until proven otherwise. Fundoscopy shows segmental disc edema with or without flame hemorrhages. [cite: Walsh & Hoyt's Clinical Neuro-Ophthalmology, 6th ed.; Kanski's Clinical Ophthalmology, 9th ed.; Harrison's Principles of Internal Medicine, 21e Ch 32]