A 68-year-old woman from Mumbai presents with gradual loss of peripheral vision bilaterally over the past 6 months. She reports that she has difficulty navigating crowded spaces and frequently bumps into objects on either side. Visual acuity is 6/6 in both eyes. Fundoscopy shows optic disc cupping with a cup-to-disc ratio of 0.8 bilaterally. Intraocular pressure is 28 mmHg in both eyes. Visual field testing reveals bilateral concentric visual field defects. Which layer of the retina is primarily affected in this condition?

Explanation

## Diagnosis: Primary Open-Angle Glaucoma (POAG) — Retinal Ganglion Cell Pathology ### Clinical Presentation Analysis **Key Point:** Glaucoma is characterized by selective loss of retinal ganglion cells (RGCs) and their axons, leading to optic nerve head damage and characteristic visual field defects. **High-Yield:** The peripheral vision loss in glaucoma occurs because peripheral RGCs (which project to the periphery of the optic disc) are damaged first, causing concentric visual field constriction. Central vision (acuity) is preserved until late stages because the macula is supplied by central RGCs that are affected last. ### Why RGCs Are the Primary Target | Feature | RGC Layer | Other Retinal Layers | |---------|-----------|----------------------| | Vulnerability to elevated IOP | Highest — axons compressed at lamina cribrosa | Relatively spared | | Mechanism of damage | Axonal compression + ischemia + excitotoxicity | Indirect ischemic effects | | Visual field pattern | Concentric defects, arcuate scotomas | Diffuse loss (if affected) | | Optic disc appearance | Cupping, pallor, rim thinning | Not directly involved | | Reversibility | Irreversible if prolonged | Variable | ### Pathophysiology of RGC Loss in Glaucoma 1. **Elevated intraocular pressure (IOP)** → compresses axons at the lamina cribrosa 2. **Axonal compression** → impairs axonal transport of neurotrophic factors (BDNF, NGF) 3. **Reduced trophic support** → RGC apoptosis and death 4. **Ischemia** → mitochondrial dysfunction and excitotoxicity (glutamate accumulation) 5. **Progressive RGC loss** → optic disc cupping and visual field defects **Clinical Pearl:** The cup-to-disc ratio of 0.8 indicates advanced optic nerve damage. The lamina cribrosa is the site of maximum RGC axonal damage because it is the narrowest point of the optic nerve and the location of highest pressure gradient. ### Visual Field Defects in Glaucoma **Mnemonic: GASP** — Glaucoma causes Arcuate scotomas, Stepwise peripheral loss, Paracentral defects. - **Concentric visual field constriction** = peripheral RGCs affected first - **Arcuate scotomas** = damage along nerve fiber bundle distribution - **Nasal step** = superior and inferior arcuate defects meet at the nasal midline - **Central vision preserved until late** = foveal RGCs spared until advanced disease ### Why Other Layers Are Not Primarily Affected **Photoreceptor layer (rods and cones):** Primarily affected in retinitis pigmentosa and age-related macular degeneration, not glaucoma. Photoreceptor loss would cause diffuse vision loss, not concentric field defects. **Inner nuclear layer:** Contains bipolar, amacrine, and horizontal cells. Damage here would cause diffuse retinal dysfunction, not the characteristic optic nerve cupping and arcuate field defects seen in glaucoma. **Retinal pigment epithelium (RPE):** Affected in macular dystrophies and geographic atrophy. RPE damage causes central vision loss and drusen, not optic disc cupping. [cite:Guyton & Hall Textbook of Medical Physiology Ch 51; Harrison 21e Ch 28]