A 58-year-old man from Delhi presents with progressive loss of his peripheral vision over 6 months. He reports that he bumps into objects on the sides while walking straight. Visual acuity is 6/6 in both eyes. Confrontation visual field testing reveals bilateral concentric field constriction. Fundoscopy shows normal optic disc and macula. Optical coherence tomography (OCT) of the macula is normal. Which layer of the retina is primarily affected in this condition?

Explanation

## Clinical Presentation Analysis The patient presents with: - Progressive peripheral (concentric) visual field loss - Preserved central vision (6/6 acuity) - Normal optic disc and macula on fundoscopy - Normal OCT of macula This pattern is classic for **retinitis pigmentosa (RP)**, a hereditary retinal dystrophy. ## Pathophysiology of Retinitis Pigmentosa **Key Point:** Retinitis pigmentosa is characterized by progressive degeneration of **photoreceptors (rods >> cones)**, beginning in the peripheral retina and progressing centripetally toward the macula. ### Why Peripheral Vision Loss Occurs First Rods are: - Predominantly located in the peripheral retina - More sensitive to light (scotopic vision) - More vulnerable to genetic mutations in RP - Affected before cones in the natural disease progression Cones are: - Concentrated in the central macula - Responsible for color vision and visual acuity - Preserved longer, explaining maintained central vision early in disease ## Retinal Layer Involvement in RP | Layer | Status in RP | Explanation | |-------|--------------|-------------| | **Photoreceptor layer** | **PRIMARY AFFECTED** | Rod and cone degeneration; bone spicule pigmentation from RPE migration | | RPE | Secondary changes | Migration and proliferation (appears as bone spicules) | | Inner nuclear layer | Preserved initially | Becomes affected only in advanced stages | | Ganglion cell layer | Preserved initially | Secondary loss in late disease | | Bruch's membrane | Preserved | Not primarily involved | **High-Yield:** The classic fundoscopic finding of "bone spicule" pigmentation represents RPE migration into the photoreceptor layer as rods degenerate — a secondary phenomenon. ## Clinical Pearl **Night blindness (nyctalopia)** is often the earliest symptom of RP because rods (which mediate scotopic vision) are affected first. This patient may not have reported it, but it would be present on detailed history. ## Why OCT of Macula Is Normal OCT shows normal macula because: - Cones are preserved in early-to-moderate RP - Peripheral rod degeneration is not visible on macular OCT - Central retinal thickness remains normal until late disease [cite:Ganong's Review of Medical Physiology 26e Ch 8]