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Visual cycle (retinoid cycle)
Neuro-Ophthalmology
Key Points
- •The visual cycle regenerates 11-cis-retinal after photoisomerisation: 11-cis-retinal → all-trans-retinal (light) → all-trans-retinol (RDH) → transport to RPE (IRBP) → esterification (LRAT) → isomerisation by RPE65 (rate-limiting step) → 11-cis-retinol → 11-cis-retinal (RDH5) → back to photoreceptors
- •RPE65 mutations cause LCA type 2 / early-onset RP - treatable with Luxturna (voretigene neparvovec), the first FDA-approved gene therapy for inherited retinal disease (AAV2 subretinal injection; requires viable photoreceptors)
- •ABCA4 mutations (Stargardt disease) impair all-trans-retinal clearance → A2E/lipofuscin accumulation in RPE → RPE death → photoreceptor loss; FFA shows pathognomonic dark choroid; FAF shows dramatically increased signal; avoid vitamin A supplements
- •Fundus autofluorescence (FAF) reflects lipofuscin/A2E content: high FAF in Stargardt (excess lipofuscin); low FAF in RPE65 LCA (no visual cycle → no A2E production) - a key diagnostic differentiator
- •Vitamin A deficiency disrupts the visual cycle at the precursor level → nyctalopia → xerophthalmia → keratomalacia → blindness; fully reversible with supplementation if treated before corneal scarring; WHO: leading cause of preventable childhood blindness
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12 more sections are included with the Clinical plan.
- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Clinical features