Mesopic and Low-Contrast Visual Acuity Deficits in Retinitis Pigmentosa: Clinical Markers for Early Functional Impairment

<b>Background:</b> Standard visual acuity (VA) is often preserved in early retinitis pigmentosa (RP), limiting its value as a marker of functional impairment. Alternative measures such as low-luminance deficit (LLD) and low-contrast deficit (LCD) may detect earlier changes in cone function. This study aimed to evaluate the diagnostic utility of these measures in RP patients under photopic and mesopic conditions. <b>Methods:</b> A prospective observational study was conducted on 57 RP patients and 54 age-matched controls. Binocular VA was assessed using ETDRS charts at 100% and 10% contrast under photopic (100 cd/m²) and mesopic (1 cd/m²) conditions. LLD and LCD scores were computed from VA differences across conditions. ROC curve analysis was used to determine diagnostic accuracy. <b>Results:</b> RP patients showed significant VA loss under reduced luminance and contrast (<i>p</i> < 0.001), independent of age. LLD under high contrast was reduced, while LLD under low contrast and LCD (both photopic and mesopic) were significantly higher than in controls. The mesopic LCD demonstrated the highest diagnostic capacity (AUC = 0.87), with a threshold of > 13 ETDRS letters yielding optimal sensitivity and specificity. Unlike standard VA, mesopic LCD correlated with functional symptoms and was unaffected by age. <b>Conclusions:</b> Low-contrast VA under mesopic conditions is a simple, reproducible, and sensitive marker for early visual dysfunction in RP. A difference > 13 ETDRS letters may serve as a clinically relevant threshold for disease monitoring and early detection in retinal dystrophies.