Full-Field Stimulus Test for Visual Function Assessment in Ultra-Low Vision with Retinitis Pigmentosa

PurposeAssessing visual function in patients with ultralow vision (ULV), particularly those with retinitis pigmentosa (RP), remains a significant challenge in therapeutic development. Full-field stimulus test (FST) provides a quantitative measure of retinal light sensitivity and may serve as a valuable clinical endpoint. We investigated FST in ULV RP by examining its associations with functional measures and daily activity-based tasks. DesignObservational, cross-sectional study. ParticipantsPatients with RP and visual acuity in the worse-seeing eye below counting fingers (CF) were enrolled. MethodsAfter dilation and 45-minute dark adaptation, FST was performed monocularly with brief full-field white-light flashes across three visits. Visual acuity was classified into four groups: no light perception (NLP), light perception (LP), hand motion (HM), and CF or better. We assessed functional vision using two tabletop object-recognition and exploration tasks, two mobility tasks, and three vision-related questionnaires. FST thresholds were compared across visual acuity groups, and associations with functional outcomes were analyzed. Main Outcome MeasuresFST thresholds and their associations with functional vision outcomes. ResultsThirty-five patients (70 eyes; median age, 62 years, range 39-84) were included. Median FST thresholds (log cd*s/m{superscript 2}) by visual acuity group were as follows: NLP, 1.13 (-0.63-2.54); LP, -0.27 (-2.70-2.91); HM, -1.13 (-6.24-0.51); CF or better, -2.82 (-5.67- -1.73) (p < 0.001). Measurable FST thresholds were obtained in 9 of 14 NLP eyes (64.3%). FST thresholds showed significant correlations with tabletop performance (r = -0.70 to - 0.46) and mobility performance (r = -0.65), whereas no significant association was observed with questionnaire scores. Test-retest variability across three visits showed no systematic bias, with a coefficient of repeatability of {+/-}0.66 to {+/-}0.82 log cd{middle dot}s/m{superscript 2}. ROC analyses identified FST cutoffs of -1.75 to -0.87 log cd{middle dot}s/m{superscript 2} at which patients first achieved nonzero functional task performance. ConclusionsFST quantifies residual visual function in ULV RP and correlates strongly with performance-based measures of functional vision in daily life. These findings support FST as a clinically meaningful endpoint for therapeutic trials in advanced RP and other severe visual impairments and highlight the value of anchoring FST thresholds to functional task performance.