Purpose: Assessing visual function in patients with ultra–low 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. Design: Observational, cross–sectional study. Participants: Patients with RP and visual acuity in the worse–seeing eye below counting fingers (CF) were enrolled. Methods: After 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 Measures: FST thresholds and their associations with functional vision outcomes. Results: Thirty–five patients (70 eyes; median age, 62 years, range 39–84) were included. Median FST thresholds (log cd·s/m2) by visual acuity group were as follows: NLP, 1.13 (–0.63 to 2.54); LP, –0.27 (–2.70 to 2.91); HM, –1.13 (–6.24 to 0.51); CF or better, –2.82 (–5.67 to 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·s/m2. ROC analyses identified FST cutoffs of –1.75 to –0.87 log cd·s/m2 at which patients first achieved nonzero functional task performance. Conclusions: FST 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.

All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. This study was sponsored by Restore Vision Inc. Y. Katada and T. Kurihara are co-founders of Restore Vision Inc. Individual investigators participating in the sponsored project were not directly compensated by the sponsor; any salary support was provided through their institution. Restore Vision Inc. provided financial support and supported study operations, but had no role in data collection, data analysis or interpretation, manuscript preparation, or the decision to submit for publication.

This study was sponsored by Restore Vision Inc. and partially supported by the Japan Agency for Medical Research and Development (AMED) under Grant Number 23be0904005j0003 and 23qfb127003j0001.

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Institutional Review Board of Keio University School of Medicine gave ethical approval for this work.

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All data produced in the present study are available upon reasonable request to the authors