Purpose: Docosahexaenoic acid (DHA) was supplemented in a single-site, placebo-controlled, randomized clinical trial designed to slow vision loss associated with X-linked retinitis pigmentosa (XLRP); the DHAX Trial. We previously reported no significant differences between supplemented and placebo groups in intent-to-treat analysis of primary ERG outcomes. Assessed herein are hypothesis-generating measures of ancillary visual function outcomes in participants fully adhering to trial protocol.

Methods: Male participants with XLRP (range, 7–31 years) received 30 mg DHA/kg/d (n= 29) or placebo (n= 22) for 4 years. Visual outcomes were measured annually and red blood cell (RBC) DHA determined every 6 months.

Results: Oral DHA supplementation increased mean RBC-DHA levels by 4-fold (P< 0.0001) over placebo. No group differences in progression were found for visual acuity (P= 0.11), shape discrimination (P= 0.18), or fundus appearance (P= 0.70). Optical coherence tomography (OCT) became available during year 2 of the trial; no group differences were seen in ellipsoid zone constriction (P= 0.87) over 2 years. Yearly rates of progression were reduced for dark-adapted thresholds (P= 0.06) and visual field sensitivity for foveal, macular, peripheral, total, and ellipsoid zone regions by DHA supplementation (P= 0.039, P= 0.031, P< 0.0001, P< 0.0001, and P= 0.033). Rates of visual field sensitivity decline were dependent on RBC-DHA (P= 0.046 to< 0.0001).

Conclusions: Supplementation of DHA significantly elevated blood DHA levels and reduced the rate of progression in final dark-adapted thresholds and visual field sensitivity. From the relationship between RBC-DHA and the rate of field sensitivity loss, we can extrapolate that an RBC-DHA level of 17% could minimize the decline in field sensitivity.(ClinicalTrials. gov number, NCT00100230.)