Docosahexaenoic acid (DHA) supplementation at a dose of 1200 milligrams per day for four years did not, on average, slow the course of retinitis pigmentosa (RP) in adult RP patients already taking vitamin A supplements (vitamin A palmitate 15,000 IU/day), an established treatment for RP. These findings, reported in the September 2004 issue of Archives of Ophthalmology, are from a randomized clinical trial conducted at the Massachusetts Eye and Ear Infirmary in Boston and supported by the National Eye Institute (NEI) and in part by the Foundation Fighting Blindness.
RP is a group of inherited diseases that cause degeneration of the retina. In RP, photoreceptors, the light-sensing cells in the retina that initiate vision, degenerate and die. Over time, RP patients begin to experience difficulty seeing at night and progressively lose vision. At this time, there is no known cure for the diseases.
DHA is a long-chain omega-3 fatty acid abundant in fish such as salmon and tuna. DHA is also present normally in very high concentrations in the retina. Study investigators wished to answer the question of whether supplementation with DHA among patients already receiving vitamin A slowed the course of photoreceptor degeneration in RP. This question was motivated because DHA likely plays an important role in normal photoreceptor function. Some patients with RP have been found to have decreased levels of DHA in their plasma and red blood cells, and some limited observational data has been consistent with this hypothesis.
Over a four-year period, patients in both the DHA treatment group and control group experienced a similar loss in central and peripheral visual fields, visual acuity, and retinal function as assessed by electroretinography.
"This was a well-designed and well-conducted clinical trial," said Dr. Paul A. Sieving, director, NEI. "While a treatment benefit of DHA was not found for this condition, this trial has provided extremely valuable clinical information about RP, particularly regarding the dynamics of the loss of visual fields in these patients."
A companion paper also published in the September 2004 issue of Archives of Ophthalmology reports exploratory, post-hoc analyses of data derived from subgroups of RP participants in the DHA trial, namely those who at entry into the trial were or were not taking vitamin A supplements. There are likely many differences, mostly unknown, between individuals with RP who take vitamin A supplements and those that do not. Findings from such analyses, however intriguing, should be interpreted with caution. Accordingly, NEI makes no specific treatment recommendations based on the results of these subgroup analyses.