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Delayed dark adaptation may herald AMD
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Delayed rod-mediated dark adaptation in older adults with normal macular health may be associated with the incidence of early age-related macular degeneration 3 years later, according to a study.
People in the early phase of AMD have slowing in the recovery of light sensitivity by rod photoreceptors after exposure to bright light. Therefore, previous evidence has supported biological probability that rod-mediated dark adaptation could be a functional biomarker of AMD.
During the earliest stage of AMD, retinal pigment epithelium cells secrete cholesterol deposits, which are clinically undetectable, into the Bruch’s membrane complex throughout the macula. The deposits create a diffusion barrier that impedes translocation of plasma lipoproteins that deliver lipophilic essentials such as vitamin A, which slows recovery of light sensitivity. Consequently, delayed dark adaptation can be the first clinical sign of AMD pathogenesis.
“We already knew that dark adaptation problems, problems with night vision, problems seeing in the dark were more of a problem for older adults with early, intermediate and advanced AMD compared to individuals who don’t have AMD. So this finding is consistent with that, but what it does is actually show that the dark adaptation or night vision problems can precede the development of AMD, and thus they are a functional biomarker for the disease or a risk factor for the disease,” lead author Cynthia Owsley, PhD, told Ocular Surgery News.
The prospective cohort Alabama Study on Early Age-Related Macular Degeneration, published in Ophthalmology, evaluated adults 60 years or older with normal macular health at baseline to determine whether slowed rod-mediated dark adaptation is associated with the incidence of AMD development after 3 years.
Mean age at baseline was 67.8 years.
“Keep in mind that older age is the strongest risk factor for the disease so that’s where the disease is likely to occur, and your risk goes up as you get older,” Owsley said.
Speed of dark adaptation was characterized by the rod-intercept value, in which an abnormal dark adaptation was considered to be 12.3 minutes or greater.
Of the 325 adults who completed follow-up, 263 had normal dark adaptation with a mean rod-intercept of 9.1 minutes, and 62 patients had abnormal dark adaptation with a mean rod-intercept of 15.1 minutes.
After adjusting for age and smoking status, the 62 patients with abnormal dark adaptation were approximately two times more likely to have AMD by the time of the follow-up visit than patients with normal dark adaptation.
Delayed rod-mediated dark adaptation can now be used as a functional biomarker for early disease. Owsley said interventions using dark adaptation as an outcome measure may be developed to prevent AMD in older adults.
“It can be used to evaluate whether the intervention is working or not from a functional standpoint,” she said. “If one treatment group gets the new drug that is hypothesized to prevent AMD and the other group is randomized to usual care, you use dark adaptation to track the progression in both groups over the years. And it can be used to evaluate whether perhaps the new drug prevented a dark adaptation delay from occurring or getting worse.”
Because various AMD clinical trials are ongoing or in developmental stages, Owsley said that dark adaptation testing, and perhaps other functional tests, should be considered as alternative endpoints to visual acuity testing.
“It’s sort of a hot research topic right now,” she said. “We need these functional vision tests to evaluate new interventions as laboratories and pharmaceutical companies are developing them.” – by Kristie L. Kahl
- Reference:
- Owsley C, et al. Ophthalmology. 2016;doi:10.1016/j.ophtha.2015.09.041.
- For more information:
- Cynthia Owsley, PhD, can be reached at Department of Ophthalmology, University of Alabama at Birmingham, Calhan Eye Hospital, 700 S. 18th St., Suite 609, Birmingham, AL 35233; email: owsley@uab.edu.
Disclosure: Owsley reports she receives royalties from a patent for the method and apparatus for the detection of dark adaptation.