Dark adaptation offers clue to earlier AMD diagnosis

By Daniel M. Keller

WASHINGTON, DC — Impairment of rod-mediated vision appears to be a harbinger of age-related macular degeneration (AMD) and may form the basis of an early diagnostic test, say researchers at the University of Alabama at Birmingham.
Rod-mediated dark adaptation is very susceptible to the effects of early AMD. Patients in a study conducted at the University of Alabama at Birmingham showed moderate to markedly slowed dark adaptation compared to normal elderly adults, said Gregory Jackson PhD at a Research to Prevent Blindness Foundation seminar.
In a pilot study, researchers measured acuity, contrast sensitivity, visual sensitivity and rod-mediated dark adaptation.

Of 14 normal elderly adults with impaired dark adaptation at baseline, 12 received a clinical diagnosis of early AMD within four years of the original test, compared to one of six patients with normal dark adaptation at baseline. The early AMD patients required an average of 16 minutes longer to complete the test.

Dark-adaptation testing appears capable of predicting a diagnosis of early AMD before detection by standard fundus grading or other common clinical tests. In a study of 24 early AMD patients, rod-mediated dark adaptation was impaired in 85% of the patients. Cone-mediated visual sensitivity, acuity and contrast sensitivity were impaired in only about 25% of the early AMD patients, Dr Jackson said.
He has proposed that photoreceptor function is a direct assay of the dysfunction of Bruch's membrane and the retinal pigment epithelium in early AMD. This dysfunction impairs the retinoid cycle, depriving the rod outer segment of 11-cis-retinal (Vitamin A) and thereby slowing dark adaptation.

The research appears to be the first report of a verifiable outcome measure which showed a distinct difference between correlatable signs of a phenotype of macular degeneration with an objective laboratory test that had predictive value, Dwight Cavanagh MD noted.
He said such a prospective test would be “extremely useful” and noted that this sort of objective (and possibly masked) test could also be very helpful in assessing experimental nutritional or pharmaceutical interventions in early AMD.

In fact, Dr Jackson is planning a prospective study to validate impaired dark adaptation as an early marker for early age-related maculopathy (ARM) and its usefulness in monitoring clinical interventions.
When useful treatments are developed, they will need to be applied before AMD causes functional impairment, William Mathers MD remarked
“These people could see well here and that's what you want to maintain before you get structural damage. This is very exciting research,” Dr Mathers said.

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