Abstract
Purpose: :
Age–related Macular Degeneration (AMD)is considered one disease but it has at least two different end stage components (neovascular and atrophic) and may represent many diseases with different aetiologies and prognosis. We used visual function testing in eyes with early AMD to better define subsets of disease that might then predict outcome. From our AMD macular function study we selected out two subsets of subjects: A) subjects who had a choroidal neovascular membrane (CNVM) in one eye and little clinical signs of AMD in the second study eye. These eyes are known to be at a very high risk of CNVM. B) Subjects with multiple large soft drusen in both eyes, some with GA in the non study eye, but no CNVM.
Methods: :
Subjects with VA better than 20/60 with any stage of AMD, as well as control subjects, were enrolled into our prospective study of visual function (Ageing Macular/Functioning Macular: AM/FM study). VA, fundus photographs and a suite of visual function tests were performed which included, flicker thresholds (4Hz and 14Hz), and dark adaptation curves. To achieve these tests a stimulus was generated on a high–resolution CRT driven by 8–bit Radeon video–card (0–255 LUT) hosted in a Macintosh G4 computer. For adaptation a 30% ganzfeld pigment bleach was achieved with a camera flash
Results: :
To date, from the 60 cases of AMD studied there were 5 subjects who had CNVM in one eye but minimal AMD changes in the study eye. Another 5 subjects had extensive bilateral large soft drusen and no signs of CNVM. Aged matched normal subjects had a rod–cone break at 9.25 minutes (95% CI 7.55–10.85) and a flicker threshold to 14 HZ of 0.0196 %contrast (95%CI 0.14–0.024) In all 10 subjects the flicker threshold to 14hz was increased (0.025–0.071). In 5/5 subjects with bilateral large soft drusen the rod–cone break in the dark adaptation curve was prolonged, (range from 19.66–31.39 minutes) However in all 5/5 eyes with very little sign of AMD in the study eye, but with CNVM in the other eye, the time to rod cone break was much closer to normal times (range from 5.44–16.61 minutes)
Conclusions: :
Increased flicker threshold is a sensitive test of early AMD. Prolonged dark adaptation can be explained by poor flux of nutrients through a thickened Bruch’s membrane. This was seen in those with large soft drusen and perhaps predisposes a person to atrophy. Prolonged dark adaptation does not appear to be a necessary finding in eyes at high risk of CNVM but which do not have high risk signs. The corollary to this finding is that a thickened BM is not required to develop CNVM and other mechanisms are at play.
Keywords: age-related macular degeneration • Bruch's membrane • choroid: neovascularization