Abstract
Purpose: :
Near–infrared autofluorescence (AF) confocal imaging (Exc.: 787 nm) has shown that the fovea exhibits higher AF than the perifovea and that the contrast of the higher–AF foveal area decreases with age (Keilhauer 2005). To investigate the nature of the fluorophore responsible for the infrared AF, we studied the spatial distribution and age–relationship of near–infrared AF of the fundus, and compared its characteristics with those of lipofuscin AF.
Methods: :
Near–infrared AF, AF[Exc.: 730 nm], and lipofuscin AF, AF[Exc.: 550 nm], were measured in 19 subjects with normal retinal status (ages: 23–65; 17 Caucasian and 2 Black subjects). Measurements were made using our fundus fluorometer at the fovea (F) and at 7deg. nasal to the fovea (7N) with a sampling field of 3deg. in diameter.
Results: :
The emission spectrum of AF[730] exhibited no distinct maximum and decreased continuously from 780 to 900 nm. Mean AF[730] was 48±15 times less efficient than AF[550]. The ratio of AF[730] at F to that at 7N decreased with age (p=0.002) confirming results obtained by imaging, and was 1.13±0.15 on average (p=0.002). For AF[550], this ratio was 0.66±0.13 (p<0.0001) and was age independent. Mean AF[730] in Blacks was higher than in Caucasians, both at F and at 7N (p=0.003), but there were no significant differences with iris color among Caucasians. Near–infrared AF[730] increased with age at 7N (p=0.02) and less markedly so at the fovea (p=0.09). Lipofuscin AF increased substantially with age at both sites (p<0.0002). After accounting for the age variations, we found no significant correlation between AF[730] and AF[550].
Conclusions: :
Together with observations made by AF[787]–imaging, our results are consistent with RPE and choroidal melanin contributing substantially to infrared autofluorescence. The higher AF obtained at the fovea may result from the higher concentration of melanin in the RPE, whereas the pigmentation dependence indicates a choroidal contribution. The age–related increase of AF[730] appears to confirm, in–vivo, the increase in fluorescence of RPE melanosomes detected ex–vivo (Boulton 1990; Docchio 1991) and the increase in quantum efficiency of melanin after (photo–) oxidation (Kayatz 2001; Sarna 2003). The slower increase of foveal AF[730] may reflect protection from short wavelengths light by the macular pigment. The ability to measure melanin, in addition to lipofuscin, may contribute to a better understanding of the aging RPE.
Keywords: retinal pigment epithelium • aging • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)