Count and size of macular drusen correlated with the parafoveal annular reflex

Age related macular degeneration is one of the most frequently occurring reasons for decreased vision in the elderly population in Western countries.¹ It is divided in a non-exudative form with formation of drusen of the retinal pigment epithelium, and the exudative form with subfoveal chorioretinal neovascularisation and subretinal exudation. Since the subretinal alterations typical for age related macular degeneration may lead to a change in the superficial contour of the retina, it was the aim of this study to evaluate whether the visibility of the macular wall reflex depends on the presence and size of macular drusen as part of age related macular degeneration.

The study included 47 rhesus monkey (Macaca mulatta) (77 eyes) for which 60° colour fundus photographs were obtained. Only fundus photographs with the central fundus region fully illuminated were evaluated. The entire study group was divided into eyes of monkeys with unilateral experimental glaucoma (n = 36 eyes) or experimental temporary unilateral occlusion of the central retinal artery (n = 18), and normal eyes (n = 23). The detectability of the parafoveal annular reflex was assessed using a score ranging between “0” for “no parafoveal annular reflex detectable” to “3” for “clear detectability of the parafoveal annular reflex” (figs 1, 2). The reproducibility of the semiquantitative assessment of the parafoveal annular reflex had been determined in a previous study.² The coefficient of variation for the re-assessment of the ophthalmoscopic detectability of the parafoveal annular reflex was 0.08. The degree of age related macular degeneration was evaluated by counting the number of drusen, separately in the foveal region and in the extrafoveal region within the temporal vascular arcade. The mean size of the drusen was graded into three grades: “1” for very small (30 μm or less) and “3” for very large (larger than 100 μm). The possibility to grade the severity of age related macular degeneration on fundus photographs has already been described in detail previously.³ For assessment of the visibility of the retinal nerve fibre layer, the fundus was divided into eight sectors: temporal inferior, temporal horizontal, temporal superior, superior, nasal superior, nasal horizontal, nasal inferior, and inferior. In each sector, the visibility of the retinal nerve fibre bundles was estimated using a subjective grading ranging from “0” for “no fibre bundles detectable” to “8” for “abundant nerve fibre bundles visible.” The technique has already been described in detail.⁴ All eyes included in this study had not undergone any experimental procedure. The study design complied with the National Institutes of Health’s as well as the University of Iowa’s institutional guidelines for the care and use of laboratory animals, and guidelines of ARVO. All experimental procedures and the fundus photography were performed at Iowa City.

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Figure 1

 Fundus photograph of a monkey with clearly visible macular annular reflex in an eye without macular drusen and good visibility of the retinal nerve fibre layer.

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Figure 2

 Fundus photograph of a monkey without detectable macular annular reflex in an eye with multiple macular drusen.

In an univariate statistical analysis, the detectability of the macular wall reflex was significantly and negatively correlated with the number and total area of macular drusen (p = 0.05), score of the visibility of the retinal nerve fibre layer as a whole and especially in the temporal horizontal fundus region (p<0.001), and age (p<0.001). A multiple linear regression analysis confirmed that the ophthalmoscopic visibility of the macular annular reflex was significantly and negatively correlated with the number and total area of the drusen in the foveal region (p<0.001), visibility of the retinal nerve fibre layer in the temporal fundus region (p<0.001), and age (p = 0.01).

The results suggest that the detectability of the macular annular reflex depends on presence and amount of non-exudative age related macular degeneration. It is parallel to other studies in which the visibility of the macular annular reflex decreased with increasing optic nerve damage, presumably because the loss of optic nerve fibres and retinal ganglion cells decreased the height of the macular annular wall and because of that, its ophthalmoscopic visibility.^2,5 From a clinical point of view, it may suggest that examination of the macular annular reflex may be useful in screening patients for age related macular degeneration. The presence of the macular annular reflex may be taken as hint for the anatomic integrity of the fovea.