Quantification of reduced macular pigment optical density in the central retina in macular telangiectasia type 2☆
Introduction
Macular pigment (MP) consists of the two different xanthophylls, lutein and zeaxanthin (Bone et al., 1988, Davies and Morland, 2004) and is highly accumulated along the axons of the cone photoreceptors in the central retina (Snodderly et al., 1984a, Snodderly et al., 1984b). A number of functions have been proposed for MP (Davies and Morland, 2004, Whitehead et al., 2006). Its absorption maximum at 460 nm results in blue light filtration (Bone et al., 1992, Sharpe et al., 1998) which may reduce photic damage and glare, minimize the effects of chromatic aberration on visual acuity (Engles et al., 2007, McLellan et al., 2002, Reading and Weale, 1974), improve fine detail distinction and enhance contrast sensitivity (Davies and Morland, 2004, Ham et al., 1976, Whitehead et al., 2006). Moreover, its neutralization of reactive oxygen species may have a protective effect on the neurosensory retina (Chucair et al., 2007, Khachik et al., 1997).
The mechanism of deposition of MP in the central retina remains unknown. MP is entirely of dietary origin and it has been shown that dietary habits or supplementation influence its retinal optical density (Berendschot et al., 2000, Bone et al., 2003, Hammond et al., 1997a, Landrum et al., 1997). The concentration (Berendschot et al., 2002) and spatial deposition (Berendschot and van Norren, 2006, Hammond et al., 1997b, Robson et al., 2003) of MP vary substantially between normal subjects. However, the distribution generally shows a peak at the foveal centre, rapidly decreasing with eccentricity, hardly leaving any MP at about 8 degrees eccentricity. MP has been suggested to play a role in the pathophysiology of various diseases including age related macular degeneration (Aleman et al., 2007, Davies and Morland, 2004, Whitehead et al., 2006). Recently, a unique distribution, namely a reduction of macular pigment optical density (MPOD) within the central retina with a surrounding ring-like structure of preserved MPOD at about 6 degrees eccentricity was suggested to be a common finding in macular telangiectasia (MacTel) type 2 (Charbel Issa et al., 2008, Helb et al., 2008).
MacTel type 2 is a rare disease that usually becomes symptomatic between the fifth and seventh decade (Gass and Blodi, 1993, Charbel Issa et al., 2007b). It is characterized by slow, but progressive loss of visual acuity, reading difficulties and/or metamorphopsia. Well defined parafoveal scotomas may develop with disease progression (Charbel Issa et al., 2007a, Schmitz-Valckenberg et al., 2008). Typically, the morphologic and functional alterations, such as telangiectatic vessels with corresponding leakage on fluorescence angiography, foveal cysts and retinal pigment epithelial (RPE-) clumping, are most pronounced on the temporal side of the foveola (Barthelmes et al., 2008a, Charbel Issa et al., 2007a, Charbel Issa et al., 2007b, Charbel Issa et al., 2008, Gass and Blodi, 1993, Gaudric et al., 2006, Schmitz-Valckenberg et al., 2008, Yannuzzi et al., 2006). Previously, MPOD distribution in MacTel type 2 was calculated from fundus autofluorescence (FAF) or reflectance images acquired at two different wavelengths of which one (488 nm) is significantly more absorbed by MP than the other (514 nm) (Charbel Issa et al., 2008, Helb et al., 2008). However, the results showing reduced MPOD may have been influenced by unknown absorbing, reflecting or fluorescent substances, e.g. a supposed lack of MP could as well be due to unknown absorbers in the retina. Moreover, it remained unclear, if MPOD was just preserved or even increased eccentric to its central reduction.
Spectral reflectometry is a method capable of distinguishing between the different spectral characteristics of the various absorbing and reflecting substances within the eye (van de Kraats et al., 2006). It allows quantification of MPOD and distinction between its constituents lutein and zeaxanthin, and is therefore a suitable method to further investigate the reduction or accumulation of MP in retinal regions of interest (van de Kraats et al., 2008).
Section snippets
Subjects
Fourteen patients with MacTel type 2 were investigated. Mean age of the patients was 62.2 years (SD 6.9 years, range 51–74 years) and mean visual acuity was 20/40 (range, 20/20–20/250; log MAR visual acuity used for calculation). Nine patients were female and five were male. Two patients had been taking supplements containing lutein and zeaxanthin (patient #13: 12 mg lutein and 840 μg zeaxanthin per day for about 5 years; patient #14: 6 mg lutein and 200 μg zeaxanthin every other day for about 2
Results
In Fig. 1 we show the MPOD maps obtained by two wavelengths FAF of all right eyes of the study population (patients with MacTel type 2 according to Table 1) and of three control subjects. The abnormal distribution in the patients' MPOD maps, i.e. reduced central MPOD signal and a ring-like structure at about 6 degrees eccentricity, is visible in all of the patients' eyes.
The upper panel of Fig. 2 shows the mean reflectance as a function of wavelength at 4 and 6 degrees eccentricity for patients
Discussion
Our study shows that reduced MPOD in the central retina can occur in retinal diseases as part of their pathophysiology. Moreover, it shows that the earlier described prominent ring at about 6 degrees eccentricity in cSLO autofluorescence and reflectance maps observed in patients with MacTel type 2 (Helb et al., 2008, Charbel Issa et al., 2008) is indeed caused by MP. Relating the amount of MP to disease progression unfortunately seems unreasonable in this study. Problems include the possibly
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Supported by the Lowy Medical Research Institute (The Macular Telangiectasia Project, www.mactelresearch.com); BONFOR Program, grant O-137.0011 (Faculty of Medicine, University of Bonn); EU FP6, Integrated Project “EVI-GENORET” (LSHG-CT-2005-512036).
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These authors contributed equally to the work.