The spatial profile of macular pigment (MP) optical density has been shown to vary considerably among subjects. The optical density of MP, measured in log units, typically peaks centrally and declines sharply with eccentricity away from the foveola.
1–3 Central MP optical density (MPOD) has been reported to be lower with age,
4 smoking,
5 in the presence of inflammation promoting conditions (e.g., diabetes),
6 in females,
7 and in the presence of light iris color.
8,9 Previous studies described MP spatial profiles with either a single peak decaying exponentially,
2,10,11 a central dip (i.e., without a central peak),
10,11 or exhibiting a secondary peak up to 2° eccentricity, also referred to as a subpeak, shoulder, bimodal, or ring-like structure.
2,10 Using psychophysical heterochromatic flicker photometry (HFP), Hammond et al.
2 found that the MP distribution of 32 Caucasian subjects was best described by an exponential fit. However, the authors also discovered that approximately 40% of subjects presented secondary subpeaks (defined as increments greater than 0.05 optical density units from the exponential fit) at 1° and 2°. More recent studies have shown similar bimodal MP spatial profiles in a significant proportion of subjects.
10,12–15 The presence of a parafoveal ring was also shown in 20% to 50% of subjects when using objective autofluorescence imaging (AFI) techniques.
10,15–17 Moreover, using AFI, the frequency of ring-like profiles was found to be significantly greater in females and in nonsmokers,
15,16 and in healthy subjects (43%) compared with patients with age-related maculopathy (23%).
15 Similar findings have also been demonstrated in ethnicities with a low prevalence of AMD, whereby 86% of African subjects presented with secondary peaks versus 68% non-Hispanic, white subjects.
17 However, it was also suggested that the lack of a central peak could possibly have an adverse effect on the protective role of MP in AMD, as the prevalence of a central dip has been found to increase with age and smoking in Caucasian subjects.
11