The Ringlike Structure of Macular Pigment in Age-Related Maculopathy: Results from the Muenster Aging and Retina Study (MARS)

Abstract

Purpose.: The role of macular pigment (MP) in age-related maculopathy (ARM) is still not clearly understood. Recent studies have reported on variations in the spatial distribution of MP optical density (MPOD) including a secondary peak (“ring”) in the slope of the MPOD profile. The authors investigated in a cross-sectional manner the presence of ringlike structures, their determinants, and their relationship with ARM.

Methods.: In all, 369 participants of the Muenster Aging and Retina Study were examined using dual-wavelength analysis of autofluorescence images. ARM was graded using digital fundus photographs according to the International Classification System.

Results.: A ringlike structure was observed in 73 (19.8%) study participants. The MP maximum of the ring was located on average at 0.85° and the minimum at 0.48° from the center of the fovea. Their concordance between pairs of eyes was highly significant. MPOD measured at eccentricities of 0°, 0.25°, and 0.5° from the fovea was significantly lower in eyes with ringlike structure, whereas it was significantly higher at 1.0° and 2.0° than that in eyes without the ring. Ringlike structures were significantly more common in females and never smokers and were found significantly less often in eyes with ARM than in healthy eyes, even after adjustment for influential factors (adjusted odds ratio, 0.347; 95% confidence interval, 0.196–0.617).

Conclusions.: Ringlike structures in the MP spatial profile are fairly common, show a high degree of bilaterality, and appeared inversely related with ARM.

The macular pigment (MP) consists of three dietary carotenoids, lutein (L), zeaxanthin (Z), and meso-zeaxanthin (MZ), which accumulate at high concentrations at the macula. ¹ L and Z are entirely of dietary origin, whereas MZ is largely derived from retinal L. The MP acts as an antioxidant and as a filter of short-wavelength light, therefore limiting photooxidative damage to retinal cells. ¹ Due to these properties, it is supposed that MP may protect against the development of age-related maculopathy (ARM) and its late-stage, age-related macular degeneration (AMD), the leading cause of blindness in people older than 50 years in the developed world. ¹  

Corroborating histologic ² and clinical studies, ^3,4 we have previously reported on a wide variation of MP distribution. ⁵ Recently, several research groups have shown that MP optical density (MPOD) does not always steadily decline from the center of the fovea to the periphery. They observed ringlike structures with a secondary peak of MP at a distance of 0.7° from the fovea. ^6,7 Others reported on “central dip” MP profiles with a dip of MPOD in 0.25° retinal eccentricity, followed by a rise at 0.5°, and finally a steady decline to the periphery. ⁸ Although the impact of those variations of MPOD profiles is still unknown, there is first evidence that the spatial distribution of MP might be associated with risk factors for ARM and AMD, respectively. ⁸  

We therefore investigated the ringlike structure in the spatial distribution of MPOD, its determinants, and its association with ARM in patients with ARM and eye-healthy controls who participated in the first follow-up examination of the Muenster Aging and Retina Study (MARS). 

Material and Methods

Subjects

MARS is a longitudinal study designed to identify medical, environmental, and genetic factors, with implications for the pathogenesis and progression of ARM. Eligibility criteria were previously described in detail. ⁹ A sample of 1060 residents of the Muenster (Germany) region with and without ARM were examined at baseline (MARS-I) of whom 828 (85.5% of all eligible) took part in MARS-II. ⁵ Median follow-up time was 2.6 years. In MARS-II, we replicated the baseline study protocol and additionally measured the MPOD. ⁵ For the present report, we used only data obtained at MARS-II in a cross-sectional manner. 

As described previously, ⁹ subjects were interviewed by trained staff members using a standardized risk factor questionnaire. Detailed information was obtained about smoking history, lifestyle, and the current use of supplements containing L and/or Z. Height and weight were measured. The spherical equivalent refractive error in each eye and the number of pseudophakic eyes were determined. 

The recruitment and research protocols were reviewed and approved by the Institutional Review Board of the University of Muenster, and written informed consent was obtained from all study participants, in compliance with the Declaration of Helsinki. 

Grading of ARM

As previously described, ⁹ 30° stereoscopic digital color fundus photographs were taken from both eyes after pupil dilatation with tropicamide 0.5% and phenylephrine 2.5%. The diagnosis of ARM features was based on grading of the photographs according to the International Classification and Grading System for ARM. ¹⁰ In accordance with the Rotterdam Study classification, ARM was classified into five severity stages. ¹¹ Eyes were classified as having no ARM (stages 0 to 1), ARM (stages 2 to 3), or as AMD (stage 4). ^12,13  

Measurement of MPOD

The autofluorescence (AF) method for measuring MPOD has been previously described. ^{14 –18} In brief, it is based on the AF of lipofuscin, which is located in the retinal pigment epithelium (RPE) cells. ^14,19 Lipofuscin is excited in vivo between 400 and 580 nm, to emit its fluorescence in the 500- to 800-nm spectral range, whereas MP absorbs blue light for wavelengths shorter than 550 nm, with a peak absorbance of 460 nm. ¹⁶ In the fovea, excitation light within the absorption range of MP is partially absorbed by the carotenoids, resulting in an area of reduced fluorescence. To measure the MPOD, the dual-wavelength approach of the AF method compares results from two excitation wavelengths that are differentially absorbed by the MP, thereby taking account of the nonuniform distribution of lipofuscin in the RPE. ¹⁶  

For quantitative imaging, we used a Heidelberg Retina Angiograph (HRA 1; Heidelberg Engineering, Heidelberg, Germany) that was modified for the measurement of MP. This approach has previously been used in clinical studies. ^{5,15,17,18,20} In brief, alignment and focusing of the fundus were performed under 488-nm illumination. Retinal bleaching was conducted to minimize the influences of absorption of incoming light by the visual pigments. ¹⁷ For bleaching, the central 20 × 20° area of the retina was illuminated with the 488-nm wavelength light of the HRA 1 for at least 30 seconds. ¹⁷ After retinal bleaching, sequences of 20° images were captured at 488 and 514 nm and MP density maps were generated by digital subtraction of the log AF images, and mean MPOD values were calculated for circles centered on the fovea as previously described. ¹⁷ In our analyses, these circles were located at eccentricities of 0.25°, 0.5°, 1.0°, and 2.0°. The reference location where MPOD is optically undetectable was selected at an eccentricity of 6° because the profile normally plateaus for eccentricities larger than approximately 4°. ¹⁶  

MP Density Profile and Ringlike Structure

To investigate the spatial distribution of MP, we analyzed the MP density maps and the radial density profiles. The latter were generated and graphically displayed by plotting the mean MPOD values that were calculated for each radius around the fovea, against the distance to the fovea ²¹ (see Figs. 1a, 1c, 1e). 

Figure 1.

 

Radial density profiles of macular pigment (MP) optical density (MPOD) (a, c, e) and associated MP density maps (b, d, f) with and without ringlike structures. (a, b) A bimodal distribution of MPOD with a secondary peak of MPOD forming the ringlike structure. (c, d) An intermediate distribution of MP without strictly monotonic decline of MPOD but with an implied plateau in the slope of the profile. (e, f) A steady decline of MPOD from the center of the fovea to the periphery without any ringlike signs.

A ringlike structure was defined as the density profile showing a bimodal pattern; the latter consists of a central peak of MPOD followed by a decline and a secondary peak of increased density on the slope of the profile (see Fig. 1a). The bimodal pattern of MPOD is also visible in the associated MP density map as a “ringlike structure,” with a central peak of MPOD surrounded by a ring of increased density (Fig. 1b). In cases where the distribution of MPOD showed no strictly monotonic decline from the center of the fovea to the periphery, and no explicit ringlike pattern of MP (but, for example, an implied plateau on the slope of the profile), the eyes were marked as having “intermediate distributions” (Figs. 1c, 1d). The absence of a ringlike structure was defined as a strictly monotonous decline of the density profile graph from the center of the fovea to the periphery without any plateauing or bimodal pattern (Fig. 1e), with the associated MP density map showing no ring patterns (Fig. 1f). 

Density profiles were analyzed for the eccentricity at which the maximum and minimum of the ring occurred, and for the respective MPOD measured at these eccentricities; the “maximum” of the ring was thereby defined as the secondary maximum of MP forming the ring and the “minimum” as the minimum density between the central peak of MPOD and the second peak. Furthermore, all MPOD profiles were analyzed for the “half width,” which indicates the eccentricity from the fovea where half of the peak of MPOD is reached. 

Study Sample

MPOD measurements were taken in 609 of 828 participants of MARS-II because the HRA 1 became available only after MARS-II had already started. AF images of inadequate quality and those with central AMD (i.e., with central geographic atrophies or choroidal neovascularizations that affect the measurement of AF ¹⁷ ) were excluded. From the 659 eyes with valid AF measurements we excluded those with missing information in relevant study variables, such that data on 571 eyes of 369 participants were available for analyses. Bilateral data were available for 202 pairs of eyes. 

Apart from the assessment of symmetry in pairs of eyes, we present patient-based analyses using the results obtained in one study eye: where measurements for single eyes only were available, we used these eyes as study eyes; if measurements for pairs of eyes were available, we used the result of the right eye, with the exception of cases with an “intermediate distribution” in the right eye and a clear attribution of the ringlike structure in the left eye (“ring” or “no ring”), in which case the latter was used in the analyses. 

Statistical Analysis

The present report is a cross-sectional analysis of data obtained at the MARS-II examination, which included measurements of MPOD, spherical equivalent refractive error, body mass index (BMI), age, smoking history, current use of MP-containing supplements, status of the lenses (phakic, pseudophakic), and classification of ARM. Spearman's rank correlation coefficients were computed to assess the association of the localization and the respective MPOD values of the ringlike structure between pairs of eyes. For analyses of single eyes, χ² tests were computed to compare categorical and t-tests for continuous variables. Pairwise U tests were used for unadjusted comparison of the three MP distribution groups. The impact of influential factors and confounders on the presence of ringlike structures was evaluated by multivariable logistic regression models. Adjusted odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated. A value of P < 0.05 was considered statistically significant. A commercial analytical software package (SAS for Windows, version 9.1; SAS Institute Inc., Cary, NC) was used for analysis. 

Results

The 369 participants included in this report had a mean age of 71.6 years. They were more often females and about half of the single study eyes were free of ARM (stage 0 to 1 according to the Rotterdam Study classification). Other characteristics of the participants are contained in Table 1. 

Table 1.

 

Characteristics of Study Participants (n = 369)

Characteristic	Value*
Mean age, y	71.6 (5.3)
Female; n (%)	227 (61.5)
Mean BMI, kg/m2	27.3 (4.1)
Smoking
Never smokers; n (%)	243 (65.9)
Ever (current, former) smokers; n (%)	126 (34.1)
Current users of supplements containing lutein and/or zeaxanthin; n (%)	56 (15.2)
Study eyes; showing a ringlike structure of MPOD; n (%)	73 (19.8)
Pseudophakic study eyes; n (%)	65 (17.6)
Spherical equivalent refractive error in phakic study eyes	+0.53 (2.38)
ARM stage according to Rotterdam Classification (in study eye):
Stage 0; n (%)	99 (26.8)
Stage 1; n (%)	95 (25.7)
Stage 2; n (%)	89 (24.1)
Stage 3; n (%)	86 (23.3)

*  Values are given as mean (SD), unless otherwise noted.

Of the 369 single study eyes, 73 (19.8%) showed a ringlike structure, 245 (66.4%) had no ring, and 51 (13.8%) displayed an “intermediate distribution.” Figure 2 shows the mean MPOD in density units (D.U.) at six eccentricities from the fovea (0°, 0.25°, 0.5°, 1.0°, 2.0°, and 6.0°) and the mean “half width” according to these three types of spatial MPOD patterns. The mean eccentricity of the minimum of ringlike structures was at 0.48 ± 0.09°, whereas the maximum was at 0.85 ± 0.15°; the corresponding mean MPOD values were 0.45 ± 0.16 and 0.56 ± 0.17 D.U., respectively (Fig. 2a). 

Figure 2.

 

Spatial profile of macular pigment in study participants (a) with ringlike structure (n = 73), (b) with intermediate MP distribution (n = 51), and (c) with no ring (n = 245). The mean MPOD in D.U. with 95% confidence interval is presented at six eccentricities from the fovea: 0° (the peak), 0.25°, 0.5°, 1.0°, 2.0°, and 6.0° (the reference); the mean MPOD values at the minimum and maximum eccentricities of the ring were additionally included in (a). The dotted vertical line indicates the mean eccentricity where half of the peak MPOD was reached within each group (“half width”).

Table 2 shows the mean MPOD at different eccentricities within the three groups. Mean MPOD measured at 0°, 0.25°, and 0.5° from the fovea was significantly lower in eyes with a ring than in those without a ring. In contrast, eyes with a ring had significantly higher mean MPOD measured at 1.0° and 2.0° compared with that of the no-ring group. Likewise, the mean “half width” was significantly higher in eyes with a ring pattern than that in eyes without a ring pattern. MPOD values of eyes with intermediate distributions were rather consistently in between the values found in eyes with and without a ring. 

Table 2.

 

Mean MPOD Measured at Different Eccentricities from the Center of the Fovea and the Occurrence of Ringlike Structures in Subjects with Ringlike Structure (n = 73), with Intermediate Distribution of MP (n = 51), and without Ringlike Pattern (n = 245)

MPOD Measurements	Units	Mean (SD)	P Value*
Peak MPOD (0°)	D.U.
Ringlike structure		0.75 (0.24)	Ref.
Intermediate distribution of MP		0.71 (0.20)	0.3122
No ringlike structure		0.86 (0.25)	0.0011
MPOD at 0.25°	D.U.
Ringlike structure		0.58 (0.20)	Ref.
Intermediate distribution of MP		0.61 (0.17)	0.4106
No ringlike structure		0.69 (0.20)	0.0002
MPOD at 0.5°	D.U.
Ringlike structure		0.50 (0.17)	Ref.
Intermediate distribution of MP		0.54 (0.16)	0.1202
No ringlike structure		0.59 (0.18)	0.0001
MPOD at 1.0°	D.U.
Ringlike structure		0.51 (0.16)	Ref.
Intermediate distribution of MP		0.46 (0.13)	0.0630
No ringlike structure		0.41 (0.16)	<0.0001
MPOD at 2.0°	D.U.
Ringlike structure		0.19 (0.08)	Ref.
Intermediate distribution of MP		0.17 (0.08)	0.2467
No ringlike structure		0.14 (0.07)	<0.0001
Eccentricity where half of the peak MPOD is reached (“half width”)	deg
Ringlike structure		1.26 (0.49)	Ref.
Intermediate distribution of MP		1.18 (0.49)	0.1377
No ringlike structure		0.90 (0.35)	<0.0001

 D.U., density units; Ref., reference group.

*   P value of pairwise U tests versus reference group with ringlike structure.

In 202 pairs of eyes, ringlike patterns were highly symmetrical: 41 (20.3%) had a bilateral ringlike structure, 108 (53.5%) had no ringlike structure, and 22 (10.9%) had an intermediate MP distribution in both eyes; by contrast, only 14 (6.9%) showed a ringlike structure in solely one eye and 17 (8.4%) had an intermediate distribution in one and no ring in the other eye (P < 0.0001). Similarly, the localization of the ringlike structure, the respective MPOD values, and further characteristics of the ring pattern showed a high degree of symmetry between right and left eyes (see Table 3). 

Table 3.

 

Characteristics of the Ringlike Structure in Pairs of Eyes with Bilateral Ring Pattern (n = 41)

Characteristic	Units	Right Eye*	Left Eye*	Spearman's Rank Correlation Coefficient†
Peak MPOD (0°)	D.U.	0.71 (0.23)	0.72 (0.21)	0.91
Eccentricity of minimum of the ring	deg	0.48 (0.09)	0.49 (0.11)	0.89
MPOD at minimum of the ring	D.U.	0.43 (0.18)	0.44 (0.16)	0.85
Eccentricity of maximum of the ring	deg	0.85 (0.14)	0.84 (0.14)	0.89
MPOD at maximum of the ring	D.U.	0.53 (0.19)	0.53 (0.17)	0.91
“Half width”	deg	1.24 (0.48)	1.20 (0.51)	0.81
MPOD at maximum of the ring/Peak MPOD (0°)	—	0.77 (0.18)	0.75 (0.17)	0.81
MPOD at minimum of the ring/Peak MPOD (0°)	—	0.61 (0.16)	0.62 (0.16)	0.74
MPOD at maximum of the ring/MPOD at minimum of the ring	—	1.27 (0.26)	1.23 (0.19)	0.48

*  Values are given as mean (SD).

†  For each Spearman's rank correlation coefficient: P < 0.0001.

We found no difference in the presence of ringlike structures in phakic and pseudophakic eyes (P = 0.6992). Likewise, we found no association between the spherical equivalent refractive error in phakic eyes and MPOD values measured at 0°, 0.25°, 0.5°, 1.0°, and 2.0°, the mean half width, and the localization of the minimum or maximum of the ringlike structure (correlation analyses, each P > 0.05). 

Females and never smokers showed significantly more often ringlike structures than did males and ever smokers (Table 4). In contrast, other potential influential factors such as current use of supplements containing L and/or Z, overweight (defined as BMI ≥ 30 kg/m²), and age older than 70 years were not significantly associated with the presence of ringlike structures. 

Table 4.

 

Association of the Ringlike Structure of MPOD with Other Factors among 369 Study Participants, χ² Test

Factor	Ringlike Structure			P Value
	Present, n (%)	Intermediate Distribution, n (%)	Absent, n (%)
Sex
Female	57 (25.1)	27 (11.9)	143 (63.0)
Male	16 (11.3)	24 (16.9)	102 (71.8)	0.0039
Smoking
Never	60 (24.7)	33 (13.6)	150 (61.7)
Ever	13 (10.3)	18 (14.3)	95 (75.4)	0.0040
Current use of supplements*
Yes	13 (23.2)	8 (14.3)	35 (62.5)
No	60 (19.2)	43 (13.7)	210 (67.1)	0.7580
Body mass index, kg/m2
<30	59 (21.1)	39 (14.0)	181 (64.9)
≥30	14 (15.6)	12 (13.3)	64 (71.1)	0.4737
Age group, y
<70	38 (23.0)	25 (15.2)	102 (61.8)
≥70	35 (17.2)	26 (12.7)	143 (70.1)	0.2328
Age-related maculopathy
No	53 (27.3)	30 (15.5)	111 (57.2)
Yes	20 (11.4)	21 (12.0)	134 (76.6)	0.0001

*  Containing lutein and/or zeaxanthin.

Interestingly, a ringlike structure was clearly less common in the presence of ARM (Table 4). Further detailed analyses with logistic regression models revealed an unadjusted OR of 0.343 (95% CI, 0.196–0.603; P = 0.0002) for the occurrence of a ringlike structure compared with the group with no ring or an intermediate distribution, when ARM was present (see Table 5, model 1). Thus, the ringlike structure seemed almost three times less common in eyes with ARM than that in healthy eyes. Adjustment for the influential factors sex and smoking did not materially alter the magnitude of this association (Table 5, model 2). 

Table 5.

 

Association of the Ringlike Structure of MPOD with Age-Related Maculopathy

Predictor Variables	OR	95% CI	P Value
Unadjusted, model 1
ARM	0.343	0.196–0.603	0.0002
Adjusted, model 2
ARM	0.347	0.196–0.617	0.0003
Sex (female)	2.214	1.178–4.162	0.0136
Smoking (ever smokers)	0.453	0.230–0.889	0.0213

 This table provides results of logistic regression models comparing the presence of a ringlike structure of MPOD with that of an intermediate distribution/no ring; n = 369 study participants.

Of note, drusen size, drusen type, and area covered by drusen (within the central foveal field [radius: 500 μm] of the standard grid used for ARM classification by fundus photographs) showed no statistically significant differences in ARM patients with a ring, without a ring, or with intermediate distribution (data not shown). Likewise, in study eyes with ringlike structure, the localization of the maximum and minimum of the ring and the respective MPOD values were not statistically significantly different between participants with and without ARM (data not shown). 

Discussion

We used quantitative analyses of AF images and identified ringlike structures in the spatial distribution of MP, which showed a high degree of symmetry between right and left eyes and occurred significantly more often in females, never smokers, and in eyes without ARM. 

The first systematic evaluation of the ringlike structure of MPOD was presented by Staurenghi G, et al. (IOVS 2003;44:ARVO E-Abstract 5188) and Delori FC, et al. (IOVS 2004;45:ARVO E-Abstract 1288). By analyzing AF images, Delori et al. ⁷ described bimodal spatial distributions of MP that were characterized by a central peak of highest MP density surrounded by a ring with high-density values at approximately 0.7° from the fovea. These results were confirmed by Berendschot and van Norren, ⁶ who additionally used the technique of reflectance spectroscopy, and by Wolf-Schnurrbusch et al., ²¹ who observed ringlike structures at 0.66°. These findings are well compatible with our observations. We determined the mean maximum of ringlike structures at 0.85° from the center of the fovea, whereas we found the minimum, like Delori et al., ⁷ at 0.48°. In the latter report, the MPOD values at the maximum and minimum were 81% and 75% of the peak MPOD, respectively, which is also in the same range of magnitude as our results in 41 pairs of eyes with bilateral ring. Moreover, Delori and colleagues ⁷ reported on the “MPOD at maximum to minimum ratio” of 1.09, which was lower than our results of 1.27 in right and 1.23 in left eyes. Thus, the differences of MPOD at maximum and minimum of the ringlike structure were slightly more pronounced in our study. 

Since unbleached cone pigment could theoretically influence the profile of the MP distribution, ⁷ retinal bleaching was performed before taking the first MPOD measurements in our study, in accordance with procedures previously described. ¹⁷  

Although the ringlike structure and its localization can be consistently observed with different imaging modalities, ⁷ the anatomic basis of the ringlike structure is not yet fully understood. Snodderly et al. ²² measured the MP density profiles in retinal layers of macaque and cebus monkeys and found in some animals a “trimodal” distribution with secondary maxima at 200–300 μm (i.e., around 0.8° eccentricity) from the fovea. The main peak was associated with MP along the photoreceptor axons, whereas the secondary maxima followed the inner plexiform layer. The authors hypothesized that profile variability in different species might be the result of differences in the shape of the foveal depression. ²² Interestingly, Kirby et al. ²³ recently reported on an association between “secondary peaks” in the MP spatial profile measured by heterochromatic flicker photometry (HFP) and wider foveae measured by optical coherence tomography (OCT) in a small group of healthy individuals (n = 16), indicating a relationship between spatial distribution of MPOD and foveal architecture. They measured the MPOD at five distinct eccentricities from the fovea (i.e., at 0.25°, 0.5°, 1.0°, 1.75°, and 7°) and defined profiles with lower MPOD at 0.25° than at 0.5° as “secondary peaks,” thus also confirming variations in spatial distribution of MPOD. On the other hand, the location of these “secondary peaks” differed from the “ringlike structures” found in our study and the mentioned studies ^6,7,21 and reasons for this difference need to be further elucidated. 

Whereas other studies reported a mean MPOD of 0.28 D.U. measured at 1.0° eccentricity from the fovea ²⁴ and 0.54 D.U. at 0.25°, 0.44 D.U. at 0.5°, and 0.32 D.U. at 1.0°, ²⁵ using devices similar to the one used in our study, our MPOD measurement results were slightly higher. On the other hand, our MPOD values are well compatible with the results of Lima et al., ¹⁵ who reported on MPOD of 0.51 D.U. measured at 0.5°, and of Trieschmann et al., ¹⁷ who reported on a mean MPOD of 0.50 D.U. measured at 0.5° eccentricity. Interestingly, in the latter study, oral supplementation with lutein raised MPOD on average by 0.1 D.U., which is comparable with our results; we found that MPOD at 0.5° was on average 0.09 D.U. higher in users of L/Z supplements compared with nonusers (0.64 vs. 0.55 D.U.). ⁵ These findings support our contention that supplement use by study subjects may explain higher values in our study. 

Regarding the symmetry in the spatial distribution of MP between pairs of eyes, Snodderly et al. ²² examined retinal sections of a monkey and found a similar shape of the MP density profile (i.e., the “trimodal” distribution mentioned earlier) in both foveae. Similarly, Staurenghi and colleagues (IOVS 2003;44:ARVO E-Abstract 5188) reported a high degree of symmetry of the ring pattern in 36 study subjects measured in vivo by AF imaging (P = 0.002). Our study confirms these results, showing a high degree of symmetry of the localization of the ring, the respective mean MPOD values, the peak MPOD at 0°, and the “half width” in pairs of eyes with ringlike structures. Thus, a substantial part of retinal MP levels may be genetically closely regulated, as suggested by Liew et al. ²⁴ in a twin study on the heritability of MPOD. Although these authors did not explicitly analyze the spatial distribution of MPOD, they observed a high degree of correlation of MPOD measured at 1° eccentricity by AF imaging that was more pronounced in mono- than that in dizygotic twins. ²⁴  

We observed significant differences in MPOD measured at eccentricities of 0°, 0.25°, 0.5°, 1.0°, and 2.0° between participants with and without a ring. These differences are likely due to diverse MP profiles: the broader distribution in individuals with a ringlike structure (i.e., a secondary peak at 0.85°) is also transferred to higher MPOD values at 1.0° and 2.0° and higher “half width” values; conversely, lower MPOD values at 0.25° and 0.5° are a reflection of the minimum of the ring at 0.48°. Interestingly, the intermediate distribution group rather consistently showed mean MPOD values in between those of the ring and the no-ring groups. The mean “half width” of the intermediate group was similar to that of the ring group, indicating a broader MPOD distribution but without a secondary peak in the slope of the MPOD profile. 

We further noted that the presence of ringlike structures was not associated with eyes being phakic or pseudophakic. Likewise, we found no association between the spherical equivalent refractive error and MPOD in phakic study eyes. The issue of refractive defect has been previously addressed, ^26,27 with results similar to those reported in our study. 

Our results of ringlike structures being significantly more common in females are compatible with the work of Delori et al. ⁷ These authors hypothesized that shapes and sizes of the fovea differ between males and females, with a more open foveal depression in females. ⁷ However, this was not supported by a recent study reporting a reduced central subfield thickness in females compared with that in males, but no sex-dependent differences in foveal pit morphology, as measured by spectral-domain (SD) OCT. ²⁸ Moreover, Berendschot and van Norren ⁶ did not find any differences between males and females with regard to the presence of ring structures. 

The relationship between age and MPOD has been previously investigated, ^5,20,29 but with inconsistent results: although some studies reported a decline of MPOD with age, ²⁰ we found a slight increase of MPOD (measured at 0.5° and 2.0°) with age, ⁵ whereas others found no age dependence at all. ²⁹  

The presence of ringlike structures was not associated with age in our elderly study individuals with a mean age of 71.6 years (range, 62–85 years). Despite examining younger subjects with a mean age of 50 years (range, 19–76 years), Berendschot and van Norren ⁶ also found no effect of age on the ring structure in the spatial distribution of MPOD. Interestingly, although not explicitly mentioning age dependence of the ringlike structure in individuals ranging in age from 20 to 70 years, Delori et al. ⁷ reported on a broadening of the MP distribution with age; the occurrence of “intermediate distributions,” which included plateaus on the slopes of the profiles; and a distribution pattern that “is often fragmented at old age.” These results seem to concur with those in our elderly study population, where we also found “intermediate distributions” in the spatial distribution of MPOD. 

To our knowledge, this study is the first to evaluate the relationship between smoking and the ringlike structure in a large study population. We report here that never smokers showed significantly more often a ringlike structure than did ever smokers. Of note, Delori et al. ⁷ studied 41 individuals and did not detect an effect of smoking on the ring pattern. To our knowledge, there are also no studies that investigated the impact of supplementation with L and/or Z on the ringlike structure. We found no statistically significant relationship between current supplementation with L and/or Z and the presence of ring patterns in our cross-sectional study. However, we did not have data on duration of supplement use and its dosage in current users, and this lack of detailed information may partly account for the absence of such an association. On the other hand, Connolly et al. ³⁰ recently reported on the disappearance of “central dips” (i.e., lower MPOD at 0.25° than that at 0.5° of eccentricity) after 8 weeks of oral supplementation with L, Z, and MZ in four study participants, measured by HFP, thus suggesting an influence of supplementation on spatial distribution of MPOD. These results might indicate that environmental factors related to oxidative stress could modify the spatial distribution of MPOD. However, further studies are obviously warranted in this area. 

The most important finding of our study was that ringlike structures were significantly more common in persons without ARM. It may be worth noting that all the studies on ringlike structures of MPOD mentioned earlier involved only persons with normal retinal status, ^6,7,21 whereas we were able to compare groups of subjects with and without ARM. As reported earlier, mean MPOD was not different between our study participants with and without ARM after adjustment for confounders and exclusion of users of L/Z supplements. ⁵  

In our study setting, 1.0° eccentricity from the fovea corresponded to 270- to 305-μm eccentricity from the fovea. Thus, the central field of the standard grid used for ARM classification, which has a radius of 500 μm, included the ringlike structures whose mean peak was found at 0.85° eccentricity from the fovea. Of note, morphologic characteristics of drusen, such as type, number, and area covered by drusen within the central radius of 500 μm of the macula, were not different between ARM patients with and without ringlike structures. Likewise, we found no differences in the localization of the ring and the respective MPOD values between participants with and without ARM when a ringlike structure was present. 

Recently, first evidence was presented that indicated that the spatial distribution of MP might be associated with risk factors for ARM and AMD. ⁸ Using HFP, Kirby et al. ⁸ observed “central dip” MP profiles (defined by a dip of MPOD at 0.25°, a rise at 0.5°, and then a steady decline to the periphery) more often in older subjects and in smokers. They concluded that the “central dip” might represent an undesirable feature of macular pigmentation. Contrary to our study, Kirby and colleagues ⁸ included persons ranging in age from 18 to 70 years without any ocular pathology. 

Interestingly, former studies that involved younger and ophthalmologically healthy subjects found the ringlike structure more frequently (in approximately half of the subjects ^6,7 ) than we did in our elderly study participants (in approximately 20%) who, in addition, relatively often showed intermediate distribution patterns. Therefore, we hypothesize that the ringlike structure of MP may be interpreted as an individual characteristic of the fovea that tends to change or even disappear in the course of life and, in particular, in combination with ARM; this may be potentially related to alterations in absorption and storage of MP, especially in the parafoveal area. 

In conclusion, we have performed extensive analyses on the ringlike structure of MPOD as a specific pattern of spatial distribution of MP, comparing persons with and without ARM. We have shown that ringlike structures exhibited a high degree of symmetry between right and left eyes and were more common in females, never smokers, and persons free of ARM. The reasons and mechanisms of these relationships are presently not clearly understood. Therefore, longitudinal studies should investigate how the spatial distribution of MPOD changes individually over the course of time. Furthermore, longitudinal studies including the techniques of high-resolution SD-OCT might help to better understand whether changes in the spatial distribution of MP are the sequelae of developing ARM or whether the ringlike structure in the spatial distribution is protective against ARM. Similarly, the impact of MP-containing supplements on the spatial distribution of MPOD and their potential effect on the occurrence or progression of ARM need to be further explored in longitudinal trials. We are confident that further analyses of the prospective long-term observation of the MARS cohort will be able to contribute to a better understanding of the processes involved. 

Footnotes

 Supported in part by Deutsche Forschungsgemeinschaft Grants HE 2293/5-1, 5-2, 5-3, and PA 357/7-1; the Intramural International Monetary Fund of the University of Muenster; the Pro Retina Foundation; and the Jackstaedt Foundation.

Footnotes

 Disclosure: M. Dietzel, None; M. Zeimer, None; B. Heimes, None; D. Pauleikhoff, None; H.-W. Hense, None