A new desktop instrument for measuring macular pigment optical density based on a novel technique for setting flicker thresholds

The macular pigment optical density spatial profile and increasing age

PURPOSE

To investigate the relationship between the central spatial profile of macular pigment optical density (MPOD) and increasing age in normal eyes.

METHODS

Ninety-eight individuals (aged 19-71 years) with good visual acuity, free from ocular disease, and with clear ocular media participated. MPOD was measured at 0.25, 0.50, 1.00, and 1.75° eccentricity from the foveal centre using a heterochromatic flicker photometry based densitometer instrument.

RESULTS

Linear regression analysis revealed that there was no statistically significant association between MPOD and increasing age for the group as a whole at 0.25, 0.50, and 1.00° eccentricity (p > 0.05 for all eccentricities). There was a small but statistically significant positive association between increasing age and MPOD at 1.75° eccentricity (p = 0.020), but age only accounted for 6 % of the variation in MPOD values. Fifteen percent of all participants had a non-exponential MPOD spatial profile.

CONCLUSION

There was no statistically significant relationship between MPOD and increasing age for three of the four locations measured. A significant proportion of individuals show an atypical MPOD spatial profile, indicating that studies on MPOD should ideally report information on the MPOD spatial profile rather than estimates from only one retinal location.

Imaging system to assess objectively the optical density of the macular pigment in vivo

This paper presents an optical system called MacPI, which implements a two-color reflectance technique in combination with various hardware and software tools to assess objectively the macular pigment (MP) optical density in vivo. The system consists of a bespoke optical design, a control architecture, driver electronics, a collection of image-processing techniques, and a graphical user interface. The deficiencies of the technique employed and the solutions implemented in the MacPI system to confront those inherent frailties are presented. An overview of the effective interpretation of the acquired data and the techniques employed by MacPI in the acquisition of that data is discussed. The result of a comparison trial with an alternative device is also presented. We suggest that appropriate design of the hardware and an efficient interpretation of the acquired data should produce a system capable of consistent, accurate, and rapid measurements, while retaining the distinction of ease of use, portability, comfort for the subject, and a design that is economic to produce. Its versatility should allow both for a clinical screening application and for further investigation and establishment of the physiological role of the MP in a laboratory-based environment.

Macular pigment in ophthalmic practice; a survey

BACKGROUND

Macular pigment (MP) might provide some protection against age-related eye disease, and it is now being measured in ophthalmic practice. The purpose of the survey described here was to determine the distribution of MP in a random population of patients in a typical UK ophthalmic practice.

METHODS

Macular pigment optical density (MPOD) was measured in 56 patients aged 11 to 87 years, mean 52 ± 19, over a 3-month period. Typically, the test requires setting flicker thresholds for a centrally and peripherally viewed blue/green alternating target. Here we describe the results when an age-based estimate of the peripheral value is used, thus avoiding the peripheral setting. In 32 observers, a comparison was made between this and values obtained with the centre and periphery method. Information on smoking habits, iris colour, diabetic status, and ethnicity were recorded.

RESULTS

The overall average MPOD for the population obtained with the centre-only approach for 56 individuals was 0.400 ± 0.165. The centre-only technique was an accurate predictor of values based on centre and peripheral measures, with 95% limits of agreement of 0.137 OD units. Pearson's correlation coefficient showed a high correlation between right and left eyes (r = 0.7 (p < 0.001)). There was a small difference between males and females that did not reach statistical significance (r = -0.22). There was a non-statistically significant age-related decline in MPOD in this particular population (r = -0.17). Dark irides were significantly associated with high MPOD (r = 0.28, p < 0.05). MPOD in Type II diabetic patients was 27% lower than that in non-diabetics (r = 0.29, p < 0.05).

CONCLUSION

The technique provides similar values of MP optical density to previous reports. As with other HFP-based methods, in a small percentage of older patients, more than one measurement is required before satisfactory results are obtained.

Inverse relationship between macular pigment optical density and axial length in Chinese subjects with myopia

BACKGROUND

Macular pigment (MP) has been the focus of much attention in recent years, due to its protective effect against macular degenerations. In this study, we investigated the association between macular pigment optical density (MPOD) and axial length (AL) in Chinese subjects with myopia.

METHODS

In total, 173 myopes (mean spherical equivalent [MSE] ≤-1.00D) were recruited for this prospective observational study. MPOD was measured in both eyes of each subject using a macular metrics densitometer. AL was measured in eyes using an IOL-Master. A raw coefficient of correlation analysis and a partial correlation analysis were used to investigate the relationship between MPOD and AL.

RESULTS

The age of the subjects ranged from 18 to 67 years. The overall mean MPOD for the cohort was 0.412 ± 0.119 (range, 0.105-0.812). The mean AL was 25.18 ± 1.08 mm (range, 23.14-28.19 mm). Using a raw coefficient of correlation, a significant inverse correlation was found between MPOD and AL (r= -0.134, p=0.012). When using a partial correlation analysis to eliminate the impact of covariant, a significant inverse correlation was also found between MPOD and AL (r= -0.142, p=0.008). Furthermore, when AL was divided into two groups: AL>26 mm and AL ≤ 26 mm, a significant inverse correlation was observed between MPOD and AL in the former (r= -0.253, p=0.029), but no significant relationship was observed between these in the latter (r=0.104, p=0.067).

CONCLUSIONS

MPOD correlated inversely with AL in this sample of Chinese subjects with myopia.

Improving the repeatability of heterochromatic flicker photometry for measurement of macular pigment optical density

BACKGROUND

Heterochromatic flicker photometry (HFP) is a psychophysical technique used to measure macular pigment optical density (MPOD). We used the MPS 9000 (MPS) HFP device. Our aim was to determine if the repeatability of the MPS could be improved to make it more suitable for monitoring MPOD over time.

METHODS

Intra-session repeatability was assessed in 25 participants (aged 20-50 years). The resulting data was explored in detail, e.g., by examining the effect of removal and adjustment of data with less than optimal quality parameters. A protocol was developed for improved overall reliability, which was then tested in terms of inter-session repeatability in a separate group of 27 participants (aged 19-52 years).

RESULTS

Removal and adjustment of data reduced the intra-session coefficient of repeatability (CR) by 0.04, on average, and the mean individual standard deviation by 0.004. Raw data observation offered further insight into ways of improving repeatability. The proposed protocol resulted in an inter-session CR of 0.08.

CONCLUSIONS

Removal and adjustment of less than optimal data improved repeatability, and is therefore recommended. To further improve repeatability, in brief we propose that each patient perform each part of the test twice, and a third time where necessary (described in detail by the protocol). Doing so will make the MPS more useful in research and clinical settings.

An evaluation of a novel instrument for measuring macular pigment optical density: the MPS 9000

PURPOSE

Of the antioxidants found in the human retina, only the macular carotenoid quantities can be estimated noninvasively (albeit in a collective fashion), thus facilitating study of their role in that tissue. The aim of this study was to evaluate concordance between macular pigment optical density (MPOD) values recorded on a commercially available instrument, the MPS 9000, with those of an already validated heterochromatic flicker photometry instrument. Also, we assessed and compared test-retest variability for each instrument.

METHODS

Macular pigment optical density at 0.5 retinal eccentricity was measured using two different heterochromatic flicker photometers, the MPS 9000 and the Macular Densitometer(TM), in 39 healthy subjects. Test-retest variability was evaluated separately for each instrument by taking three readings over a 1-week period in 25 subjects.

RESULTS

There was a moderate positive correlation for MPOD at 0.5° of retinal eccentricity between the MPS 9000 and the Macular Densitometer described by the linear equation y = 0.763x + 0.172 (r = 0.68, p < 0.001, r(2) = 0.46); however, a paired-samples t-test showed a significant difference in terms of mean values, with a bias of lower MPOD values being yielded by the MPS 9000 (t = -4.103, p < 0.001). Bland-Altman analysis indicated only moderate agreement between the two instruments, reflected in 95% limits of agreement of 0.1 ± 0.27. Inter-sessional repeatability, expressed as a coefficient of repeatability, ranged from 0.18 to 0.21 [mean (±SD): 0.19 (0.02)] for the MPS 9000 and from 0.11 to 0.12 [mean (±SD): 0.12 (0.01)] for the Macular Densitometer.

CONCLUSION

The results demonstrate that the MPS 9000 consistently yields MPOD readings, which are lower than that found with the Macular Densitometer, and exhibits substantial test-retest variability.

Randomized, double-blind, placebo-controlled study of zeaxanthin and visual function in patients with atrophic age-related macular degeneration: the Zeaxanthin and Visual Function Study (ZVF) FDA IND #78, 973

BACKGROUND

The purpose of this study is to evaluate whether dietary supplementation with the carotenoid zeaxanthin (Zx) raises macula pigment optical density (MPOD) and has unique visual benefits for patients with early atrophic macular degeneration having visual symptoms but lower-risk National Institute of Health/National Eye Institute/Age-Related Eye Disease Study characteristics.

METHODS

This was a 1-year, n = 60 (57 men, 3 women), 4-visit, intention-to-treat, prospective, randomized controlled clinical trial of patients (74.9 years, standard deviation [SD] 10) with mild-to-moderate age-related macular degeneration (AMD) randomly assigned to 1 of 2 dietary supplement carotenoid pigment intervention groups: 8 mg Zx (n = 25) and 8 mg Zx plus 9 mg lutein (L) (n = 25) or 9 mg L ("Faux Placebo," control group, n = 10). Analysis was by Bartlett's test for equal variance, 3-way repeated factors analysis of variance, independent t test (P < 0.05) for variance and between/within group differences, and post-hoc Scheffé's tests. Estimated foveal heterochromic flicker photometry, 1° macular pigment optical density (MPOD QuantifEye(®)), low- and high-contrast visual acuity, foveal shape discrimination (Retina Foundation of the Southwest), 10° yellow kinetic visual fields (KVF), glare recovery, contrast sensitivity function (CSF), and 6° blue cone ChromaTest(®) color thresholds were obtained serially at 4, 8, and 12 months.

RESULTS

Ninety percent of subjects completed ≥ 2 visits with an initial Age-Related Eye Disease Study report #18 retinopathy score of 1.4 (1.0 SD)/4.0 and pill intake compliance of 96% with no adverse effects. There were no intergroup differences in 3 major AMD risk factors: age, smoking, and body mass index as well as disease duration and Visual Function Questionnaire 25 composite score differences. Randomization resulted in equal MPOD variance and MPOD increasing in each of the 3 groups from 0.33 density units (du) (0.17 SD) baseline to 0.51 du (0.18 SD) at 12 m, (P = 0.03), but no between-group differences (Analysis of Variance; P = 0.47). In the Zx group, detailed high-contrast visual acuity improved by 1.5 lines, Retina Foundation of the Southwest shape discrimination sharpened from 0.97 to 0.57 (P = 0.06, 1-tail), and a larger percentage of Zx patients experienced clearing of their KVF central scotomas (P = 0.057). The "Faux Placebo" L group was superior in terms of low-contrast visual acuity, CSF, and glare recovery, whereas Zx showed a trend toward significance.

CONCLUSION

In older male patients with AMD, Zx-induced foveal MPOD elevation mirrored that of L and provided complementary distinct visual benefits by improving foveal cone-based visual parameters, whereas L enhanced those parameters associated with gross detailed rod-based vision, with considerable overlap between the 2 carotenoids. The equally dosed (atypical dietary ratio) Zx plus L group fared worse in terms of raising MPOD, presumably because of duodenal, hepatic-lipoprotein or retinal carotenoid competition. These results make biological sense based on retinal distribution and Zx foveal predominance.

The relationship between peripherally matched invariant hues and unique hues: a cone-contrast approach

A characteristic shift in hue and saturation occurs when colored targets are viewed peripherally compared with centrally. Four hues, one in each of the red, blue, green, and yellow regions of color space, remain unchanged when presented in the peripheral field. Apart from green, these peripherally invariant hues correspond almost exactly in color space with the unique hues. We explore this puzzling observation using asymmetric color-matching and color-naming experiments and computing cone contrasts for peripheral and central stimuli. We find that the difference between cone contrasts for the peripheral and central stimuli reaches a maximum at the chromatic axis corresponding to peripherally invariant green. We speculate that the effect is linked to a weakened signal from M-cones and probably associated with a reduced number of M-cones in peripheral retina.

Assessment of macular pigment optical density (MPOD) in patients with unilateral wet age-related macular degeneration (AMD)

PURPOSE

To compare the macular pigment optical density (MPOD) of patients with unilateral wet age-related macular degeneration (AMD) with the MPOD of bilateral dry AMD patients and healthy elderly individuals.

METHODS

The MPOD of 34 patients with unilateral wet AMD was measured in their fellow eye that had the dry form of the disease (study group). The MPOD of the study group was compared with the MPOD of 33 patients with bilateral dry AMD (patients' control group) and 35 elderly subjects without any signs of retinal disease (control group). None of the subjects was under carotenoid supplementation. The MPOD was measured with Heterochromatic Flicker Photometry [QuantifEYE™- MPS 9000 (ZeaVision(©))]. The statistical package SPSS v 17.0 was used for the analysis.

RESULTS

The overall mean MPOD was 0.52 (SD 0.15). Patients with unilateral wet AMD have significantly higher levels of MPOD in their fellow eye compared with patients with bilateral dry AMD (0.58 versus 0.48, p = 0.026). Mean MPOD of patients with bilateral dry AMD does not differ significantly from that of healthy elderly subjects (0.48 versus 0.50, p = 0.865). In this population sample, no correlation with age was observed, while women have slightly but significantly higher levels of MPOD (0.55 versus 0.49, p = 0.029).

CONCLUSION

In the present study, the mean MPOD at the fellow eye of patients with unilateral wet AMD was found to be significantly higher than that of patients with bilateral dry AMD, while no other significant difference emerged between groups. Further investigation is demanded to clarify the role of macular pigment in AMD progression.

The Macular Assessment Profile test - a new VDU-based technique for measuring the spatial distribution of the macular pigment, lens density and rapid flicker sensitivity

The measurement of macular pigment optical density (MPOD) in the eye is often carried out using optical techniques based on heterochromatic flicker photometry (HFP). These require the use of two spectrally-narrow beams, one at the wavelength of maximum absorption of the macular pigment (MP) and the other in the long wavelength region of the visible spectrum where MP absorption is negligible. A new technique for the measurement of MPOD spatial profiles has been developed by overcoming the current shortcomings associated with the use of visual displays. The new Macular Assessment Profile (MAP) test makes use of a 'notch' filter and a photometric model to measure and compute the peak MPOD value. Two other useful parameters are also computed from the same measurements. These describe the subject's sensitivity to rapid flicker and the absorption of blue light by the lens. MPOD profiles, lens density, rapid flicker sensitivity, and red/green (RG) and yellow/blue (YB) colour thresholds were measured in 54 normal subjects aged 18-61 years. The results confirm previous findings on ageing effects and demonstrate the complete absence of correlation between MPOD and the subject's YB chromatic thresholds. In contrast, RG chromatic sensitivity improves with higher levels of MPOD.

Measuring macular pigment optical density in vivo: a review of techniques

BACKGROUND

Macular pigment has been the focus of much attention in recent years, as a potential modifiable risk factor for age-related macular degeneration. This interest has been heightened by the ability to measure macular pigment optical density (MPOD) in vivo.

METHOD

A systematic literature search was undertaken to identify all available papers that have used in vivo MPOD techniques. The papers were reviewed, and all relevant information was incorporated into this article.

RESULTS

Measurement of MPOD is achievable with a wide range of techniques, which are typically categorized into one of two groups: psychophysical (requiring a response from the subject) or objective (requiring minimal input from the subject). The psychophysical methods include heterochromatic flicker photometry and minimum motion photometry. The objective methods include fundus reflectometry, fundus autofluorescence, resonance Raman spectroscopy and visual evoked potentials. Even within the individual techniques, there is often much variation in how data is obtained and processed.

CONCLUSION

This review comprehensively details the procedure, instrumentation, assumptions, validity and reliability of each MPOD measurement technique currently available, along with their respective advantages and disadvantages. This leads us to conclude that development of a commercial instrument, based on fundus reflectometry or fundus autofluorescence, would be beneficial to macular pigment research and would support MPOD screening in a clinical setting.

Macular pigment optical density measurements: evaluation of a device using heterochromatic flicker photometry

PURPOSE

Accurate assessment of the amount of macular pigment (MPOD) is necessary to investigate the role of carotenoids and their assumed protective functions. High repeatability and reliability are important to monitor patients in studies investigating the influence of diet and supplements on MPOD. We evaluated the Macuscope (Macuvision Europe Ltd., Lapworth, Solihull, UK), a recently introduced device for measuring MPOD using the technique of heterochromatic flicker photometry (HFP). We determined agreement with another HFP device (QuantifEye; MPS 9000 series: Tinsley Precision Instruments Ltd., Croydon, Essex, UK) and a fundus reflectance method.

METHODS

The right eyes of 23 healthy subjects (mean age 33.9 ± 15.1 years) were measured. We determined agreement with QuantifEye and correlation with a fundus reflectance method. Repeatability of QuantifEye was assessed in 20 other healthy subjects (mean age 32.1 ± 7.3 years). Repeatability was also compared with measurements by a fundus reflectance method in 10 subjects.

RESULTS

We found low agreement between test and retest measurements with Macuscope. The average difference and the limits of agreement were -0.041 ± 0.32. We found high agreement between test and retest measurements of QuantifEye (-0.02 ± 0.18) and the fundus reflectance method (-0.04 ± 0.18). MPOD data obtained by Macuscope and QuantifEye showed poor agreement: -0.017 ± 0.44. For Macuscope and the fundus reflectance method, the correlation coefficient was r = 0.05 (P = 0.83). A significant correlation of r = 0.87 (P<0.001) was found between QuantifEye and the fundus reflectance method.

CONCLUSIONS

Because repeatability of Macuscope measurements was low (ie, wide limits of agreement) and MPOD values correlated poorly with the fundus reflectance method, and agreed poorly with QuantifEye, the tested Macuscope protocol seems less suitable for studying MPOD.

Correspondence between retinal reflectometry and a flicker-based technique in the measurement of macular pigment spatial profiles

A comparison of macular pigment optical density (MPOD) spatial profiles determined by an optical and a psychophysical technique is presented. We measured the right eyes of 19 healthy individuals, using fundus reflectometry at 0, 1, 2, 4, 6, and 8 deg eccentricity; and heterochromatic flicker photometry (HFP) at 0, 0.5, 1, 2, 3, 4, 5, 6, and 7 deg, and a reference point at 8 deg eccentricity. We found a strong correlation between the two techniques. However, the absolute estimates obtained by fundus reflectometry data were higher than by HFP. These differences could partly be explained by the fact that at 8 deg eccentricity the MPOD is not zero, as assumed in HFP. Furthermore, when performing HFP for eccentricities of <1 deg, we had to assume that subjects set flicker thresholds at 0.4 deg horizontal translation when using a 1-deg stimulus. MPOD profiles are very similar for both techniques if, on average, 0.05 DU is added to the HFP data at all eccentricities. An additional correction factor, dependent on the steepness of the MPOD spatial distribution, is required for 0 deg.