Macular pigment is the collective name for three carotenoids, lutein, zeaxanthin and meso-zeaxanthin, which are found at higher concentrations in the retina than anywhere else in the body, and to the exclusion of all other carotenoids. They are only accessible to the body by dietary intake of foodstuffs or supplements containing them, with high levels being found in certain fruits and vegetables, such as kiwi fruit, corn and spinach, as well as egg yolks.
Analysis of donor maculae is possibly the most unequivocal approach for assessing the distribution of macular pigment in the retina, and pioneering work by Snodderly and colleagues in the 1980s achieved this. Using primate monkeys and the technique of microdensitometry, it was confirmed, as expected, that macular pigment reaches its peak in the centre of the retina. There was then a sharp decline to negligible levels at approximately 1 mm (4°) from the central fovea. In 1988, Bone et al., using high-performance liquid chromatography (HPLC), investigated the spatial distribution of macular pigment in human donors; in this case, it was found to reach negligible levels at 7° eccentricity. Within the retinal layers, macular pigment is primarily located in the photoreceptor axons and to a lesser extent in the inner plexiform layer.
The macular carotenoids have an absorption spectrum of 400–540 nm, peaking at approximately 460 nm. This spectral peak, along with the spatial distribution and retinal layer localization of macular pigment contribute to its proposed function as a blue-light filter. Short-wavelength (blue) light is more damaging to the retina than longer-wavelength light, so by attenuating the amount of blue light reaching the photoreceptors, macular pigment may protect the macula from this photo-damage; the higher the density of macular pigment (macular pigment optical density, or MPOD), the greater the amount of blue-light filtering that will occur.
A second proposed function of macular pigment is that it protects the macula against oxidative stress by acting as an antioxidant.
These blue-light filter and antioxidant functions have led to the school of thought that having a high MPOD could help to protect against the eye disease age-related macular degeneration (AMD), the most prevalent cause of severe visual impairment in Western society. As a result, there have been a multitude of studies investigating possible links between MPOD and AMD, using a variety of measurement techniques. Some of these studies have supported an MPOD–AMD association and some have not. This inconsistent evidence is not too surprising, given the apparent multifactorial nature of AMD. What’s more, it is highly likely that an individual’s MPOD is equally multifactorial, but as one of the few potentially modifiable risk factors for AMD, its continued investigation is extremely important.
Macular pigment optical density may be measured in vitro or in vivo. In vitro measurement involves the techniques of HPLC or microdensitometry. However, they can only be performed on excised retinas, and so are clearly not suitable for widespread use. This review therefore details the most common techniques currently used to measure MPOD in vivo. These in vivo techniques are noninvasive, and are normally categorized under one of two headings: psychophysical (requiring a response from the subject) or objective (requiring minimal input from the subject). Together, these techniques have established that MPOD varies widely between individuals, from virtually no macular pigment to greater than 1 log unit optical density, with average levels ranging from 0.16 to 0.69, depending on the method and/or the study population.
Methods and Results
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.
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.
Discussion and Conclusions
There are currently two main psychophysical techniques for measuring MPOD in vivo, and three main objective techniques. All take advantage of the spectral absorption properties of macular pigment, but in very diverse ways. This diversity may be useful for macular pigment research, but it does present difficulties for those wishing to compare MPOD values between techniques. For instance, does the value represent the peak density of macular pigment, the density of macular pigment at a certain point within the fovea, or the total amount within the target area?
If macular pigment measurement is to become commonplace in large populations, then equipment investors will have an important decision to make with regard to the method they choose to employ. Unfortunately, as each MPOD technique has its own benefits and limitations, there is no clear ideal choice, as highlighted by Beatty, van Kuijk and Chakravarthy. Heterochromatic flicker photometry is probably the most affordable choice. It is also an established, valid and reliable method, particularly when protocols are followed as per 'customized' HFP. There is, however, the problem that some individuals find this task very difficult, and their results cannot necessarily be relied upon.
A commercially available objective technique would therefore be desirable, possibly through adaptation of a scanning laser ophthalmoscope or fundus camera. The former is often used in a hospital setting, and the latter is commonly found in optometric practice.
A future objective technique could make use of FR or AF to assess MPOD, although AF may be preferred over FR because it is less influenced by light scatter and appears to have better reliability. Both have the facility to measure the spatial distribution of macular pigment, and this seems to be an increasingly useful advantage. The main issue associated with macular pigment screening using an objective technique is the need for pupil dilation, although several non-mydriatic devices have now been developed.
The researchers conclude that the measurement of MPOD is best conducted using an objective technique based on FR or AF, but we acknowledge that a commercial instrument capable of this is not currently available. The development of such an instrument will aid research in this area and provide a better understanding of the relationship between MPOD and AMD, as well as supporting MPOD screening in a clinical setting. Source:Graefes Arch Clin Exp Ophthalmol. 2011 Jan 8. [Epub ahead of print]