On my visit with Dr. Dan Press in Park Ridge, we’ve been discussing (among other things) the Macular Pigment Ocular Densitometer (MPOD), a device he’s begun to consider regarding the impact of nutrition on athlete’s vision after discussions with a colleague, Dr. Jarrod Davies. Our friend, Dr. Graham Erickson, who spoke on Sports Vision at this year’s COVD meeting, has done a webinar addressing the connection between dietary zeaxanthin, MPOD, and visual performance oriented toward athletes.
Our conversation took me back to a blog six years ago on the subject, with regard to the relevance of MPOD in amblyopia. In that post I speculated on whether the Macula Integrity Tester (MIT)/Haidinger Brush could be conceptualized as “the poor person’s MPOD”, both being subjective measures of macular pigmentary properties (density in the case of MPOD, and configuration in the case of the MIT).
Looking more closely at the subject brings further evidence that subjective and objective imaging of the macular region might shed more light not only on the differential diagnosis of amblyopia, but on an opto-metric that may be useful in providing feedback during therapy as well as in documenting structural improvement. The first piece of evidence I’ll offer, at least in organic amblyopia, is a study published two years ago in IOVS showing a direct linkage between visual acuity, MPOD, and perception of the Haidinger Brush.
Another intriguing piece of evidence comes through an abstract from 2016 European Association for Vision and Eye Research Conference, This study of MPOD measurements in children with strabismus suggests that there is a significant difference in MPOD between the two eyes in patients with unilateral strabismus, whereas patients who were alternators had similar MPOD values in both eyes. This strengthens the argument that structural differences exist between the two eyes at the retinal level that have previously gone unrecognized or under-appreciated. One can gain a sense of how pervasive this is through a marvelous PhD thesis from the University of Bradford on The Measurement of Retinal Structure in Amblyopia Using Optical Coherence Tomography, and the fact that that structural abnormalities support the binocular rather than monocular nature of amblyopia. This and other sources suggest that part of the suppression or adaptation process in amblyopia is that the brain “dumbs down” the preferred eye in an effort to reduce the performance differences between the two eyes.
Recognizing that there is always a “chicken vs. egg” element to these considerations, if we accept that structural retinal abnormalities exist in amblyopia that negatively impact function, can amblyopia therapy improving function have a salient impact on retinal structure? The findings of a recent study in Current Eye Research indicate that children with hyperopic anisometropic amblyopia have significant alterations in choroidal thickness, and that these changes partially regress after amblyopia treatment.
From a functional standpoint, we often emphasize the use of the MIT/Haidinger Brush in amblyopia therapy as providing increased awareness of and feedback about the stability of fixation. But the fact that fixation stability is abnormal even in the fellow eye of patients with amblyopia, further supports the use of such therapy in both eyes.
Accommodative and ocular motor stability training in amblyopia with the MIT is, of course, predicated on the ability to perceive the Haidinger Brush in the first place. Why is it so difficult for some patients to see the Haidinger Brush through the amblyopic eye? The likelihood is that the amount or proper configuration of xanthophyll plays a significant role in addition to the rotating polarization of the dichroic carotenoid. If MPOD shows that the macular pigment (xanthophyll) is too sparse, the blue filter isn’t canceling sufficiently with the yellow pigment to create the expected black bow-tie/propellor appearance (which is why we need to add more or deeper blue filters). If OCT shows the choroid is thickened, the bow-tie/propellor appears as more of a blob that a thin-centered dark object. Viewed in this manner, accurate perception and use of the HB/MIT isn’t a side-show in amblyopia therapy, but a powerful illustration of improved function altering structure.