OCT 09, 2014
This editorial addresses a study in the same issue that found a significant relationship between choroidal thickness and visual acuity. The editorial concludes that, with increasing longevity worldwide coupled with the epidemic increase in high myopia, choroidal loss could become one of the most significant causes of moderate and even severe vision loss.
The editorial’s author, Richard F. Spaide, MD, notes that the choroid has the highest blood flow and densest packing of vessels of any tissue in the body and has the lowest oxygen extraction. Other tissues in the body extract much of the oxygen carried by hemoglobin, and those tissues seem to do just fine. Looking at these parameters suggests that the choroid’s capacity may be much greater than what it really needs. So how could a thinner choroid make any real difference in function?
He explains that while the choroid does show some reserve in functional capability, studies have shown that even with small decreases in atmospheric oxygen tension, visual function is not optimal. He writes, “Given that the evolutionary pressure for survival fine-tuned the eye across many divergent and oftentimes competing species in parallel, that all of them ended up with a design that includes a highly vascularized choroid suggests an anatomic optimum, at least for individuals through reproductive age.” Nevertheless, he says we have very little knowledge of how choroidal thickness and delivery of oxygen are related.
He notes that while the study on choroidal thickness and vision demonstrates a statistically significant change in vision with small decrements in subfoveal choroidal thickness, these changes are extremely small and have no clinical utility. However, he says that a much more important finding was that of a threshold in subfoveal choroidal thickness below which function shows a meaningful decline. A choroid thinner than 30 μm was associated with a visual acuity loss of 0.31 logMAR. He says that this level of thickness may be seen in older individuals, is common in those with pathologic myopia, and is nearly universal in people with both.
Dr. Spaide concludes that the magnitude of visual acuity change identified in the study is very similar to the association found between acuity and thinner choroids in eyes with high myopia, andthe extension of this finding to eyes without high myopia increases the pool of patients likely to have vision loss related to a thin choroid.