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  • Choriocapillaris Flow Features Follow a Power Law Distribution

    By Lynda Seminara and edited by Richard K. Parrish II, MD
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    Journal Highlights

    American Journal of Ophthalmology, October 2016

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    Spaide     hypothesized that the pattern of flow voids seen in the choriocapillaris during in vivo optical coherence to­mography angiography (OCT-A) could be related to ocular and even systemic microvascular disease states. Howev­er, there are technical limitations that make it difficult to image the chorio­capillaris in vivo: light scattering within the overlying tissue, and inadequate lat­eral resolution in both fluorescein an­giography and OCT. In this paper, the author presents a method that allows analysis of the structure and pattern of the flow signal in the choriocapillaris.

    OCT-A and subsequent analysis were performed on 104 eyes of 80 patients ranging in age from 24 to 99 years (median, 71 years) to mathematically model blood flow in the choriocapil­laris. The choriocapillaris was sampled as a 10-μm-thick section starting 31 μm or 34 μm posterior to the segmen­tation of the retinal pigment epitheli­um and Bruch’s membrane complex.

    Although OCT-A cannot resolve individual choriocapillary vessels, a visible pattern of bright and dark areas suggests their structure, with the bright areas representing blood flow; and the dark areas, flow voids. Evalua­tion of the distribution of the number of flow voids reveals a potential mathe­matical relationship that could be used to grade choriocapillaris flow.

    Flow voids were identified with the Phansalkar method of thresholding and were measured, counted, and binned by size. The log-transformed data were fit to a slope-intercept equa­tion. Results showed that the occur­rence of flow voids in the choriocap­illaris is a fractal process that follows a power-law distribution, with many small flow voids and progressively fewer larger flow voids.

    In the choriocapillaris, capillary branches are organized as discrete lobules, and blood flow is segmented. In this study, flow voids generally were smaller than the lobules, suggesting that flow abnormalities occur sub­lobularly. The power law distribution implies that the presence of numerous low-flow segments in the microvas­culature increases the likelihood that flow abnormalities will occur in neigh­boring segments.

    The author concluded that this study has a number of new findings, which consequently suggest new research possibilities. In particular, the blood flow in the choriocapillaris has been found to have a mathematically defined structure that has been impli­cated, but not shown, in living tissue before. Flow voids in the choriocap­illaris may be predictive of systemic microvascular abnormalities and may contribute to macular diseases such as late AMD.

    The original article can be found here.