Dyeless, non-invasive optical coherence tomography angiography (OCT-A) is an easy, fast, software-generated newer investigative tool for retinal and uveitic diseases. In the past few years, the structural and functional components of retinal diseases were studied by OCT-A with revolutionary development. OCT-A stressed an understanding of histology and histopathology of retinal and choroidal layers. Retinal vessel density mapping particularly in the macula is the proportion of blood vessel areas with blood flow measured over the total area.
The macula in the posterior pole has a thickness of 160–190 μm where it forms a slight depression having the fovea and within it, the foveola. The fovea has the highest concentration of cones with a high density of xanthophyllic pigments and pigment epithelial cells which are densely filled and have a high density of pigment granules. OCT-A can involve split-spectrum amplitude-decorrelation angiography which helps in the clear differentiation of superficial and deep vascular plexuses. The superficial vascular plexus is located in the ganglion cell layer and nerve fiber layer, whereas the deep vascular plexus is located in the inner nuclear and external plexiform layers. The superficial plexus has vascular distribution represented by white linear structures against the black background in centripetal arrangement forming a web by secondary retinal vessels. The deep plexus consists of a close interlaced pattern around the avascular foveal zone. Both superficial and deep vascular plexuses are interconnected with small vessel anastomoses.
The present article by the author(s) to document macular vessel densities by OCT-A is a step forward in intermediate uveitis (IU) among South Indian uveitic populations. The correlation of this study with the baseline normative OCT-A data is important. IU is fairly common in uveitis patients in almost all patterns of uveitis studies within India and abroad. There may be various causes of IU which include both infectious and noninfectious diseases. The most important cause of IU is idiopathic (pars planitis) followed by sarcoidosis and tuberculosis. The study has shown the following parameters of superficial capillary plexuses: 15–70 μm (below the internal plexiform layer) and deep capillary plexus 3 μm (below the internal plexiform layer) – 15 μm (below the internal plexiform layer). In this study, inclusion and exclusion criteria in selecting the cases for the studies were made. Macular microvascular changes in patients with IU using OCT-A were analyzed, and they tried to establish a relationship between the measurements of those plexuses with disease duration and disease activities. Sixteen patients were investigated in the study (Eight males and eight females); the average age of the patients was 28.80 ± 12.80 years. Macular thicknesses were measured using a Heidelberg Spectralis (Germany) machine where superficial and deep macular vessels were examined. The parameters were compared with those of healthy volunteers. Macular thickness was found to be higher in the patient group (P < 0.001). Superficial and deep vessel densities were almost lower in all quadrants in the IU patient group (P < 0.05). Macular thickness was increased in active disease but none of the OCT-A parameters showed a significant difference between the active and inactive diseases. It was concluded that macular vessel density reduction was noted in both superficial and deep plexuses in IU. These parameters could provide critical data to observe the IU cases in the follow-up.
Some of the limitations of the study could be its fewer number of cases in the study. Moreover, If the biomarkers of some etiological cases, for instance, serum angiotensin-converting enzyme levels in sarcoidal IU, could be correlated with those involved in microvascular changes, it would give more important clues in the disease-specific IU. IU is also known for its complications in the macula. Studying the temporal associations in IU or posterior uveitis with microvascular macular changes would be an interesting aspect to look at in future large-scale studies. Epi-retinal membrane formation is one of the important structural complications in IU. This was not seen in the present study which can affect the microvascular component in the macula.
Fractal dimensional analysis of OCT-A refers to newer grayscale image modalities that demarcate superficial and deep plexuses in some of the retinal diseases, particularly diabetic retinopathy. In diabetic eyes, the vascular density was also significantly reduced in superficial and deep plexuses, and applied fractal analysis to OCT-A imaging has potential pathological parameters in microvascular characteristics. Fractals are the patterns found in nature and biological systems that show the same level of complexity and are compared from the usual general pattern drawn in a scale in which they are measured. The best part of the fractal geometric analysis is that a mathematical framework can be drawn on those biological structures. Fractal analysis in OCT-A for retinal vasculature compared with future three-dimensional color fundus photographs and fluorescein angiography can tell us about the microvascular changes in a better way. They can prognosticate severe retinal and uveitic diseases, calculate retinal threat stratification, and monitor disease management practices.
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