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Imaging Choroidal Disorders: The Future
Seventy percent of the total blood flow to the eye travels through the vascular network that is the choroid. Its primary function is to provide nutrients to the outer retinal structures, as well as the foveal avascular zone. However, there is evidence to suggest that the choroid has other roles including; thermoregulation in ocular tissues, modulation of intraocular pressure via vasomotor control of blood flow, and drainage of aqueous humor via the uveoscleral pathway.
Anatomy of the Choroid
The outer retina is dependent on the nutrients provided to it by the choroidal vasculature that consists of Bruch’s membrane, the choriocapillaris layer (CC), Sattler’s layer which contains medium diameter vessels, Haller’s layer that contains large diameter vessels, the suprachoroidal potential space, and finally the choroidal–scleral interface.
Diseases that Affect the Choroid
Dilation of the choroidal vessels may lead to an increased choroidal thickness as seen in central serous chorioretinpathy (CSCR), and this may be associated with an increase in the choroidal hydrostatic pressure and vascular permeability. The reverse of this situation is choroid thinning, when there is a reduction in the nourishment to the retina as in old age and in age-related macular degeneration (AMD).
The high blood flow in the choroid also predisposes it to the embolic spread of metastatic tumors and infectious diseases including cytomegalovirus and toxoplamosis. The choroid is also the site of a number of inflammatory disorders involving the posterior segment of the eye including Vogt–Koyanaghi–Harada disease, birdshot chorioretinopathy, and choroidal granulomas in sarcoidosis or tuberculosis.
Current Techniques
Dye-Based Angiography
Traditional imaging modalities, including fluorescein angiography (FA), indocyanine-green angiography (ICGA), and ultrasonography, offer only an incomplete view of the in-vivo structure of the choroid and offer only poor quantitative data for research due to the poor resolution and repeatability inherent in the technology. Clinically, FA and ICGA are still considered the gold standard for imaging the choroidal vasculature. These imaging modalities are dynamic and allow direct visualization of the filling of large choroidal vessels and eventual leakage and/or pooling of dye. Of the two modalities, ICGA is superior for visualizing the choroidal vasculature as it has a longer wavelength of fluorescence than FA, this helps to enhance the visualization of structures beneath blood, exudates or retinal pigment epithelium (RPE) detachments in greater detail. ICGA has also been shown to reveal more detail in choroidal neovascularization (CNV) and has a greater sensitivity at detecting choroidal polyps than FA.
However, in both these techniques the microvascular architecture and feeder vessels of the choroid are often obscured in conditions such as neovascular AMD (nAMD) by hyperfluorescence in the late phase of dye transit, which can limit precise assessment of CC and the choroid. In addition, these modalities are invasive involving the use of intravenous contrast that can result in systemic side effects and rarely anaphylaxis.
Laser Doppler Flowmetry
Laser doppler flowmetry is a technique that is primarily used in research to allow the noninvasive measurement of choroidal blood flow by measuring the speed and number of the erythrocytes moving in a sampling volume. This in-vivo technique has provided evidence that decreased flow in the choroidal vasculature may be linked to various diseases including diabetic retinopathy, AMD, and retinitis pigmentosa.
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