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Explore the underdiscovereddthe choroid of the eye

The choroid of the eye is primarily a vascular structure containing large membrane-lined lacunae, nonvascular smooth muscle cells, and intrinsic choroidal neurons. It lies between the retina and sclera, extending from the ora serrata anteriorly to the optic nerve posteriorly, and is composed of three layers: the choriocapillaries, the medium vessel layer, and the large vessel layer. Choroidal blood flow constitutes 75e85% of the total blood flow that perfuses the eye, which is responsible for vascular support and temperature modulation of the outer retina. More than 90% of the oxygen delivered to the retina is consumed by the photoreceptors, which are extremely metabolically active in order to maintain ion homeostasis. A normal choroidal vasculature is essential for the health of retinal pigment epithelium (RPE) and photoreceptors, and an abnormal choroidal vasculature plays a critical role in the pathophysiology of a variety of retinal diseases such as diabetic retinopathy and age-related macular degeneration. In a clinical setting, choroidal vasculature often is assessed with fluorescein angiography and indocyanine green angiography that evaluate the time sequence and distribution pattern of the injected dye that fills the choroid. However, these technologies have limitations in that they provide neither quantitative blood flow data nor structural information of the RPE and choroid, and patients may develop adverse reactions to the injected dye. Laser Doppler flowmetry has been used by some researchers to measure choroidal blood flow, but the device is not available in most areas of the world. In recent years, various versions of spectral-domain optical coherence tomography (SD-OCT) that use an interferometer with a highspeed spectrometer to simultaneously measure light echoes from all time delays, have become available on the market. Coupled with improvement in the quality of light sources, the cross-section image with an axial resolution of 5e7 mm of the choroid can be assessed noninvasively under normal and pathologic conditions. However, the outer limit of the choroid abutting against the sclera cannot usually be reliably identified using standard SD-OCT. In order to more accurately measure the choroidal thickness (CT), techniques such as image averaging, mechanical eye tracking, or enhanced depth imaging usually have to be adopted. Assessment of CT by SD-OCT may play a role in the differentiation of age-related maculopathy from polypoidal choroidal vasculopathy, of central serous chorioretinopathy from other causes of serous retinal detachment, and may help in the determination of disease