Retinopathy

Excerpt

The retina contains photoreceptor cells that function in the process of visual transduction, i.e., transforming light signals to nerve impulses eventually transmitted from the optic nerve to the brain forming an image. There are two types of photoreceptor cells, rod, and cones. The cone cells are for color vision and the rod cells, which function well in low light conditions, i.e., scotopic vision. The macula is the small part of the retina responsible for central vision (see Figure), and it has a very high density of cone cells thereby providing high visual acuity. Retinopathy is not a specific disease but refers to pathological alterations to the retina resulting from a variety of causes, environmental conditions, and genetic factors. The retina is especially susceptible to the effects of oxidative stress damage since it has the highest content of polyunsaturated fatty acids of any tissue as well as the highest oxygen consumption.

In mammals, the retina is supplied with oxygen and nutrients by the retinal and choroidal circulatory systems, and most major causes of retinopathy involve damage to these systems. The retinal circulation supplies the inner (towards the center of the eye) half of the retina and the choroidal circulation supplies the outer half of the retina. Light must pass through the retinal capillaries (and some neuronal layers) before striking the photoreceptors (see Figure). The choroid lies behind the retinal pigment epithelium (RPE). The RPE, the macula, and anterior segment of the optic nerve depend on the choroidal circulation for oxygen and nutrients (see Figure). Lack of sufficient blood flow in the retinal circulation can lead to optic neuropathy and vision loss. Rupture of the retinal capillaries can lead to bleeding into the vitreous humor (vitreous hemorrhage) and vision loss.