Photocoagulation Therapy Health Article

Structure of the human eye

To fully understand how photocoagulation therapy works, it is helpful to have a basic picture of the structure of the human eye. The retina is the innermost tunica, or covering, of the posterior part of the eyeball. It is made of several layers of cells, one of which contains the rod and cone cells that are sensitive to light. Behind the retina are the other two tunicae of the eye, the choroid and the sclera. The sclera is a tough white layer of tissue that covers the exterior of the eyeball. At the front of the eye, the sclera is continuous with a transparent area of tissue known as the cornea.

At the back of the eye, the retina is continuous with the optic nerve. The macula, which is a yellowish oval-shaped area that is the central point of vision, lies in the center of the retina. In front of the retina is the vitreous body, which is also known as the vitreous humor, or simply the vitreous. The vitreous body is a clear gel that consists primarily of water and collagen fibers.

Types of retinal detachment (RD)

RHEGMATOGENOUS. A rhegmatogenous RD is the most common of the three types of retinal detachment. The word rhegmatogenous comes from a Greek word that means "tear." A rhegmatogenous RD typically occurs in older people. As the vitreous body in the center of the eyeball ages, it shrinks and pulls away from the retina. This separation is called a posterior vitreous detachment (PVD). A PVD is not the same thing as a retinal detachment, although it may slightly increase the risk of an RD. In places where the retina is still attached to the vitreous body, a small hole or tear can develop. Over time, fluid can seep into the area around the hole or tear and thus enlarge the area of detached tissue.

TRACTION. Traction RDs are most often found in adults with diabetic retinopathy or infants with retinopathy of prematurity (ROP). Diabetic retinopathy is a disorder that develops when the patient's diabetes affects the small blood vessels in the eye. Although diabetic retinopathy is more severe in patients with type 1 diabetes (insulin-dependent), it can also occur in patients with type 2. Retinal detachment is most likely to occur in a subtype of the disorder known as proliferative diabetic retinopathy. The term proliferative refers to the abnormal growth of new blood vessels along the surface of the vitreous body. These new blood vessels can bleed into the vitreous body and form scar tissue that pulls on the retina. Eventually, the scar tissue can exert enough pulling force to cause a retinal detachment.

In ROP, a traction RD can develop because premature birth interrupts the normal development of the blood vessels in the baby's eyes. After the baby is born, some of these blood vessels grow along the retina, bleed into the vitreous body, and form scar tissue similar to that found in diabetic retinopathy. Retinal detachment in ROP can be treated with photocoagulation.

EXUDATIVE. Exudative RDs occur when tissue fluid builds up in the space between the retina and the choroid underneath it. If enough fluid leaks into this space, it can push the retina away from the choroid and cause it to detach. Exudative RDs are associated with certain inflammatory disorders of the eye; tumors, including melanoma (cancer) of the choroid; and a congenital disorder known as Coats' disease, which affects the growth of the blood vessels in the retina.