Last updated: 02/17/2009

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.

Risk factors for retinal detachment

Retinal detachment is associated with a number of different factors and conditions, including:

• extreme nearsightedness
• genetic factors (retinal detachment tends to run in families)
• premature birth (the risk of ROP is highest in premature infants weighing less than 2.2 lb [1 kg] at birth)
• type 1 or type 2 diabetes
• cataract surgery
• sickle cell disease
• Coats' disease
• Eales' disease
• Marfan's syndrome
• breast cancer or melanoma
• leukemia
• history of previous retinal detachment
• age (Retinal detachment is most common in people between the ages of 40 and 70.)
• traumatic injury to the eye
• laser in situ keratomileusis surgery (LASIK, a procedure done to correct vision without the need for glasses or contact lenses)

Photocoagulation therapy for retinal detachment is usually performed with an argon laser. A laser is a device that produces high-intensity, narrowly focused monochromatic light by exciting atoms and causing them to give off their energy in phases. The word laser comes from "light amplification by stimulated emission of radiation." An argon laser uses ionized argon to generate its light, which is in the blue-green portion of the visible light spectrum.

In a laser photocoagulation treatment, the patient is asked to sit in front of the instrument. After applying anesthetic eye drops, the ophthalmologist places a contact lens on the patient's eye and focuses the laser beam through it. He or she operates the laser by foot. The patient may see a brief burst of blue-green light. When the laser beam reaches the retina at the back of the eye, its light is absorbed by the pigment in the cells and converted to heat, which seals the edge of the retinal detachment against the underlying choroid. The procedure is short, taking about 10–30 minutes.