How the Eye Sees
Light rays enter the eye through the cornea, pupil, and lens. These light rays pass through the vitreous (a clear, gel-like substance that fills the middle of the eye.) The light rays are focused on the retina, a light-sensitive tissue lining the back of the eye. The macula is a very small area at the center of the retina that gives us our fine pinpoint central vision. The area of the retina surrounding the macula gives us our peripheral or side vision. The retina converts the light rays into signals that are sent through the optic nerve to the brain.
With Non-Proliferative Diabetic Retinopathy, or NPDR, damaged blood vessels in the retina begin to leak fluids, including small amounts of blood, into the retina. Sometimes, deposits of fat may enter the retina. These deposits are called “hard exudates”.
With Fluorescein angiography, a small amount of yellow dye is injected into your arm, where it circulates through all the blood vessels in the body, including those in the retina. A special camera with a blue flash is then used to take a series of pictures of the retina. Any blood vessels that have been affected by NPDR will show up in these images, as well as areas of abnormal leakage or “ischaemia”.
With OCT, a special camera is used to photograph your retina. It measures the thickness of the retina, and is also very sensitive at detecting fluid. This diagnostic information helps your doctor to determine why your vision is blurred, and whether treatment should be started.
Laser for Macular Edema
For macular edema, the laser is focused on the retina outside the center of the macula. The laser is not applied directly to the center of the macula, since this would reduce central vision. The main goal of treatment is to prevent more vision loss by sealing off leaking blood vessels that interfere with the proper function of the macula.
With PDR, many blood vessels in the retina close, preventing adequate blood flow to the retina. The retina responds to this problem by trying to grow new blood vessels However, these new abnormal vessels do not provide proper blood flow. They can also bleed as well as lead to scar tissue, which may cause the retina to wrinkle or even detach. If similar vessels grow abnormally in the front of the eye, they can block the drainage channels of the eye and cause high pressure and possibly glaucoma. Both the central and side vision can be affected by PDR.
Treatment for PDR is often done with laser, and it is called RPR, or Panretinal Photocoagulation. It is also known as Scatter Photocoagulation. The treatment is usually performed in an office setting. For comfort during the procedure, an anaesthetic is applied to the eye. The laser is applied to the peripheral retina, avoiding the central macula. This causes the blood vessels to shrink, and often prevents them from growing again in the future. It also decreases the chances of the blood vessels bleeding into the vitreous, or causing a retinal detachment. In some cases, multiple laser treatments may be necessary.
Anti-VEGF drugs target a specific chemical in your eye. This chemical, called vascular endothelial growth factor, or VEGF, is critical in causing abnormal blood vessel growth on the surface of the retina, as well as in other parts of the eye. Several drugs have been developed that can block the trouble-causing VEGF. An anti-VEGF drug can help reduce the growth of abnormal blood vessels, which helps to prevent: bleeding, scar tissue, and other problems that can cause vision loss.
The anti-VEGF drug is administered directly to the eye, in an outpatient procedure. After the eye has been numbed with an anaesthetic, the anti-VEGF drug is injected into the eye with a very fine needle. Some people may need multiple anti-VEGF injections over a period of months.
The abnormal blood vessels can bleed into the vitreous, the clear gel in the middle of the eye. This bleeding, called a vitreous hemorrhage, can prevent light rays from reaching the retina.
Ultrasound imaging is used to create a picture of the retina. Sound waves are sent through the eye, and bounce back. A computer is used to read the returning waves and build a picture of the retina. From these images, your ophthalmologist can determine if surgery is necessary.
Vitrectomy surgery is usually performed in the operating room on an outpatient basis. An operating microscope and small surgical instruments are used to enter the inside of the eye. Blood and scar tissue are removed. At the same time, a laser may be used to prevent further bleeding, and abnormal blood vessel growth. To help the retina heal in place, the ophthalmologist may place a gas bubble or silicone oil in the vitreous space. The gas bubble will gradually dissolve on its own. Removal of the silicone oil requires an additional procedure. Use of a gas bubble or silicone oil is reserved for eyes with retinal detachment or advanced scar tissue. After vitrectomy, the results of your surgery may not be apparent for months.