Diabetic Eye Disease
The retina is a thin clear layer of brain tissue one-fifth of a millimeter thick. The retina coats the inner wall of the eye and is much like the “film” in the camera; light hits the retina and the retina sends a signal to the brain, via the optic nerve.
If we think about how amazing our vision is in the sense of the detail, color, and motion we can see, we realize that the retina is quite an tissue to capture and convey such a rich visual signal. To perform that job, the retina needs a tremendous amount of oxygen.
The macula is the “center” of the retina which accounts for our sharpest vision–our 20/20 central vision that allows us to read books and road signs, watch TV, play baseball or tennis, etc. The remainder of the retina gives us “peripheral” vision.
To understand how diabetes takes away vision, we must first talk about how diabetes damages damages the retina. Diabetes is a “microangiopathy”– “micro” for small, “angio” for blood vessels, and “pathy” for illness. The small blood vessels in our body are called capillaries. The retina has a dense capillary network. These capillaries are so small we cannot see them directly, but we can get an indirect sense of their health based on looking at the retina with our lenses and based on performing an angiogram of the eye.
The high blood sugars of diabetes cause metabolic damage to these small blood vessels. Eventually capillaries start to “drop out”–on other words, those capillaries are lost and no longer carry blood and oxygen to the tissue. This lack of blood flow is called “ischemia.” The retina starts to lose capillaries and does not receive enough oxygen.
If we make the macular edema go away with our treatments, vision may improve but may not return to 20/20, as the cells in the macular may not fully recover (“macular ischemia”).
With the loss of capillaries, the oxygen-hungry retina may release special hormones (one of which is VEGF, vascular endothelial growth factor) that cause abnormal blood vessels to grow on the surface of the retina (unlike macular degeneration, where the abnormal blood vessels usually grow underneath or rarely within the retina). These abnormal new blood vessels on the surface of the retina (“retinal neovascularization”) can bleed into the vitreous gel portion of the eye (“vitreous hemorrhage”). This vitreous hemorrhage can cause floaters, “cobwebs”, and loss of vision. Sometimes the eye will reabsorb the blood on its own over the course of a few weeks and vision will improve. Sometimes the blood is too thick and must be removed surgically with a “vitrectomy.”
Sometimes fibrous scar tissue may develop along the surface of the retina in conjunction with the abnormal new blood vessels. This scar tissue is attached to both the retina and to the vitreous gel. The vitreous gel may then shift an the scar tissue thus pulls ad detaches the retina (“tractional retinal detachment”). Vitrectomy surgery may be performed to remove the scar tissue and re-attach the retina.
Since the retina has lost capillaries and is not receiving sufficient blood flow and oxygen (retinal ischemia and hypoxia), the ischemic retina makes hormones inside the eye that contribute to these problems. To reduce the production of these hormones, we may recommend “PRP laser” for panretinal laser photocoagulation. In the office, we perform hundreds or thousands of tiny laser burns to the peripheral retina, not the central retina (macula). We are essentially sacrificing part of the peripheral retina to save the macula. By lasering a good portion of the peripheral retina, we reduce the production of these hormones inside the eye that cause these secondary problems. However, the laser essentially ablates part of the peripheral retina, the the patient may notice reduced peripheral or “side” vision as well as reduced night vision. The PRP laser may even make the central vision drop slightly. Its important to keep in mind that the goal of the PRP laser is not to improve vision. Vision may nevertheless even worsen after the PRP laser. The goal of the PRP laser is to reduce the risk of severe vision loss months or years later by reducing additional abnormal blood vessel growth, scar tissue formation, and bleeding inside the eye.
These abnormal hormones produced by the ischemic retina may also cause abnormal blood vessels to grow in the front part of the eye (“iris” neovascularization). These abnormal blood vessels in the front of the eye can, over time, cause a special diabetic glaucoma known as NVG for neovascular glaucoma. When PRP laser is performed in timely fashion this complication is usually avoided.