How Does Diabetes Affect Glaucoma?
The Optic Nerve
Glaucoma is known as the “silent thief” because of how slowly it affects a person’s vision. It is so slow, in fact, that visual loss is usually undetectable by the patient until it is too late.
For diabetics, glaucoma can be complicated by neovascularization. But in order to understand clearly how diabetes affects glaucoma, we need to understand what glaucoma is exactly.
What is Glaucoma?
In glaucoma, one of two things happens to the aqueous:
- Too much aqueous is produced (very rare, usually related to medications)
- Drainage is hindered by an obstruction or narrowing of the drainage tubes
Both of these problems lead to a buildup of aqueous fluid in the eye causing the eye pressure to increase. The eye needs pressure in order to keep its shape. However, too much pressure can cause permanent damage to the optic nerve and lead to blindness.
The optic nerve is the gathering of retinal fibers into a bundle that exits the eye and travels to the occipital lobe of the brain. Each area of the retina corresponds to a specific section of the optic nerve. The retina is, essentially, one big neuron with a very long axon or tail.
Optic Nerve Cupping
When the pressure in the eye rises, it forces the optic nerve to “cup.” This creates a bowl-like appearance to the optic nerve which results in the loss of peripheral vision.
If we think of the optic nerve as a doughnut, the center hole would be the cupping created by the increased pressure and the doughnut would be the rim of the optic nerve. The size of the doughnut hole grows as the pressure in the eye rises, which leads to permanent vision loss.
Types of Glaucoma
There are two main types of glaucoma:
- Primary (chronic) Open Angle Glaucoma
- Acute Narrow Angle Glaucoma
Primary open angle glaucoma results from an obstruction (either partial or complete) of the eye’s drainage system, specifically the trabecular meshwork. This meshwork is porous like a sponge, and as we age the pores narrow causing the flow of aqueous to slow.
In patients with glaucoma, there is more narrowing of the pores than there would be in someone without the disease. Open angle glaucoma progresses slowly and very rarely has any symptoms. Damage is caused by sustained increased pressure over a long period of time.
Narrow angle glaucoma is not as common as open angle, but it is more vicious. The “angle” is the space between the iris and the back of the cornea (see image below).
In patients with narrow angle glaucoma, this structure, like its name suggests, is very narrow and does not allow the aqueous to easily flow through the drainage system, called the trabecular meshwork, and then out of the eye.
This type of glaucoma occurs suddenly and is very painful. If not treated, narrow angle glaucoma can cause permanent blindness in 24 to 48 hours. Symptoms of this type of glaucoma include:
- Excruciating pain that occurs suddenly
- Rainbows and halos around lights, especially at night
- Hazy vision (caused by corneal edema, or swelling)
- Sudden complete loss of vision
There are other types of glaucoma; however, a discussion of these types would be beyond the scope of this article.
Sudden Onset Glaucoma
The Effects of Diabetes on Glaucoma
Diabetes has a tendency to complicate an already complicated disease. It can either exacerbate the condition in someone who already has glaucoma, or it can be the cause.
The most important thing to remember for diabetics is to keep the blood sugar levels under control. Inconsistent blood sugar levels cause the blood vessels all over the body to weaken, but because the eye’s blood vessels are so delicate they are often the first ones affected.
When the vessels in the eye weaken, they begin to breakdown and leak. This leakage not only damages the retina, but also prevents oxygen from getting to its destination. In an effort to fix this, the body grows new blood vessels. This process is called neovascularization. Unfortunately, these new blood vessels are even weaker than the parent vessels and are more likely to rupture and leak.
Neovascularization doesn’t just affect the retina. These new blood vessels can grow into the iris and then the angle eventually cutting off the flow of aqueous through the trabecular meshwork, which will effectively close the angle. This is called neovascular glaucoma and can lead to a rapid rise in pressure and permanent blindness if left untreated.
Treatment of Neovascular Glaucoma
Neovascular glaucoma is not an easy disease to treat. The main treatments available are:
- Eye drops
- Anti-VEGF injections
- Laser therapy
- Trabeculectomy surgery
- Aqueous drainage implants
The goal of treatment for this disease is to get the aqueous out of the eye. One option is to use eye drops to decrease the production of aqueous, allowing time for the fluid to drain. Eye drops are most effective in the earliest stages of the disease. They do not, however, prevent development of neovascularization. They only reduce the amount of aqueous produced.
Anti-vascular endothelial growth factor (anti-VEGF) medications, such as Avastin or Lucentis, are being used more frequently to treat neovascular glaucoma in the early stages. These medications help to prevent the further growth of new blood vessels, slowing the progression of the disease. When paired with eye drops, the visual prognosis can be improved significantly.
Laser therapy involves the destruction of the cilliary body that produces aqueous in an effort to limit the production of the fluid. However, long term effects and benefits are not well established. Laser treatment can improve the outcome of surgical procedures if done pre-operatively.
Trabeculectomy surgery is one of the most invasive, but effective, treatments for both neovascular and primary open angle glaucoma. It involves creating a “bleb,” or permanent drain, at the limbus (the junction between the cornea and the sclera – see image).
There is a downside to this form of treatment, however. The bleb can become infected, or obstructed. In the case of infection, antibiotic eye drops are quite effective. If the bleb becomes obstructed however, the procedure may need to be repeated.
Probably the most effective treatment for neovascular glaucoma is the aqueous drainage implants. A shunt is placed in the eye at the pars plana, which is part of the cilliary body (behind the iris). Although this is an invasive procedure, the risk of infection after surgery is much lower than that of the trabeculectomy.
The best treatment for neovascular glaucoma is prevention. Diabetics have several risk factors for neovascular glaucoma, the most common of which is diabetic retinopathy. To prevent retinopathy and neovascular glaucoma it is imperative to keep blood sugar levels stable and within normal range.
If you suspect you have loss of vision or have noticed visual disturbances or symptoms, you should contact your ophthalmologist right away. This article is in no way a substitute for the advice of your physician.
A Brilliant Explanation of Glaucoma
I have written extensively about eye health in general. For more information about the relationship between diabetes and vision, you may like to read this article:
© 2012 Melissa Flagg COA OSC