04 May Retinal Ischemia: Supply vs. Demand
Retinal ischemia causes VEGF to be liberated inside the eye. In cases of diabetes this can lead to diabetic retinal detachment or neovascular glaucoma. Treatments included pan-retinal photocoagulation (laser) or anit-VEGF medication such as Lucentis or Avastin.
Ischemia results when oxygen supply does not meet oxygen demand to any tissue. Other examples are the heart and legs. With strenuous activity, O2 demand increases. If the body can’t compensate, chest pain or leg pain develops.
In cases of proliferative diabetic retinopathy, the retina becomes ischemic due to the loss of the microcirculation of the eye. The small capillary beds that help feed the retina often get blocked and close off. Blood flow is reduced, and therefore, so are oxygen levels.
In response to this lack of blood, or oxygen, supply, the ischemic retina produces Vascular Endothelial Growth Factor (VEGF). VEGF causes abnormal blood vessels to grow on the surface of the retina and other structures inside the eye. This can lead to blindness by causing a diabetic retinal detachment or glaucoma.
Stop the Ischemia
If we were to stop the ischemia, the VEGF production would cease. If we can change conditions so that supply is equal to demand, the imbalance is gone.
But we don’t know how to improve oxygen supply to the retina. Presently, laser treatment and anti-VEGF medications are employed.
Laser Treatment Reduces Demand
As we are unable to increase oxygen supply, laser photocoagulation (specifically, pan-retinal photocoagulatoin, aka PRP) is used to decrease demand. By ablating, a fancy term for killing, retinal tissue, we are in effect, reducing the demand.
If enough laser is performed, the overall O2 requirements will decrease. Ischemia is stopped and VEGF is no longer produced.
The eye becomes stable. A diabetic retinal detachment and glaucoma are prevented.
anti-VEGF Medications Block VEGF
Anti-VEGF medications such as Lucentis and Avastin block VEGF from doing its job. In doing so, neovascularization can not be initiated and diabetic retinal detachment and glaucoma are avoided.
Is the eye stable?
What Does This Mean? Actually I am not sure. The end result of laser (PRP) and anti-VEGF treatments are the same; preventing retinal detachment and glaucoma.
The use of laser for proliferative diabetic retinopathy is old hat. It has been saving the sight of diabetics for about 40 years. It is a good treatment because it fixes the problem. PRP, when properly performed, stabilizes the eye by eliminating the ischemia. It secondarily stops VEGF by halting the initial O2 imbalance.
I am not sure; however, if intraocular injections of Lucentis/Avastin actually fix the problem. There is no mechanism to fix the ischemia. VEGF is still liberated as the ischemia still exists. Therefore, careful monitoring and repeated injections are needed.