In proliferative diabetic retinopathy (PDR), abnormal blood vessels and scar tissue grow on the surface of the retina and also attach firmly to the back surface of the vitreous. The vitreous then pulls on the scar tissue and can cause the retinal blood vessels to bleed into the vitreous cavity (called vitreous hemorrhage). This can cause immediate and severe visual loss. Often, the hemorrhage will clear by itself. It if doesn't clear, a vitrectomy can be performed to remove the blood-filled vitreous.
When the vitreous pulls on the scar tissue, it can detach the retina. This is called a traction retinal detachment. When the detachment involves the macula, central vision is lost. Also, scar tissue may wrinkle the retina and cause visual loss.
The patient can regain some vision only if the scar tissue is removed from the surface of the retina and the retina is reattached. This is accomplished by vitrectomy. The surgeon removes the vitreous and scar tissue from the surface of the retina so that the pulling of the retina is stopped, thereby releasing the traction. Removal of the scar tissue also reduces or eliminates wrinkling of the retina. In treating cases, a scleral buckle may also be placed around the eye at the time of surgery.