When Should Eyes With Proliferative Diabetic Retinopathy Receive Panretinal Laser Photocoagulation?
Diabetic retinopathy is the leading cause of blindness in patients 20 to 64 years old in developed countries. In eyes with proliferative diabetic retinopathy (PDR), fibrovascular proliferation results from ischemia and release of vasoproliferative factors. Proliferation extends beyond the internal limiting membrane (ILM) and results in vitreous hemorrhage (VH) and vitreoretinal traction, which can lead to severe vision loss (SVL).
Panretinal photocoagulation (PRP) is effective in stabilizing PDR and reducing the risk of severe visual loss. The goal of PRP is to induce regression of existing neovascular tissue and to prevent progressive neovascularization.
The Diabetic Retinopathy Study (DRS) was a randomized, controlled, prospective clinical trial evaluating PRP in eyes with clear media and high-risk PDR. High-risk PDR was defined as neovascularization of the disc (NVD) involving 25% to 33% of a disc area with VH, moderate to severe NVD with or without VH, or neovascularization elsewhere (NVE) on the retina with VH.1 The DRS demonstrated a 50% reduction in the risk of severe visual loss, defined as visual acuity of less than 5/200 on 2 consecutive follow-up examinations 4 months apart.2 If there is concurrent clinically significant macular edema (CSME), this should be treated either before or at the same time as the first session of PRP.
When discussing the procedure with the patient, I emphasize that we are trying to prevent SVL and other neovascular complications. I also discuss the potential secondary effects such as decrease in peripheral vision, night vision, color vision, as well as possible temporary loss of accommodation. I also discuss the potential for the development or progression of CSME, choroidal effusions, and VH and the possible need for vitrectomy surgery.
In addition to high-risk PDR, neovascularization of the iris (NVI) is another indication for PRP. These eyes have a significant risk of developing neovascular glaucoma due to neovascularization of the angle (NVA). PRP is indicated prior to glaucoma surgery as it can control progression of NVA and, in some cases, avoid the need for glaucoma filtration surgery. Furthermore, PRP helps decrease postoperative VH after glaucoma surgery.
I perform PRP as soon as possible, preferably on the day of diagnosis (especially in eyes with anterior segment neovascularization) and usually no later than 1 week. The key is to make the patient as comfortable as possible and make it as convenient as possible. I try to perform it under topical anesthesia in 2 sessions if the patient can tolerate it. I prefer slit-lamp delivery using a contact lens, typically a wide-field lens. The laser power settings vary depending on the lens, laser, degree of fundus pigmentation, and degree of vitreous hemorrhage. In cases where VH makes treatment difficult, krypton or red laser wavelengths may allow better penetration through hemorrhage. In general, I begin with a spot size of at least 300 µm and duration of 0.1 second. The goal is to achieve white burns approximately 1 to 1.5 burn-widths apart.
In the first session, I ideally treat the posterior retina from the area around the arcades, approximately 1 disc diameter from the optic nerve and from the temporal macula, and anteriorly into the inferior one-half of the retina. I do the latter, if possible, in case there is subsequent VH, which usually settles inferiorly. I will then complete the laser treatment in a second session. The more anterior retina can be accessed with a 3-mirror lens or indirect laser.
If the patient cannot tolerate the procedure with a topical anesthetic, I will administer a peribulbar or retrobulbar block with 2% lidocaine. In these eyes, I will try to do as much as possible in the first session and in some cases, the entire treatment if possible. Comfort is of the utmost importance as one wants the patient to return for future exams and treatments.
Vascular endothelial growth factor (VEGF) antagonists have become a useful adjunct in select, difficult circumstances. VEGF is believed to be the prominent factor released in response to retinal ischemia and drives angiogenesis. In eyes where there is NVA causing corneal edema from elevated intraocular pressure, dispersed red blood cells in the anterior or posterior segment, or suboptimal dilation due to NVI, intraocular injection of VEGF antagonists may provide quick resolution (1 to 2 days) to facilitate PRP. These agents effectively “turn back the clock” in PDR and temporarily induce regression of neovascularization. VEGF antagonists do not achieve permanent regression and therefore are not a substitute for PRP in these cases. Another caution is the potential for progression of traction retinal detachment due to rapid conversion of active fibrovascular tissue to contracting fibrotic tissue. Unless vitreoretinal surgical intervention is planned within 1 week, injection of these agents should be used with caution in eyes with traction or large patches of NVE due to PDR.
PRP for PDR is a time-tested, effective therapy and remains the cornerstone of treatment for PDR and its neovascular complications.
1. Four risk factors for severe visual loss in diabetic retinopathy. The third report from the Diabetic Retinopathy Study. The Diabetic Retinopathy Study Research Group. Arch Ophthalmol. 1979;97(4):654-655.
2. Photocoagulation treatment of proliferative diabetic retinopathy: clinical application of diabetic retinopathy study (DRS) findings. DRS Report 8. The Diabetic Retinopathy Study Research Group. Ophthalmology. 1981;88(7):583-600.