The Course of Response to Focal Photocoagulation for DME

Focal photocoagulation is the only treatment that has been demonstrated to be beneficial for diabetic macular edema. In the ETDRS, focal photocoagulation of eyes with macular edema reduced the risk of moderate visual loss (decrease of 15 or more letters) by approximately 50% (from 24% to 12%, three years after initiation of treatment). For eyes with center-involved DME and visual acuity worse than 20/40 that were treated with focal photocoagulation, the 15-letter improvement rate at 1 year was 11% and at 3 years was 16%.

In the ETDRS, focal photocoagulation treatment for diabetic macular edema involved direct treatment to discrete lesions between 500 microns and 3000 microns from the center of the macula that were thought to be causing retinal thickening or hard exudates with or without "grid" treatment to other macular areas of retinal thickening or non-perfusion. The lesions treated directly included microaneurysms, identified on fluorescein angiography, that either filled or leaked, intraretinal microvascular abnormalities (IRMA), or pruned capillaries that leaked fluorescein. Grid treatment was applied in the ETDRS to areas of thickened retina that showed diffuse fluorescein leakage or areas of non-perfusion identified as capillary dropout on fluorescein angiography. Areas of non-perfusion in the macula could be treated with grid at the discretion of the treating ophthalmologist. Areas that had both discrete lesions and diffuse leakage or capillary dropout would receive a combination of direct and grid treatment. Re-treatment was applied at four month intervals if clinically significant macular edema persisted, one or more treatable lesions were identified, and the investigator believed these lesions were responsible for the edema. The median number of focal laser treatments applied in the ETDRS was 3.8.

The mechanism of action of focal photocoagulation is not fully understood; however, it is clear that the retinal pigment epithelium (RPE) absorbs the majority of the laser energy and thermal injury occurs at the level of the RPE. Studies have shown that photocoagulation can eventually result in retinal and apparent RPE atrophy 200-300% larger than the original laser spot size. These areas of expanded atrophy can lead to loss of central vision, central scotomata, and decreased color vision. Consequently, many retinal specialists today tend to treat with lighter, less intense laser burns than originally specified in the ETDRS.

In addition to the concern regarding the spread of intense laser burns, there are a number of other reasons that retinal specialists today have modified the treatment procedures originally specified in the ETDRS protocol. These reasons include the advent of new lasers and the clinical observation that different techniques, such as focal photocoagulation with lighter burns or grid treatment alone, may be similar in beneficial effect as the original ETDRS treatment protocol. A modified ETDRS focal photocoagulation protocol adapted from the original ETDRS approach, has been adopted as the standard laser technique for DME used in DRCRnet studies.

There are limited data on the course of visual acuity and central retinal thickness after a single focal photocoagulation session for DME. In prior DRCRnet DME treatment protocols that included a laser arm, according to the re-treatment protocol eyes received a second focal photocoagulation session at 3.5- 4 months (which was the first follow-up visit) unless there was substantial improvement defined as at least a 50% decrease in retinal thickening of the central subfield measurement on OCT. As a result, it is unknown what proportion of eyes with lesser degrees of improvement would have continued to improve and the time course for further improvement following the initial photocoagulation session given additional time. In one study conducted by DRCRnet of eyes that had not been previously treated for DME, among 113 eyes in the modified ETDRS laser treatment group with baseline OCT central subfield >250 microns, a 50% or more reduction in OCT central subfield thickening was present at 3.5 months in only 28 (25%). The table below categorizes the 85 eyes that did not meet this measure of improvement at 3.5 months with regard to improvement in visual acuity of at least 5 letters and/or reduction in central subfield thickness of at least 10%. The 5 letter reduction was selected based on the 95% confidence interval for change determined in a study that evaluated the validity and reliability of the electronic ETDRS visual acuity testing procedure that is used in DRCRnet protocols. The 10% threshold was selected based on the DRCRnet OCT reproducibility study which found that a 10% change in central subfield thickness was likely to be real. Forty-seven (42%) eyes that met the protocol requirement for repeat photocoagulation at the first follow-up visit had an improvement in either visual acuity (of at least 5 letters), central subfield (of at least a 10% reduction), or both at this follow-up visit.

Other DRCRnet protocols provide data on the course following a single photocoagulation session through 4 months of follow up. In a pilot study designed to evaluate peribulbar corticosteroids for mild DME (OCT central subfield thickness = 250 microns and visual acuity 20/40 or better at baseline), modified ETDRS focal photocoagulation was the treatment given to the control group. Follow-up visits occurred after 1, 2, and 4 months before the eye was eligible to be retreated. Twenty-one of the 37 eyes in the laser group had not been previously treated with focal photocoagulation for DME and 17 eyes had been previously treated with focal photocoagulation. A 50% or more reduction in OCT central subfield thickening occurred in 11 (30%) of the 37 eyes at 17 weeks. Fourteen (38%) eyes that would have met the criteria for re-treatment had an improvement in either visual acuity (of at least 5 letters), central subfield (of at least 10%), or both at 17 weeks.

In another pilot study evaluating intravitreal bevacizumab for DME (OCT central subfield thickness = 275 microns and visual acuity 20/32 or worse at baseline), modified ETDRS focal photocoagulation was the treatment given to the control group. Follow-up visits occurred after 3, 6, 9, and 12 weeks before the eye was eligible to be retreated. There were 7 eyes in the laser group that had not been previously treated with focal photocoagulation for DME and 12 eyes that had been previously treated with focal photocoagulation. Among these 19 eyes, a 50% or more reduction in OCT central subfield thickening occurred in 3 (16%) at 12 weeks. Ten (53%) eyes that would have met criteria for re-treatment had an improvement in either visual acuity (of at least 5 letters), central subfield (of at least 10%), or both at 12 weeks.

The data from these three protocols indicate that a substantial number of eyes receiving focal photocoagulation (either an initial course or repeat application) will show improvement in central retinal thickness after 3-4 months that is at least 10% but is less than 50% of the baseline thickening. It is for these eyes that further knowledge of the course of retinal thickening and visual acuity without additional interventions is needed to assess whether the present requirements for re-treatment are more aggressive than they need to be.