Full scatter PRP is a treatment approach which was first established on a widespread basis in the 1970s in the Diabetic Retinopathy Study (DRS) 3 and then further explored in the 1980s and early 1990s in the Early Treatment Diabetic Retinopathy Study (ETDRS).4 The theory behind PRP for proliferative diabetic retinopathy (PDR) is that by destroying the peripheral retina, it decreases the stimulus for the growth of new abnormal blood vessels. The DRS enrolled 1,742 patients, 876 of whom were randomized to the argon group and 875 to the xenon group.5 In the end, the harmful effects of xenon coagulation were more significant than for argon, thus argon laser became the standard of care.

The current treatment recommendations which incorporate the DRS findings and those of later large scale trials include: high risk proliferative diabetic retinopathy which is defined as: mild neovascularization of the optic disc (NVD) with vitreous hemorrhage, moderate to severe NVD with or without vitreous hemorrhage, or moderate neovascularization elsewhere in the retina (NVE) with vitreous hemorrhage (Figs. 1a–b and 2). High risk proliferative diabetic retinopathy is also defined in any case with three of the following four risk factors: vitreous or preretinal hemorrhage, presence of new vessels, location of new vessels on or near the optic disc, and moderate to severe extent of new vessels. In the DRS, the risk of severe visual loss (defined as a visual acuity of less than 5/200) was 26% for patients with high risk proliferative diabetic retinopathy versus 7% in patients without the aforementioned high risk characteristics after 2 years.6 PRP reduced this risk of severe visual loss by 50%. In addition to these criteria, many studies including ETDRS have suggested that PRP may be indicated for patients with severe nonproliferative diabetic retinopathy in special high risk situations such as poor compliance history, impending pregnancy, or impending cataract surgery.4

Macular edema is responsible for a major part of visual loss in diabetic retinopathy. Many of the current treatment paradigms are based on the results of the ETDRS.4 The study enrolled 3,711 patients between 1980 and 1985 who had either (a) no macular edema with visual acuity better than 20/40 or (b) macular edema with visual acuity better than 20/200. Clinically significant macular edema (CSME) was defined in the study as one of the following: thickening of the retina at or within 500 μm of the center of the macula, hard exudates at or within 500 μm of the center of the macula if associated with thickening of the adjacent retina, or a zone of retinal thickening of greater than or equal to 1 disc area within 1 disc diameter of the center of the macula (Figs. 3a–c and 4a–b). The ETDRS results demonstrated that eyes with CSME benefited from focal laser photocoagulation by reducing the risk of moderate visual loss by at least 50% and increasing the chance of visual improvement.7 This effect was maintained over time with moderate visual loss at 3 years of follow-up in 24% of treated patients treated versus 12% of untreated patients. The study concluded that patients with CSME and good vision should be considered for treatment based on other factors such as status of the fellow eye, anticipated cataract surgery, proximity of exudates to the fovea, or the presence of high-risk PDR.8

For many years, laser photocoagulation was the only proven treatment for choroidal neovascularization associated with neovascular macular degeneration and other retinal conditions. In the late 1980s and early 1990s, the Macular Photocoagulation Study (MPS), a series of eight multicenter, randomized, prospective trials examining the use of argon and krypton laser for choroidal neovascular membranes was published.9 This study evaluated the use of laser for extrafoveal and juxtafoveal lesions in three conditions: neovascular ARMD, presumed ocular histoplasmosis (POHS), and idiopathic choroidal neovascularization. One major drawback of these trials was the narrow eligibility criteria: no more than 15–20% of neovascular ARMD cases present with well-defined choroidal neovacularization as required by the trial.

In general, in all of the MPS trials, treatment did not decrease the patient’s chance of maintaining stable visual acuity, but the proportion of eyes, treated or untreated, that maintained good or stable visual acuity was very small. The reason for this inadequate treatment effect is that despite treatment, many eyes continued to lose vision because of persistent or recurrent neovascularization that extended into the foveal center. Subfoveal recurrences were not treated with laser in these trials due to concerns about permanent central visual loss. Due to its deleterious effect on normal surrounding neural retina, the use of argon laser for choroidal neovascularization near the fovea sharply decreased with the advent of photodynamic therapy (see below) and anti-vascular endothelial growth factor therapies. However, argon laser does continue to have a rarely used but important role in that it is highly effective for extrafoveal isolated choroidal neovascular lesions. Patients should always be informed that this treatment induces a permanent scotoma.

Argon laser can be applied for PRP from a slit-lamp based or indirect ophthalmoscopic system (Fig. 5). The slit lamp-based system is used for a seated patient and consists of a modified slit-lamp (specialized microscope for ophthalmoscopic exam) mounted on a table. The retina is visualized by using a wide-field contact lens (Fig. 6). The indirect system is composed of a headset worn by the ophthalmologist which emits the laser directly. For this system, the patient is typically reclined in an examining chair and the retina is visualized by using a 20-Diopter or 28-Diopter lens (Fig. 7).

The standard technique for PRP currently involves the placement of 800–1,600 laser burns with a 500 μm spot size, spaced 0.5 burn widths apart from each other with 0.1–0.2 s of duration.10 Intensity is regulated so that mild white bleaching is obtained (Fig. 8). The treatment reaches from the temporal arcade to the equator, and up to 2 disc diameters temporal to the macular center. Typically one disc diameter or space is spared around the optic nerve to avoid central visual field defects.

Focal laser is typically applied using a slit-lamp based system for a seated patient (Fig. 5). A magnifying contact lens is held by the ophthalmologist for detailed viewing of small retinal features (Fig. 6). Focal laser treatment typically consists of 50–100 μm laser burns of 0.05–0.1 s duration applied to microaneurysms between 500 and 3,000 μm from the center of the macula with the clinical endpoint defined as a color change to a mild whitening. For more diffuse macular edema, a grid pattern is typically applied in the following manner: 100–200 burns of a 50–200 μm spot size spaced 1 burn width apart within 2 disc areas of the fovea. In the ETDRS, an average of 3–4 treatment sessions 2–4 months apart were required.7 The grid technique has been demonstrated in several more recent studies to be more effective than milder focal techniques in reducing retinal thickening based on detailed measurements taken with the optical coherence tomography (OCT) and thus continues to be the standard of care.11

Laser for choroidal neovascular membranes is typically applied using a slit-lamp based system for a seated patient. A magnifying contact lens is held by the ophthalmologist for detailed viewing of small retinal features. In the MPS studies, argon laser was applied to cover the choroidal neovascular membrane (location judged on fluorescein angiogram) and 100 μm beyond the edge of the lesion, but was never applied closer than 200 μm from the center of the fovea. The laser was set initially at a 200 μm spot size, 0.2–0.5 s duration, and 100–200 mW power. The power was adjusted to achieve an intensity sufficient enough to produce a uniform whitening of the overlying retina.

The complications of PRP in the DRS were generally mild and included a decrease in visual acuity of 1 or more lines in 11% and peripheral visual field loss in 5%.12 The DRS and ETDRS also indicated that macular edema can be worsened by PRP leading to moderate visual loss.4

The side effects and complications of focal laser in the ETDRS included: paracentral scotoma, transient increased edema/decreased vision, choroidal neovascularization, subretinal fibrosis, photocoagulation scar expansion over time, and inadvertent foveal burns.4,7

Complications from argon laser for choroidal neovascularization include: hemorrhage, perforation of Bruch’s membrane, retinal pigment epithelial tear, and arteriolar narrowing.9,13

Stay updated, free articles. Join our Telegram channel

Join