Panretinal Photocoagulation: Practical Guidelines and Considerations

JOHN R. MINARCIK, MD ∙ DANIEL M. BERINSTEIN, MD

With the advent of new therapies for neovascularization, significant attention has been recently directed toward vascular endothelial growth factor (VEGF) antagonists. Despite the promise of these new immunologic-derived pharmaceutical agents, the standard of care for retinal neovascular disease is still a well-executed panretinal photocoagulation (PRP). PRP has been accepted as a long-lasting and efficacious means of reducing pathologic VEGF levels for treatment of proliferative diabetic retinopathy (PDR) and other ischemic retinal vascular diseases.¹ It remains the gold standard treatment for preventing visual loss in PDR, and successful treatment can be achieved by implementing a suitable treatment regimen with attention to certain technical details. Factors to consider in optimizing a suitable PRP outcome include patient comfort, laser type and settings, method and strategy of delivery, and avoidance of complications.

PATIENT COMFORT

Patient comfort is critical in order to successfully perform PRP. Most patients will have some level of discomfort; however, maximizing positioning and ergonomics will reduce anxiety and movement, thus decreasing the overall duration of the PRP. During treatment, the type of anesthesia should be tailored toward the individual patient. At a minimum, all patients will benefit from topical anesthetic drops (ie, tetracaine or proparacaine). The drops should be instilled into the treated as well as fellow eye to relieve dryness and photophobia induced from holding the lids open for prolonged durations.²

The authors have also found that a topical anesthetic further facilitates maintenance of gaze with the fellow eye, minimizing Bell's reflex or unintentional wandering. Some patients may require more invasive anesthesia, such as retrobulbar (or peribulbar) orbital injections. Retrobulbar injections offer the benefits of a more complete anesthesia, akinesia, and potentially a favorable transient anatomic proptosis for patients with deep orbits. Although uncommon, the risks of retrobulbar injection can be serious³ and include strabismus, retrobulbar hemorrhage, globe perforation, optic nerve injury, or even life-threatening brainstem anesthesia.

In general, it is the authors' experience that empiric retrobulbar injections should only be reserved for particular clinical challenges that preclude proper delivery of therapy to sight-threatening retinopathy.

INDIRECT OPHTHALMOSCOPY VS SLIT-LAMP

Obtaining the best possible view of the retina greatly facilitates the delivery of PRP. Frequently in the case of PDR, the ocular media can be obscured by vitreous hemorrhage, cataract formation, or corneal edema. One of the main advantages of indirect ophthalmoscopy is superior visualization for a greater area of the fundus, especially in cases of poor media. Certain techniques can assist in maximizing the view. In the case of a small pupil, a 28-D viewing lens can be employed.

Topical glycerin is another very efficacious, although short-acting, adjunct for laser treatment through microcystic corneal edema caused by high intraocular pressure, as is commonly encountered in neovascular glaucoma. Treatment of the anterior retina, especially in conjunction with scleral depression, is also best achieved with indirect ophthalmoscopy.

Scleral depression offers the added assistance of stabilizing the globe and lids while administering the laser. In general, the indirect method is a faster method, but this can be dependent on the physician's experience and individual preference (Figure 1).

Figure 1. A. Proliferative diabetic retinopathy with clinically significant macular edema. B. PRP applied using an indirect ophthalmoscopy laser delivery system.

Slit-lamp delivery, conversely, has the advantage of more precision and control, especially if treating around the arcades and optic nerve. Optimal spot spacing can be controlled with a micromanipulator, and the closer view can assist in avoiding intraretinal hemorrhages or subtle areas of tractional retinal detachment.

Additionally, contact lenses can offer some stabilization of Bell's reflex or wandering eye movements and stabilize the lids for those prone to excessive blepharospasm. Either a wide-field or mirrored contact lens may be an appropriate option, although consideration of the patient's orbital anatomy, media opacities and tendency to squeeze may help identify the best-fitting lens for a particular case. Some surgeons suggest that photocoagulation delivered by slit lamp is more comfortable overall, as the laser power and fine focus can be titrated to a therapeutic effect on minimum energy settings.

Each means of laser delivery has both merits and challenges. In general, the authors prefer initial treatments with indirect laser, with adequate coverage of the peripheral retina. If more posterior filling of laser is needed on subsequent treatments, then the slit-lamp delivery may be superior to deliver laser close to the arcades, temporal to the macula, near the optic nerve, or in between old laser spots.

POWER, SIZE, NUMBER, AND SESSIONS

The Diabetic Retinopathy Study (DRS) and Early Treatment of Diabetic Retinopathy Study (ETDRS) were instrumental in developing clear indications and guidelines for scatter laser in PDR. Specifically, the recommendations in the ETDRS for an initial treatment consisted of 1,200 to 1,600 burns of moderate intensity, 500-μm size, one-half to one-spot diameter spacing at 0.1-second duration, divided over at least two sessions.⁴ These guidelines are helpful as a frame of reference, but reasonable modifications may be applied to different clinical scenarios and do not necessarily represent an absolute start or endpoint of therapy.

In a practical sense, the clinical goal is to administer enough laser burns to ischemic retina to induce regression of active neovascularization and prevention of new lesions or hemorrhage. This should include 360° treatment in the case of PDR, with adequate spacing to avoid excessive compromise of peripheral vision. For this reason, some favor slightly greater interspot distance, perhaps one- to one-and-a-half–spot diameter spacing initially. In cases of sectoral ischemia, such as in neovascular branch retinal vein occlusion or hemicentral retinal vein occlusion, one may treat only the ischemic portions, as defined by fluorescein angiography or clinical appearance (Figure 2).

Figure 2. A. Peripheral nonperfusion and neovascularization secondary to a branch retinal vein occlusion. B. Subsequent regression of neovascular tissue two months following sectoral PRP.

It is tricky to assign standard power settings, since burn adequacy is clearly dependent on multiple variables, such as media clarity, fundus pigmentation, laser type, and method of delivery. A common wavelength used for PRP procedures is the 532-nm green laser. Longer wavelengths, such as the 810-nm diode laser, may offer the advantage of better penetration through blood or media opacity.

Regardless of the laser type or power, the goal is to achieve an adequate blanching with medium gray spots. Avoiding intense white spots is prudent, as these can induce hemorrhage and become foci for retinal breaks or choroidal neovascularization.

By slit-lamp delivery, the size of the spots should range from 200 μm to 500 μm, favoring a smaller spot size in the posterior pole. Smaller than 500-μm spot sizes require lower energy settings to achieve the same intensity burn, so care should be taken to appropriately adjust the power down to avoid laser-induced complications.

Similar care should be taken to avoid excessive energy uptake near heavily pigmented areas, such as old laser scars. Larger spot sizes tend to cause more discomfort to the patient, and so it is the authors' preference to use 300-μm spot size setting when using the slit lamp. Using a 532-μm laser, a typical starting power setting for a 300-μm spot of 0.1-second duration might be around 250 mW, but this is highly dependent on the operator's laser, the status of the ocular media, and the pigmentation of the retina.

Often, with indirect laser, spot size cannot be precisely adjusted, but approximately 50% retinal coverage (approximately one spot width apart) is an adequate starting pattern. Similarly, the total number of spots will be greater when smaller than 500-μm spots are used.

Adjustment of the laser settings can assist in minimizing patient discomfort. Reducing the duration of the pulse, for example, from 100 msec to 70 msec, can make a favorable difference. Using somewhat unconventional parameters, one study validated this method by demonstrating effective PRP treatments, with statistically significant reduced patient pain using 20-msec pulses, with compensatory increases in power.² Reduction of spot size can also make the treatment more tolerable to the patient, with the smaller spot's size of equal blanching intensity delivering less overall energy per pulse than larger ones.

Dividing the PRP treatment into two or more sessions can help minimize the occurrence of adverse effects.⁵ More sessions are preferred when there is a greater risk of macular edema, decreased patient tolerance, or less clinical urgency. Some may treat one quadrant at a time in the appropriate clinical situation. Having a two-week interval between treatments is thought to be beneficial in minimizing macular edema and choroidal effusion,^6,7 although a shorter interval such as a week is probably reasonable in lower-risk cases. In certain scenarios, a strong argument can be made for single-session PRP. These may include poor patient reliability for follow-up, decreased community access to specialist eye care, or impending loss of visualization due to a new or active hemorrhage.

LASER PATTERN AND LOCATION

There are many considerations in optimizing the pattern and location of the laser spots in PRP. Such variations include commencing treatment in the posterior pole and extending to the equator or treatment of the anterior retina with a larger nontreatment zone around the arcades. Ultrawide-field fluorescein angiography can clearly delineate areas of peripheral capillary nonperfusion and guide the surgeon in applying scatter laser to this designated area.⁸

Some evidence suggests that more peripheral scatter laser treatments are less likely to induce or exacerbate macular edema.⁹ For this reason, there is a possible benefit to preferential distribution of the PRP treatment to the peripheral ischemic retina. Well-spaced scattered patterns are efficacious in achieving the goal of inhibition of angiogenesis in a less destructive manner. Confluent laser patterns directed at patches of neovascularization should be avoided due to concern of hemorrhage or subtle pre-existing tractional retinal detachment. Careful avoidance of the long ciliary nerves at the 3 and 9 o'clock positions will decrease the pain felt by the patient during the session and minimize that chance of post-PRP mydriasis or accommodation loss.

It is advantageous to treat the inferior retina first, whether it is a single or multiple sessions. For a single session, it has been our experience that patients are able to maintain a downward gaze longer at the beginning of the procedure rather than at the end. For patients undergoing multiple sessions, a vitreous hemorrhage could develop between sessions, causing the settled blood in the vitreous cavity to obscure the inferior retina and prevent adequate laser treatment.

Relatively new advancements in laser systems offer a more rapid and precise PRP treatment. New photocoagulation systems are available that can automatically apply multiple short-pulse successive spots in an adjustable predetermined patterns, such as the Pascal pattern scan laser (OptiMedica, Santa Clara, CA).¹⁰ While the authors have no extensive clinical experience with this device, the merits of quicker and more precise treatments show promise for expediting and improving comfort in photocoagulation treatments.

CONCLUSION

With the high prevalence of diabetic retinopathy and other ischemic eye conditions, the well-validated, tried-and-true PRP remains one of our most efficacious treatment options. Photocoagulation standards, based on traditional DRS and ETDRS protocols, are now commonly adjusted in an effort to decrease the ocular morbidity to the patient while achieving the same therapeutic result. The overall goal with PRP is to achieve maximum preservation of vision while maintaining patient comfort and minimizing ocular morbidity. RP

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