Panretinal Photocoagulation

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Overview

Laser therapy has a long-standing history in the treatment of various ocular conditions. The initial developments included the xenon arc laser created by the Carl Zeiss Laboratory in the early days of retinal therapy, followed by the argon laser discovered by William Bridges. Despite these innovations, comprehensive studies assessing the efficacy of laser treatments, particularly panretinal photocoagulation (PRP), did not materialize until decades later. One pivotal study was the Diabetic Retinopathy Study (DRS), which assessed the impact of PRP on individuals suffering from proliferative diabetic retinopathy (PDR). This research compared the results of xenon arc and argon lasers to untreated controls, revealing that laser treatment significantly benefitted patients with PDR. Moreover, it established that argon lasers posed fewer adverse effects than their xenon counterparts, setting the stage for argon lasers to become the preferred option for treatment. Subsequent findings from the Early Treatment Diabetic Retinopathy Study (ETDRS) further refined the understanding of optimal timing for intervention with PRP in the progression of non-proliferative diabetic retinopathy (NPDR). Today, while advancements in both laser technology and other therapeutic methods for retinal disorders are on the rise, PRP remains the cornerstone therapy for PDR.

Laser Physics and Biological Interactions

The principle behind PRP lies in the absorption of laser light by the retinal pigment epithelium (RPE) and the choroid beneath it. The unique pigments in the RPE are efficient at absorbing light across nearly all wavelengths, particularly yellow, green, or red light when conducting PRP. The absorbed laser energy is transformed into thermal energy, causing the tissue temperature to rise by roughly 20 to 30 degrees Celsius. This thermal effect results in tissue damage, specifically causing denaturation of proteins, leading to localized retinal cell death and necrosis. Over time, these thermally altered regions evolve into scars characterized by pigmentation changes, visibly marking the laser impact on the RPE. On average, hundreds of micro-burns are strategically administered to the retina to target and destroy the ischemic extramacular areas, effectively decreasing the amount of ischemic retinal tissue. In doing so, PRP lessens the production of vascular endothelial growth factor (VEGF), a key player in neovascularization processes within the eye.

Procedure

Administration of PRP typically employs either a slit lamp system or a laser indirect ophthalmoscope (BIO).

Slit lamp: The laser is integrated with a standard ophthalmic slit lamp, allowing for coaxial laser energy delivery. The patient is seated with their chin resting on a chin-rest, and a specially designed contact lens positioned against the cornea with a coupling agent focuses the laser onto the retina. A wide-angle or mirrored lens facilitates optimal focusing through the cornea, anterior chamber, and lens onto the retina.

Headlamp: In this configuration, the patient may either sit or lie supine. The attending physician wears an indirect headlamp, with the laser aligned coaxially. A handheld lens is utilized to examine the retina and direct the laser. The physician’s head movements guide the targeting beam. The procedure typically involves the application of topical anesthesia (proparacaine or tetracaine) in both eyes. In cases requiring special attention, such as infants or non-compliant patients, stronger anesthesia options like systemic anesthesia or sedatives may be employed. Approximately 1,000 typical-sized burns are made across one to four treatment sessions, depending on specific protocols. For PRP using standard argon lasers, the treatment parameters involve burn sizes ranging from 200 to 500 micrometers, with pulse durations around 100 milliseconds and power settings of 200-250 mW. The objective is to attain burns that appear grey while avoiding denser white burns, allowing for protocol adjustments to meet the desired therapeutic outcomes.

Indications and Evidence

PRP is primarily indicated for the treatment of retinal ischemia and neovascularization across a variety of causes, though it is most frequently used for proliferative diabetic retinopathy.

The DRS, a landmark study by the National Eye Institute, investigated whether PRP effectively halts the progression of PDR and prevents vision loss compared to no intervention.

The study encompassed fifteen medical centers in the United States, enrolling numerous patients with severe NPDR in both eyes or PDR in one eye. Participants received PRP treatment in one eye, with the contralateral eye serving as a control, and were randomly allocated to either xenon arc or argon laser treatments.

The findings indicated that PRP recipients experienced significantly enhanced outcomes compared to those who received no treatment, with more than a 50% reduction in the risk of severe visual loss (SVL). Notably, untreated eyes showed a 16.3% vision loss rate, while treated eyes demonstrated only a 6.4% loss over a two-year follow-up. Subgroups with high-risk characteristics benefitted the most from treatment.

Additionally, the study compared the effectiveness of xenon arc to argon laser treatment, concluding that the argon laser was superior, leading to a decline in the use of xenon systems.

While the DRS confirmed the superiority of PRP over no treatment, it left unanswered questions regarding the optimal timing for intervention during the disease’s progression. To address this, the ETDRS sought to determine if early PRP was more advantageous than deferred therapy. The ETDRS adhered to the photocoagulation guidelines from the DRS.

The data from the DRS indicated significant visual outcomes; at two years, 11% of treated eyes and 26% of control eyes exhibiting high-risk retinopathy developed severe visual loss, with these figures increasing to 20% and 44%, respectively, after four years. For instances of early proliferative retinopathy, the two-year severe visual loss rates stood at 3% for treated versus 7% for untreated eyes, with those figures adjusting to 7% and 21% at the four-year mark. The DRS emphasized caution against treating early-proliferative retinopathy, due to potential side effects from the laser.

Consequently, the ETDRS recommended postponing scatter photocoagulation in mild NPDR cases to mitigate the risks of treatment against minimal benefits, initiating PRP once NPDR transitioned into severe stages or PDR.