Early Experience With the PASCAL Method of Photocoagulation

System combines the efficacy of conventional treatment with unprecedented precision, safety and efficiency.

BY PAULO E. STANGA, MD, FRCOphth

Now is a time of significant advances in the diagnosis and treatment of diabetic retinopathy. In the areas of retinal imaging and pharmacological and laser therapy, our capabilities have been expanding. At Manchester Royal Eye Hospital in England, we have been using and studying a very exciting new technology, PASCAL Pattern Scan Laser Photocoagulation.

The PASCAL® Photocoagulator (OptiMedica, Santa Clara, Calif.) is a 532-nm frequency-doubled Nd:YAG diode-pumped solid-state laser. In contrast to conventional laser treatment, the PASCAL Method semi-automates retinal photocoagulation procedures by delivering, with a single foot pedal depression, multiple laser burns in a rapid predetermined sequence in the form of a pattern array. To achieve this, the pulse duration of each burn is reduced to 10-20 milliseconds. Up to 56 spots can be delivered in approximately 0.6 seconds. Based on each patient's disease state and anatomical features, the physician can employ precise treatment patterns, such as square arrays (2 × 2, 3 × 3, 4 × 4, 5 × 5), triple arcs, full and focal/modified macular grids, octants, quadrants or single-spots. Precision is further enhanced by the ability to customize parameters, such as the spacing between spots.

PASCAL's technological features are designed to provide benefits for both patients and physicians. Patients may experience less discomfort, less collateral tissue damage, less inflammatory response and preservation of the retinal nerve fiber layer. Energy delivery is fast, reducing the risk of a change in patient fixation leading to a foveal burn. Also, patients may be more likely to return for subsequent treatments because their first treatment was not painful.

Figure 1. This image obtained with the 3D OCT 1000 (Topcon, Paramus, NJ) depicts a combined representation of a cross-sectional scan showing subfoveal fluid and the corresponding topography map showing the area of retinal thickening. The pin marks the location of the thickened fovea.

For physicians, the PASCAL Method offers more precision and control — and thus more safety — than conventional photocoagulation. It also offers efficiency, by significantly reducing treatment time. Single-session panretinal photocoagulation (PRP) is possible. At our center, in some cases, we have been able to combine single-session PRP with a full grid treatment with good results.

Figure 2. Imaging with the 3D OCT 1000 (Topcon), obtained 1 hour post PASCAL treatment, shows that the laser coagulative effects are limited to the outer retina. Burns have been circled for easier identification.

WORKING WITH NOVEL TECHNOLOGY

As we began working with the PASCAL Photocoagulator, we set out to address a series of questions. How strong should the laser burn be? Should we burn full thickness retina? Should we target only the retinal pigment epithelium (RPE)? What if we do not see some of the burns? Should we go back and re-treat immediately?

With PASCAL, we attempt to make very light burns. We have observed, for example, in a patient with thickening of the macula temporal to the fovea, that not all of the burns in a pattern were visible. Red-free photography more clearly showed them. Using autofluorescence an hour later, all of the burns were visible, including those used to titrate the treatment intensity. The burns looked hypo-autofluorescent; they lacked an autofluorescence signal. In the same patient, Fourier domain optical coherence tomography (OCT) indicated that the burns were confined to the outer retina. This correlates well with the histopathology work of Mark Blumenkranz, MD, and his colleagues, who similarly found the PASCAL treatment effect in such cases to be confined to the outer retina.^1,2

In another case, in which we carried out a horseshoe-shaped grid treatment, some of the burns were not visible 1 hour after the procedure. We may be tempted in such a situation to go back and deliver more treatment. However, red-free photography and autofluorescence imaging clearly showed the horseshoe-shaped area of treatment. Our interpretation of the autofluorescence images was that the hypo-autofluorescence was secondary to the coagulated outer retina blocking the normal autofluorescence. One month after this treatment, we could barely see pigmentation of the burns. They were there, however, appearing hyper-autofluorescent; they showed a stronger than normal autofluorescence signal.

Our interpretation of the autofluorescence images was that the hyper-autofluorescence represented the accumulation of lipofuscin that resulted from either targeted photoreceptor or RPE cells. Fourier domain OCT showed that the PASCAL burn was confined to the RPE or the space between the RPE and the junction between the inner and outer layers of the photoreceptors.

The burns delivered with the PASCAL Photocoagulator do not look the same over time as those delivered via standard argon laser. By two years after PASCAL laser treatment, the neovascularization has regressed and the treated areas do not look as pigmented as we would expect to find them in a patient treated with standard argon laser. Auto-fluorescence of the same area showed that the RPE was targeted over an area larger than that visible on biomicroscopy.

EARLY EXPERIENCE WITH THE PASCAL PHOTOCOAGULATOR

We conducted a retrospective observational study involving 75 procedures in 60 patients treated between November 2006 and May 2007.³ We looked at the laser parameters used for effective photocoagulation with the PASCAL system, especially the power required to produce therapeutic burns. We compared PASCAL parameters with traditional argon photocoagulation parameters in patients who received treatment with both systems. We also looked at outcomes.

For the purposes of our study, we divided the patients into 4 groups (Group 1, PRP; Group 2, focal and modified grid macular laser; Group 3, macular grid; and Group 4, retinopexy for retinal breaks and degenerations). If an eye had been treated previously for the same indication with a conventional single spot laser, we recorded the power, the number of burns, the spot size and the burn duration in order to compare the settings needed with each system. Using the PASCAL system for PRP and retinopexy, we delivered moderate intensity burns that produced retinal blanching. Macular burns were lighter. Because the lenses used can have implications for the laser power used, it is important to note that we used the Mainster PRP 165 and the Volk Area Centralis. All patients treated with the PASCAL Photocoagulator required only topical anesthesia.

Perhaps this group of patients is too small to draw firm conclusions, but our results were good. Successful outcomes were achieved in 31 of the 34 procedures in Group 1; 24 of the 26 procedures in Group 2; 5 of the 7 procedures in Group 3; and 8 of the 8 procedures in Group 4.

As mentioned previously, we were able to compare parameters for eyes that were treated with both conventional and PASCAL photocoagulation. For PRP, the average power used with the conventional photocoagulator was 235 mW while the average power used with the PASCAL system was 396 mW. The difference in power was highly significant (p<0.001). For macular grid treatments, the PASCAL system was similarly used at significantly higher power than the conventional laser, 143 mW and 100 mW, respectively (p<0.001).

The higher power levels required with the PASCAL system did not result in any complications. This likely reflects the reduced laser energy per burn reaching the eye secondary to its shorter duration. Furthermore, we did not observe any side effects on vasculature if the treatment array involved an area traversed by blood vessels. We did not see any bleeding of retinal or choroidal origin, and we did not observe any side effects when re-treating in cases where we were unable to avoid previous burns.

In our paper, we also noted that we successfully performed single-session PRP in 5 patients using a mean of 1498 burns, with no complications. In addition, 5 patients were successfully treated with a macular grid using only pattern arrays.

MOVING FORWARD WITH IMPROVED SAFETY, PRECISION AND COMFORT

To summarize the results of our study of the PASCAL photocoagulation system, the outcomes were comparable to those achieved using conventional burn durations. We needed higher treatment powers, which is likely due to PASCAL's brief exposure times. However, we did not observe any side effects related to the higher power. Furthermore, treatment with the system was comfortable for patients and physicians, and much more efficient than conventional laser photocoagulation. The array method of multiple burns even made it feasible to carry out single-session PRP during a busy clinic day.

Our experience to date indicates that the PASCAL Pattern Scan Method represents significant progress toward improving the safety, precision, comfort and efficiency of photocoagulation procedures, which continue to be an integral part of retinal practice.RP

Paulo E. Stanga, MD, FRCOphth, is a consultant ophthalmologist and vitreoretinal surgeon for Manchester Royal Eye Hospital in Manchester, England, UK.