Pattern Scan Multispot Laser Photocoagulation in the Treatment of DME

With a growing body of evidence and a number of models, pattern-scan technology takes off.

ROSS CHOD, MD • ANKIT DESAI, MD • LEVENT AKDUMAN, MD

Laser photocoagulation, once widely accepted as the gold standard in the treatment of myriad retinal disease processes, including retinal vascular disorders and retinal tears, has undergone an exciting evolution over the past 10 years.

With the introduction of intravitreal injections (anti-VEGF agents and steroidal agents/implants) for retinal vascular diseases, use of laser photocoagulation has decreased to a certain extent. However, it still holds a significant place in the management of these disorders and can be effective in combination with injection therapy.

From the inception of harnessing light energy for retinal tissue photocoagulation, an idea cultivated by Gerhard Meyer-Schwickerath in the 1940s, through the creation of lamp-based delivery systems in the 1950s and then laser-based delivery systems in the 1960s, certain previously unavoidable hardships plagued these treatment systems.¹ In particular, these conventional photocoagulation systems required the time-consuming and painful spot-by-spot application of light energy to the retina.

A PARADIGM SHIFT

In 2006, however, OptiMedica (Santa Clara, CA) introduced a new, more efficient system for the delivery of laser light energy to the retina. This joystick-controlled, slit-lamp-based laser, known as the Pattern Scan Laser (PASCAL) photocoagulation system, allows the physician to apply a multiplicity of laser burns simultaneously to the diseased retinal tissues.

The original pattern scan multispot laser systems used a 532-nm, frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state laser to create an impressive assortment of laser spot arrays, which the physician could manipulate at the time of the procedure, depending on the needs of the patient (Figure 1).

Figure 1. The control touchscreen of the Nidek MC-500 Vixi laser has various pattern array options to meet the clinical needs of the patient, as do all pattern scan multispot lasers.

Available Pattern-scanning Platforms

A number of pattern scan multispot lasers with FDA approval are available on the market. OptiMedica, the creator of the original PASCAL multispot laser system, sold its retina section to Topcon Medical Laser Systems, Inc. (Santa Clara, CA), in 2010. Topcon continues to offer the traditional 532-nm green-wavelength PASCAL system, as well as a more recently developed 577-nm wavelength yellow laser.

However, the yellow wavelength laser might suffer from reduced scatter while traversing the ocular media, as well as increased transmittance through lenticular or corneal opacifications and decreased absorption by retinal xanthophyll pigments.² these drawbacks, however, make it safer for the treatment of macular lesions, such as those seen in DME.

Quantel Medical (Bozeman, MT) is also a leading producer of multispot laser systems. It offers both a 532-nm laser system, the Vitra Multispot Laser, as well as a 577 nm-wavelength laser system, known as the Supra Scan Laser. Carl Zeiss Meditec (Dublin, CA) offers the Visulas 532s VITE, a similar 532-nm laser system.

The MC-500 Vixi, produced by Nidek (Fremont, CA), is a multicolor pattern scan multispot laser. It offers a 647-nm wavelength red laser, in addition to the 532-nm and 577-nm wavelength options, all on one machine.

The 647-nm wavelength red laser provides an advantage in cases presenting with vitreous hemorrhage due to its poor hemoglobin absorption and consequent enhanced transmittance through blood.

With the MC-500 Vixi, the physician can choose to photocoagulate with any single wavelength or with a simultaneous combination of any two of the three offered wavelengths.

Adjustable Parameters

The PASCAL system is fully capable of creating conventional single-spot burns for more deliberate application, when desired. The adjustable settings of the system include powers ranging from 0 to 2,000 mW, as well as pulse durations that are adjustable from 0.01 to one second.

The physician can choose spot sizes of 60, 100, 300, or 400 μm, measured on the corneal plane. A touchscreen control panel allows the physician to manipulate these parameters conveniently while at the slit lamp (Figure 2).³

Figure 2. The control touchscreen of the NidekK MC-500 Vixi laser, with various adjustable setting options, including power, pulse duration, and laser colors in green, yellow, or red.

The FDA approved the PASCAL laser photocoagulation system for standard photocoagulation procedures in 2005. Such standard procedures primarily include panretinal photocoagulation for proliferative diabetic retinopathy, macular grid and focal laser treatments for clinically significant diabetic macular edema (DME or CSME) (Figure 3), and PRP for PDR (Figure 4), the two most notable vision-threatening ocular manifestations of diabetes.

Figure 3. Post-treatment fundus photo after pattern scan multispot laser photocoagulation of a patient with DME. Arced arrays of grid pattern laser can be seen in the superotemporal macula with focal laser spots dispersed throughout.

Figure 4. Two examples of how grid pattern laser provides for more efficient PRP in the setting of PDR.

A GROWING PROBLEM

With diabetes having become a global epidemic, the need for laser surgical intervention will continue to rise in concert with the prevalence of the disease. As the prevalence of diabetes rises to a projected 7.7% of adults by 2030 — an absolute increase from 285 million to an estimated 439 million adults, worldwide⁴ — diabetic retinopathy has already become the leading cause of new cases of legal blindness among all adults age 20 to 74 years old.⁵

Estimates published by JAMA in 2010 suggested a prevalence for diabetic retinopathy and vision-threatening diabetic retinopathy, or DME, at 28.5% and 4.4% of US adults with diabetes, respectively.⁶

ETDRS Definition of CSME

The Early Treatment Diabetic Retinopathy Study provided a lasting and well-recognized definition of CSME as one of the following:

• retinal thickening within 500 μm of the center of the macula;
• presence of hard exudates within 500 μm of the center of the macula, if associated with adjacent retinal thickening; or
• retinal thickening comprising an area equal to or less than 1 optic disc area, if any part of the zone of thickening lies within a disc diameter of the center of the macula.

A chronic disease, DME has an incidence of 20.1% in patients diagnosed with diabetes before 30 years old and 39.3% among patients diagnosed after 30.⁷ The DCCT reported that 27% of patients develop DME within nine years of the onset of diabetes.⁸

EVIDENCE-BASED MEDICINE

The Diabetic Retinopathy Study (DRS) and ETDRS both illustrated the efficacy of laser photocoagulation in the treatment of clinically significant DME and PDR.

Upon the discovery of a diagnosis of CSME, investigators found that immediate macular grid pattern laser application achieved reduced rates of moderate vision loss by 50%.⁹ The use of conventional single-spot laser became the gold standard for treatment of this vision-threatening pathologic complication of diabetes.

Additionally, the ETDRS and DRS provided the first widely accepted technical parameters for the safe application of laser photocoagulation burns to retina affected by DME and PDR.

Because PASCAL laser systems were not yet available at the time of the ETDRS, the study provided conventional laser recommendations with regard to power, duration, and spot size.

The current modified ETDRS laser application recommendations for macular grid treatment are for visible (gray-white) burn intensity, with a long pulse and a 50-msec duration.⁸

Negative Consequences

Unfortunately, conventional, long-pulse duration laser photocoagulation is not necessarily a benign procedure. Laser scars can enlarge due to the unintended lateral spread of heat energy, resulting in collateral tissue damage.

In addition, these procedures incur the possible side effects of subretinal fibrosis and neovascularization. The goal of decreasing or eliminating such complications has driven interest in refining laser application techniques.

For example, Jain et al used rabbit eyes to demonstrate a decrease in lateral and axial spread/extent of retinal lesions when using short-pulse duration laser photocoagulation. They also noted less variability of burn lesions despite variations in laser power, compared to long-pulse duration photocoagulation.¹⁰

Issues of Energy Uptake

Additionally, ubiquitous variability of retinal tissue laser energy uptake, secondary to the differences in the anatomy and tissue pigmentation of patient eyes, can be problematic.

Adjusting the power of the laser as needed can accommodate this variability in energy uptake, allowing the physician to apply a consistent laser burn despite these inherent patient-to-patient and eye-to-eye variabilities. Researchers continue to investigate the technical parameters for PASCAL laser photocoagulation in the clinical setting of DME.

Sanghvi et al, for instance, described their experience with regard to laser settings, as well as outcomes, in applying macular focal and grid laser in DME with the PASCAL system, compared to conventional single-spot laser application.

They used the PASCAL system to treat 10 eyes that had been previously treated with a conventional laser system and compared the power requirements. Their study demonstrated increased power requirements with the PASCAL laser, compared to conventional laser systems.

The mean power used with the PASCAL system was 143 mW, compared to 100 mW with conventional laser. This increased power requirement of the PASCAL system is considered secondary to the decreased pulse duration, and it accordingly decreased laser exposure time when used in the PASCAL system to create a burn of equal quality. Sanghvi et al reported no complications in their study, likely due to reduced fluence (power x time/area) per burn.¹¹

HEMORRHAGE AND PAIN

One study reported retinal hemorrhages after PASCAL laser treatment as a common phenomenon, due to variability in laser energy uptake.¹² However, the authors observed no major complications or permanent sequelae.

Interestingly, decreased pulse duration and exposure time have proved advantageous in retinal laser application. Al-Hussainy et al compared patients’ graded perception of pain with conventional 0.1-second pulse duration laser treatment with short-pulse duration settings of 0.02 seconds.

The authors adjusted power to create an equivalent, visible, gray-white, 300 µm burn, with the 0.1-second exposures requiring an average of 0.178 W and the 0.02 second exposures requiring an average 0.49 W.

Graded on a scale from 0 to 10, with 0 indicating no pain and 10 indicating the most severe, the subjects reported an average pain grade of 5.11 with long-pulse duration laser and 1.41 with short-pulse duration laser treatments.

This investigation highlighted a significant decrease in pain perception due to laser treatment with a reduction in pulse duration, despite the increased power required to create the same quality of burn.¹³

Visual Outcomes

Furthermore, Muqit et al demonstrated comparable clinical and visual outcomes between DME patients treated with ETDRS-recommended conventional argon laser parameters and those treated using short-pulse duration (0.02-second) settings with the PASCAL system.

The study found 89.3% and 62.3% success rates for focal and grid PASCAL treatment of CSME, respectively. The authors reported no ocular complications or adverse effects despite the increased power requirements for the PASCAL.

They believed this finding was due to the increasingly confined and highly loyal target retinal tissue damage associated with the higher-power, short-pulse duration burns, as well as minimization of burn overlap allowed by the PASCAL system.¹⁴

Bandello et al compared the efficacy of EDTRS-recommended (gray-white) burns to a technique of creating barely visible (light gray or threshold) laser burns, using conventional long-pulse duration laser in the treatment of DME. They reported effective clinical outcomes with the threshold technique.¹⁵

More recently, Muqit et al investigated the clinical efficacy and morphologic features of barely visible burns created with 0.01-second pulse duration PASCAL photocoagulation in DME. The authors reported that clinical regression of macular edema occurred in eight out of 10 treated eyes, as well as high fidelity of retinal photocoagulation lesions.¹⁶

Comparison with Single-spot Laser

Macular photocoagulation with the PASCAL system, utilizing a 0.01-second pulse duration, is becoming a clinical standard.¹⁰ In direct comparison with a conventional single-spot laser system of equal wavelength energy, PASCAL photocoagulation showed similar clinical efficacy in the treatment of PDR and DME.¹⁷

Modi et al investigated the clinical efficacy of PASCAL photocoagulation in the treatment of 21 eyes with macular edema, 18 secondary to PDR and three secondary to branch RVO.

Central foveal thickness improved in 71% of eyes, and the clinical efficacy was comparable to historical results achieved with conventional single-spot laser application systems, based on short-term follow-up.¹⁸

Safety Profile

The safety profile of the PASCAL system has proved quite favorable. Velez-Montoya et al performed a retrospective analysis of 1,301 consecutive PASCAL application sessions. They found 20 cases with significant complications: 17 cases (1.3%) of retinal bleeding; two cases (0.15%) of choroidal detachment; and one case (0.07%) of exudative retinal detachment.

The authors believed that these complications, which occurred most commonly in the anterior retina, were due to the difficulty of focusing the large array of spots secondary to the radius of curvature of the globe at an off-axis angle. This difficulty could have resulted in laser uptake variability and poor control of laser burns.

The study found no statistically significant differences between those patient eyes that incurred complications and those that did not.¹⁹ Overall, the authors determined the PASCAL system to be safe for use. However, they recommended caution when applying laser in the far periphery.

CONCLUSION

Multispot laser photocoagulation systems provide an attractive and efficient alternative to conventional singlespot laser systems for the treatment of DME. Recent evidence has indicated comparable safety and efficacy to conventional photocoagulation systems.

Additionally, the multispot laser technique offers practical advantages, including increased speed of photocoagulation sessions and enhanced patient comfort. Pattern scan multispot laser systems represent an exciting evolutionary step in the realm of macular grid, panretinal, and focal laser photocoagulation in the treatment of DME. RP

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