Retinal burns of subthreshold intensity created using micropulsed diode laser, which remain clinically invisible, have been shown to be successful in treating macular edema while minimizing the risk of collateral damage to the retina. A study was conducted to determine whether spectral domain optical coherence tomography (SD-OCT) could be used to detect subthreshold retinal burns created using the 532-nm green wavelength laser. A series of retinal burns of gradually decreasing intensity were created in 10 eyes. Retinal burns produced with duration of laser exposure of 0.03 second or less, although clinically invisible, were detectable on the SD-OCT scan as increased retinal reflectivity confined to the outer retinal layers. This series demonstrates the potential of using SD-OCT imaging to verify delivery of subthreshold laser burns.
From Birmingham and Midland Eye Centre, City Hospital (AB, SE); and Birmingham and Midland Eye Centre, Aston University (JMG), Birmingham, United Kingdom.
Presented as a poster at the Association for Research in Vision and Ophthalmology annual meeting, May 3–7, 2009, Fort Lauderdale, Florida.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Ajay Bhatnagar, FRCOphth (London), FRCSEd, Birmingham and Midland Eye Centre, Birmingham B18 7 QH, United Kingdom. E-mail: email@example.com
Received: September 04, 2009
Accepted: February 25, 2010
Posted Online: December 01, 2010
In treatment protocols for macular edema practiced currently that are based on the Early Treatment Diabetic Retinopathy Study, the end point of laser photocoagulation is clinically visible retinal whitening.1 Micropulsed 810-nm diode laser has been reported to be successful in treating macular edema by creating subthreshold, clinically invisible burns, thus minimizing collateral damage to the retina.2,3 With green laser, the most commonly used wavelength for treating macular edema, the amount of thermal damage to retinal structures depends to a great extent on the time duration of the laser pulse.2 This can be minimized by reducing the duration of laser pulse to create subthreshold burns. A drawback of subthreshold treatment is the absence of an immediate visible local effect, making it difficult to verify that the desired laser energy has indeed been delivered. We conducted a study to determine whether spectral domain optical coherence tomography (SD-OCT) could be used to confirm delivery of subthreshold laser burns created using the 532-nm green laser.
Design and Methods
Ten patients with planned panretinal photocoagulation for proliferative diabetic retinopathy were included in this study. Before initiating panretinal photocoagulation, six parallel rows of 6 to 10 laser burns using 532-nm wavelength laser were made in a 3- to 4-mm region of each patient’s retina (Fig. 1). Each laser spot was 200 μm in diameter. The first row was created at “threshold” power with exposure of 0.1 second to create a gray-white spot. For each subsequent row of spots, the duration of exposure was reduced to 0.07, 0.05, 0.03, 0.02, and 0.01 second, respectively, while keeping the power and spot size constant. The area of retina irradiated with laser was imaged using the Topcon SD-OCT (Topcon Inc., Paramus, NJ) within a few minutes of laser treatment.
Figure 1. Color Photograph Showing Six Parallel Rows of Laser Burns Created Nasal to the Optic Disc. The Top Row Is Burns Created Using the 532-nm Green Laser Set at “threshold” Power with Exposure of 0.1 Second. For Each Subsequent Row of Burns, the Duration Was Reduced to 0.07, 0.05, 0.03, 0.02, and 0.01 Second, Respectively, Keeping the Power and Spot Size Unchanged.
Laser burns created using threshold power were seen on the OCT scan in all cases as a homogenous diffuse increase in reflectivity extending across the full thickness of retina. On reducing the duration of exposure to the laser, the retinal burns became less distinctly visible and almost invisible on biomicroscopic examination at a duration of 0.03 second or less. Although clinically undetectable, these burns were visible on OCT scan as focal areas of increased reflectivity confined to the outer retinal layers (Fig. 2).
Figure 2. (A) Clinically Visible “threshold” Laser Burns Are Clearly Seen on the Spectral Domain Optical Coherence Tomography (SD-OCT) Scan as Homogenous Diffuse Increase in Retinal Reflectivity Extending Across the Full Thickness of Retina. (B) Subthreshold Laser Burns that Are Less Distinctly Visible Clinically Are Detected on the SD-OCT Scan as Focal Areas of Increased Reflectivity. (C) Further Reduction in the Duration of Laser Exposure to 0.03 Second Creates Burns that Are not Visible Clinically but Are Detectable on the SD-OCT Scan as Focal Areas of Increased Reflectivity Confined to the Outer Retinal Layers.
The optimal goal of retinal laser photocoagulation is to produce a therapeutic effect while minimizing collateral damage to adjacent retinal and choroidal tissue. Recently, interest in retinal laser treatment has turned to using shorter laser pulse durations of 20 milliseconds, which have been shown to be more comfortable for patients and to have theoretical advantages over longer exposures in terms of retinal collateral damage.4,5
OCT has been shown to detect changes in retinal reflectivity profile immediately following threshold and subthreshold laser treatment using the micropulsed 810-nm diode laser.6 In our series, we have shown that reduction in the duration of laser pulse using the 532-nm green laser creates subthreshold retinal burns that may not be visible ophthalmoscopically but are detectable using the OCT. This suggests that in the clinical setting, green wavelength laser with short pulse duration can be used to deliver subthreshold and threshold laser burns, with theoretical advantages over standard laser settings, which can be confirmed by OCT. Further studies are required to confirm the effectiveness of this treatment protocol in managing macular edema.
- Early Treatment Diabetic Retinopathy Study Research Group. Early photocoagulation for diabetic retinopathy. ETDRS Report No. 9. Ophthalmology. 1991;98(suppl):766–785.
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- Luttrull JK, Musch DC, Mainster MA. Subthreshold diode micropulse photocoagulation for the treatment of clinically significant macular oedema. Br J Ophthalmol. 2005;89:74–80. doi:10.1136/bjo.2004.051540 [CrossRef]
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- Jain A, Blumenkranz MS, Paulus Y, et al. Effect of pulse duration on size and character of the lesion in retinal photocoagulation. Arch Ophthalmol. 2008;126:78–85. doi:10.1001/archophthalmol.2007.29 [CrossRef]