Comparing Conventional Grid Laser Photocoagulation and Subthreshold Micropulse Laser in Diabetic Macular Edema Using OCT Angiography

Laser photocoagulation was previously a mainstay treatment for DME before the introduction of anti-vascular endothelial growth factor (anti-VEGF) agents injection. Vascular endothelial growth factor (VEGF) is an important mediator of blood-retinal barrier breakdown, which leads to fluid leakage and the development of macular edema [5]. Observing that intraocular VEGF levels are increased in DME, using VEGF inhibitors (anti-VEGF) was found to be beneficial in reversing vision loss from macular edema [6]. In recent years, many large-scale studies [7], [8], [9], [10] had proven that anti-VEGF injections resulted in superior improvements in visual acuity and central subfield thickness than laser photocoagulation in treating DME. This has led to the decline of conventional focal laser as a first-line therapy.

However, in our clinical setting, laser photocoagulation may still be preferred in selected clinical scenarios in treating DME. In Hong Kong, anti-VEGF agents were self-financed items for patients in the public sector of our healthcare system. These medications could be a huge financial burden to patients with low financial support, and therefore they might prefer laser therapy instead.

Furthermore, anti-VEGF intravitreal injections have been reported to have detectable levels in systemic circulation, which can lead to systemic complications. A retrospective study [11] of 1173 patients showed that bevacizumab has a risk of leading to systemic events including acute blood pressure elevation (0.59%), cerebrovascular accidents (0.5%), myocardial infarctions (0.4%), and iliac artery aneurysms (0.17%). Hence, patients with recent history of cardiovascular accidents or significant cardiovascular comorbidities and patients who could not tolerate intravitreal injections might also find laser therapy a better option in treating DME.

Therefore, it is still useful to compare the effectiveness of conventional focal/grid laser versus subthreshold micropulse in treating DME in our clinical context. Previous studies [12], [13], [14] had mainly demonstrated non-inferiority of subthreshold micropulse laser in terms of best-corrected visual acuity (BCVA), contrast sensitivity and central retinal thickness. Nonetheless, majority of the studies demonstrated that laser scars were much more frequently identified in conventional laser than micropulse laser-treated eyes.

Optical coherence tomography angiography (OCT-A) is a new, non-invasive imaging technique to visualize the retinal vasculature and choroidal vascular layers in the macular area. It employs motion contrast imaging to high-resolution volumetric blood flow information, generating angiographic images in seconds. The principle of OCT-A involves the comparison of decorrelation signal between sequential Optical Coherence Tomography (OCT) B-scans taken at precisely the same cross-section, therefore constructing a map of blood flow. Given that only erythrocyte movements in the blood vessels are represented and axial bulk motions are eliminated, determining a vascular decorrelation signal enables visualization of 3-dimensional retinal and choroidal vascular network without the administration of intravenous dye and thus reducing the risk of potential adverse events [17], [22].