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].
The authors believe that OCT-A can be used as a new assessment tool in comparing the efficacy
of conventional focal laser versus subthreshold micropulse laser in the treatment of DME. We
hypothesize that subthreshold micropulse laser is superior to focal laser in treating
patients with DME in terms of OCT-A parameters. We expect the reduction in the studied OCT-A
parameters (i.e. FAZ area, area of cysts, number of microaneurysms, etc.) in patients
receiving subthreshold micropulse laser will be greater than focal laser by 30%.