Subthreshold Lasers in Brief

by Dr. Sashwanthi Mohan and Dr. Komal Agarwal

Lasers are widely used for the treatment of retinal disorders. However, conventional laser photocoagulation produces visible thermal burns, which leads to destruction of the retinal tissue. Today, subthreshold laser is a category of photocoagulation method utilized to treat retinal or macular diseases that, unlike conventional laser treatment, provides therapeutic benefits without inducing visible laser scars in the treated area.

Using subthreshold laser enables clinicians to minimize potential harm to the surrounding healthy tissues while still achieving the desired therapeutic outcomes. Thus, it is an alternative to conventional laser, especially for the treatment of central macular and foveal pathologies.

Conventional laser photocoagulation is a long-duration continuous photothermal laser that leads to coagulative necrosis of the photoreceptor-RPE choriocapillaris complex. This can cause collateral damage to the retina and complications, such as extensive scarring, choroidal neovascularisation, subretinal fibrosis, and visual field loss.

Subthreshold laser, on the other hand, does not cause any visible damage, preserves the retinal pigment epithelium (RPE) and photoreceptors, and allows for treatment close to the fovea. It does this by modulating heat-shock protein expression in the RPE and regulating cytokine expression without damage to the retinal tissue.

Table 1 shows the different types of subthreshold lasers available today.1-4

Selective Retinal Therapy (SRT)

Short high-energy pulses of very short duration (< 5 microseconds), which leads to ‘microbubble formation’ that selectively targets RPE and spare surrounding tissues.

Selectively damages RPE cells leading to ‘retinal rejuvenation’ and preserves photoreceptors and neural tissue.

Micropulse Laser

Delivers laser energy using short pulses of duration (100-300 microseconds) in a repetitive fashion leading to an ‘on’ and ‘off’ time. The ratio of ‘on’ to ‘off’ is duty cycle. Microsecond pulses cause intracellular microbubbles around melanosomes.

Duty cycle below 100% and ‘off’ time use allows cool down time for the tissue and prevents retinal damage.

Selective Nanosecond Laser (SNL)

Endpoint Management (EpM)

Transpupillary Thermotherapy (TTT)

Ultra-short pulses of a few nanoseconds to selectively target RPE and spare surrounding tissues by formation of transient microbubbles around melanosomes. Shorter duty cycle than a micropulse laser.

Rapid application of multiple laser spots array called pattern scanning with shorter pulse duration of 1030 microseconds using a 577 nm laser.

Low irradiance and long duration pulse of 1 minute applied over a large spot (0.5- 3 mm).

Subthreshold laser is indicated in the following guidelines5 and conditions: Central serous chorioretinopathy (CSCR), diabetic macular edema (DME), retinal vein occlusion (RVO)

The evidence on the eligibility of patients requiring and responding to subthreshold laser is still developing. A major amount of present evidence on subthreshold laser is based on small studies that are retrospective in nature with wide variability in associated macular edema (ME), agerelated macular degeneration (AMD), proliferative diabetic retinopathy (PDR), macular telangiectasia (MacTel)Type 2, and retinal and choroidal tumors (*TTT for these conditions). treatment parameters and titration of power.

Selectively damages RPE cells leading to ‘retinal rejuvenation’ and preserves photoreceptors and neural tissue. It delivers only 0.2% of energy per pulse compared to conventional laser.

Provides visible titration endpoints, large number of spots can be placed in a fast manner, predefined patterns based on retinal landmarks.

Useful for treating tumors of the retina and choroid.

The Subthreshold Ophthalmic Laser Society (SOLS) in 2022 has reported consensus guidelines (Table 2) for subthreshold laser application settings for DME and CSCR6 in an attempt to standardize care.

Evidence also suggests that retinal thickness can affect the efficacy of subthreshold laser in various retinal conditions. Such conflicting evidence on efficacy might be largely due to the variations and non-standardization in different clinical parameters used by the clinicians in different clinical settings. With the latest guidelines by SOLS, we expect better evidence to support the use of subthreshold laser in a set clinical setting.

Subthreshold laser is developing into a useful non-invasive adjunct for the treatment of macular and retinal diseases. Advantages of minimal to no collateral damage, ease of repeatability, and noninvasiveness give the therapy significant potential to develop into a major therapeutic alternative or adjunct in certain retinal diseases.

1. Ong J, Selvam A, Maltsev DS, Zhang X, Wu L, Chhablani J. Subthreshold laser systems: a narrative review of the current status and advancements for retinal diseases. Ann Eye Sci. 2022;7:15.

2. Chehade L, Chidlow G, Wood J, Casson RJ. Short-pulse duration retinal lasers: a review. Clin Exp Ophthalmol. 2016;44(8):714-721.

3. Sabal B, Teper S, Wylęgała E. Subthreshold Micropulse Laser for Diabetic Macular Edema: A Review. J Clin Med. 2022;12(1):274.

4. Li J, Paulus YM. Advances in Retinal Laser Therapy. Int J Ophthalmic Res. 2018;4(1):259-264.

5. Brader HS, Young LH. Subthreshold Diode Micropulse Laser: A Review. Semin Ophthalmol. 2016;31(1-2):30-39.

6. Chhablani J; SOLS (Subthreshold Ophthalmic Laser Society) writing committee. Subthreshold laser therapy guidelines for retinal diseases. Eye (Lond). 2022;36(12):2234-2235.

Dr. Sashwanthi Mohan is a specialist ophthalmologist and vitreoretinal specialist at Medcare Eye Centre, Dubai. She completed her DNB Ophthalmology from L.V. Prasad Eye Institute in Hyderabad, India, and was awarded the Dr. G. Venkataswamy Gold Medal for Ophthalmology by the National Board of Examinations, followed by a vitreoretinal fellowship from Sankara Nethralaya, Chennai, where she was awarded as the best outgoing vitreoretinal fellow. She has a keen interest in research and has many peer-reviewed publications to her name. She is also interested in education and has an educational website called Ophthalmobytes. She is a fellow of the international council of Ophthalmology (FICO) and a member of the Royal College of Surgeons, Edinburgh (MRCS).

sashu23@gmail.com komal.agarwal.vr@gmail.com

Dr. Komal Agarwal is an experienced vitreoretinal specialist in Prakash Netra Kendra in Lucknow, India, with a special interest in retinopathy of prematurity (ROP) and pediatric retinal disorders. After completing her MBBS and post-graduation, she joined a fellowship program at the prestigious LV Prasad Eye Institute, India. She underwent further training in pediatric retinal disorders including ROP (both medical and surgical) during her fellowship. She continues to work at LV Prasad Eye Institute as faculty in both medical and surgical retina. Dr. Agarwal is actively involved in clinical research. Her prime area of research interest is in pediatric retina and ROP. She has more than 45 research publications in peerreviewed journals, both national and international. She has also written five book chapters. Dr. Agarwal has presented her work at various national and international conferences.