Subthreshold laser, or sub visible laser photocoagulation, encompasses all types of laser treatment that show no visible signs of damage to the eye of the examiner. The actual damage may range from significant cell damage and subsequent mild scarring (but no visible damage at the time of application) to no cell death whatsoever. This treatment method is currently most commonly used in cases of retinal vascular diseases with macular edema, such as diabetic macular edema (DME), central serous chorioretinopathy (CSR), and macular edema secondary to branch retinal vein occlusion (BRVO). More recently, it has been thought to be helpful in proliferative diabetic retinopathy (PDR) and glaucoma.
Retinal laser photocoagulation, in its earliest form, was first introduced in 1961 and has since been the standard of care to treat many retinal disorders including diabetic macular edema (DME), central serous retinopathy (CSR), proliferative diabetic retinopathy (PDR), vascular occlusions, and retinal tears, among other pathologies. Lasers are amplified light caused by stimulated emission of radiation. Lasers are produced by using an energy source to excite the electrons in the lasing medium and release them. Photons are emitted when these electrons return to ground state, and are reflected and amplified between the mirrors to excite more electrons. When the population of atoms in the excited state exceed those that are in a lower-energy state, light amplification occurs. Some of the amplified light is allowed to escape through the partial reflecting mirror, and this results in emitted laser energy. In retinal photocoagulation, the laser is used to produce the photothermal effects that treat retinal disorders. During retinal photocoagulation, the laser energy is absorbed primarily by melanin in the retinal pigment epithelium (RPE) and the choroid, as well as the hemoglobin in blood. It was found that at 532 nm wavelength, approximately half of the laser energy falling on the retina is absorbed by the RPE, while the rest is absorbed in the choroid.
When energy from the laser is absorbed by the retinal pigment epithelium (RPE), light energy is converted into thermal energy. As the temperature rises above 65°C, cellular proteins denature and coagulative necrosis occur A traditional laser burn creates a heat wave that spreads outward from the intended burn site in the RPE and/or choroid. If the thermal wave reaches the overlying neurosensory retina with a high enough temperature to damage the natural transparency of the retina, it can create a white scar.
Laser properties (wavelength, power, and duration) are modified based on the intended retinal therapy. Conventional retinal laser photocoagulation for diabetic retinopathy, retinal vascular diseases, and retinal tears typically uses a continuous wave laser in green, yellow, or red range at a wavelength of 514 nm to 532 nm, a pulse duration of 100 ms to 200 ms, and a power between 100 mW to 750 mW. Spot sizes are also varied between 100 um to 500 um, depending on the application.
Several variations of continuous wave laser treatment exist, but primarily Panretinal photocoagulation (PRP), Focal, and Grid laser treatments are used. Panretinal laser is used to destroy large peripheral areas of retina in the presence of vascular ischemia. Focal laser treatment is used to treat focal DME by aiming the laser energy directly at the affected area and closing the leaking microaneurysms lesions with photothermal energy. Grid laser treatment is used to treat diffuse DME and edema related to retinal vein occlusions.
The pathophysiology underlying a number of retinal vascular disorders for which typical laser treatment is prescribed includes inflammation and hypoxia. These conditions stimulate angiogenic factors, such as vascular endothelial growth factors (VEGF) or inflammatory cytokines. When the VEGF overcomes the naturally produced inhibitor Pigment Epithelium Derived Factor (PEDF) from the RPE, it promotes vascular permeability that causes the leakage of fluid into the retinal tissue. Laser photocoagulation can be used to destroy the numerous metabolically demanding photoreceptors in poorly perfused portions of the retina to limit ischemia and decrease the production of angiogenic factors. 
Development of Subthreshold Laser Therapy
Conventional continuous wave laser photocoagulation is intended to damage the retinal tissue as treatment for the disease, usually to reduce ischemic effects (PRP). After healing, retinal atrophy and thinning occur in the regions of laser application, sometimes causing scotomas and post-laser lesion enlargement. Atypical effects (complications) are subretinal and choroidal neovascularization, subretinal fibrosis, and progressive visual field loss.
These concerns regarding the destructiveness of laser therapy have led to numerous investigations to provide the same therapeutic effect of lasers while minimizing the damaging effects of lasers. Further studies have shown that full thickness retinal damage is not needed to obtain the beneficial effects of lasers. Advancement of laser technology, along with these studies, has led to the development of subthreshold (low power) lasers. Various optical and thermodynamic principles are applied to minimize retinal damage. “Subthreshold” refers to photocoagulation that does not produce clinical or histologic evidence of retinal damage.
In the 1990s, Thomas Friberg, MD, FACS, developed the Iridex Micropulse laser, which fragments laser emission into a sequence of short (microsecond) pulses with a low duty cycle (90%+ off time). This initiated interest in subthreshold laser, and now other types of non-micropulse sub-threshold laser protocols exist. In contrast to conventional longer duration millisecond continuous wave laser therapy, which delivers the same magnitude of energy throughout the entire exposure time (millisecond time scale) and causes significant collateral damage, subthreshold laser delivers similar therapeutic benefits with improved overall retinal sensitivity. Since it does not cause any visible damage, it allows for several retreatments in close proximity to the fovea. Most subthreshold treatment strategies incorporate a higher density of laser spots (and much lower power) than conventional focal and grid macular laser treatments.
In general, therapeutic benefit is thought to be derived by inducing thermal stress on RPE cells. The response of the RPE to mild thermal stress causes the therapeutic cellular cascade to be activated. Since this response is activated by the cells surviving the hyperthermal treatment, the goal of the subthreshold therapy is to maintain the temperature rise below the threshold of irreversible thermal damage. Heat generation in tissue is determined by a variety of laser parameters such as exposure time, power, wavelength, and the optical absorption and scattering characteristics of the treated tissue. Consequently, several techniques have evolved in subthreshold laser treatment.
Types of Subthreshold Laser
Although sub-threshold laser denotes any laser application below the visible threshold, there are three principal types of modern subthreshold laser: Micropulse, Selective Retinal Therapy (SRT), and Topcon PASCAL with EndPoint Management. Transpupillary Thermal Therapy is mentioned briefly.
In the 1990s, IRIDEX Micropulse 810 nm laser emitted bursts of short, repetitive pulses that last for microseconds, allowing for significant cooling in between these short pulses. This contrasts with conventional continuous wave laser, in which energy delivered continuously for milliseconds causes thermal damage. SDM laser emission allows for effective subthreshold tissue-sparing laser treatments without visible burns to the RPE. For the Micropulse system, the ‘‘on’’ time (typically 100μs to 300μs) is the duration of each micropulse and the ‘‘off’’ time (generally 1,700μs to 1,900μs) is the interval between consecutive micropulses. Short “on” times that limits thermal elevation and thermal dissipation to the adjacent tissue and long “off” times that allows the tissue to cool down before the next pulse and allowing the RPE to return to its baseline temperature before the start of the next pulse. This eliminates cumulative or continuous thermal build-up allow for heat dissipation and heat transfer to adjacent tissue decreasing collateral damage to the neurosensory retina and prevents coagulative necrosis. The surgeon is able to vary the power based upon the endpoint that is necessary. When a low duty cycle is used, the cooling interval allows the RPE to return to its baseline temperature before the start of the next pulse. This eliminates cumulative or continuous thermal build-up. 
In addition, the lack of chorioretinal scarring allows for both overlapping application of the laser and frequent retreatment of the same retinal areas, without causing noticeable retinal scarring. However, SDM laser therapy has its disadvantages as well. There is always the possibility of undertreatment or lack of treatment efficacy in difficult cases. The inability to record treated areas and the inadvertent retreatment of areas can also present challenges. The surgeon must carefully document which areas have been treated. One solution to this dilemma is indocyanine green angiography-assisted SMD photocoagulation. SMD laser-treated areas appear dark from the resultant quenching of indocyanine green fluorescence.
Selective Retinal Therapy (SRT)
Selective Retinal Therapy is another established subthreshold laser therapy used to treat retinal disorders while preventing severe damage to the retina. In SRT, RPE cells are selectively damaged without affecting the photoreceptors or choroid by using laser pulse durations of microseconds or nanoseconds. Since the pulse duration is so short, the resulting effect serves to vaporize a single cell or small groups of melanosome rich RPE cells, while preserving the photoreceptors and neural retina. This selective damage to the RPE stimulates cell migration and proliferation of surrounding RPE into the treated retinal areas. It is thought to improve the metabolism of the diseased retina.
It is likely that efficacy in the SRT treatment is due to the release of inflammatory markers, including matrix metalloproteinases 2 and 9. Multifocal ERG and histology has shown no significant retinal change 30 days after SRT laser application. Several pulse laser systems are available for SRT. Recent SRT approaches employ energy- ramping, rapidly scanning lasers that use reflectometric dosimetry. The efficacy of SRT has been demonstrated in the treatment of diabetic retinopathy, diabetic macular edema, and central serous chorioretinopathy. 
Continuous wave laser with EndPoint Management (PASCAL)
This laser utilizes rapid application of an array of laser spots (pattern scanning) with a shorter pulse duration of 10 to 30 ms. At the correct power, moderate temperature rise induces changes in various proteins, which denature at characteristic rates specific to protein species. The denaturation leads to cellular damage, but not death. The EndPoint Management system utilizes a mathematical formula derived from the Arrhenius integral. The rate of decrease in concentration of a critical component for cellular metabolism D(t) with temperature T(t) is modelled as:
dD(t) = -D(t)*A*exp(-E’/R*T(t)) * dt
where E’ and A are the activation energy and rate constant parameterizing the process respectively, and R is the gas constant (8.3 J/(K∙mol)). Tissue damage, or the decrease in critical molecular component D(τ) relative to its initial value D0 over the pulse length 𝓣, is encapsulated in the Arrhenius integral Ω.
Ω(t) = -ln (D(𝓣)/D(t)) = A0Texp(-E’/R.T(t))*dt
The therapeutic range of Ω for sub-lethal thermal treatment is determined using experiments on heat shock protein (HSP) expression following sub-lethal retinal exposures in mice, as well as a computational analysis of clinical laser settings. For Ω < 0.1, there is very little effect on the cells, while cells in which Ω > 1 are at risk of being irreversibly damaged. The same model, when applied to micropulse exposures, also demonstrates that continuous wave laser treatment of the same duration and same average power produces a similar average temperature and Arrhenius integral. This implies that micropulsing may not be necessary, and that similar tissue effects can be produced using more conventional continuous wave lasers, as long as power and duration are appropriately controlled.
Topcon’s Endpoint Management (EpM) software uses a computational model of retinal heating and an Arrhenius damage model to determine optimal laser parameters. The algorithms modulate power and duration concurrently to maximize the margins between visible and subvisible photocoagulation endpoints, providing linear control over an inherently nonlinear process.
EpM strategy begins retinal treatment with the titration of the laser power to a minimally visible retinal burn (threshold) at the edge of the macula. This gives a consistent baseline and ensures repeatable results between patients. The threshold pulse energy is designated to be the 100 percent level on the EpM settings, and the treatment pulse energy is then defined as a percentage of this level. From clinical trials on animals, it has been established that a 30 percent energy level is a safe and non-damaging level for the retina. Hence, EpM treatment adjusts laser power and pulse duration to this level, which optimizes the therapy to induce HSP expression while preventing thermal damage to the RPE. Since each spot within the high-density laser spot pattern is treated with a pulse shorter than 10 ms, a large number of spots can be quickly placed. The visible titration endpoints may be placed at the corners of a pattern and used as reference lesions throughout the course of treatment. This provides visible feedback on dosage and positioning of treated areas.
Transpupillary Thermal Therapy (TTT)
This is a form of subthreshold laser that has very low irradiance and long duration (order of tens of seconds). This is rarely used except for certain cases of retinal tumors.
The indications and acceptance of non-damaging sub-threshold laser continue to evolve. Efficacy has been shown in published papers for the following common pathologies, however laser is not currently the first line treatment for any of these conditions and efficacy can often be disappointing.
- Central Serous Retinopathy
- Macular edema from retinal vein occlusions
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