Subthreshold Micropulse Laser Therapy for Retinal Disorders

Keywords:medical, laser, fiber,  Time:22-03-2016

Since its inception, retinal photocoagulation has become more refined, effective, and safe. It has become the first line of treatment for numerous chorioretinal disorders, and its efficacy has been validated by many clinical studies. Laser photocoagulation may produce its therapeutic effects by destroying oxygen-consuming photoreceptor cells and retinal pigment epithelium (RPE), thereby reducing the hypoxic state of the retina. This concept is increasingly being challenged as therapeutic agents such as steroids and anti-vascular endothelial growth factor (anti-VEGF) agents can reduce edema without destroying photoreceptors.

The heat from conventional laser therapy is conducted to surrounding structures such as the neurosensory retina and the choroid, which can result in collateral thermal damage. The “grayish” endpoint of a conventional retinal laser burn signifies that the thermal spread has reached the overlying neurosensory retina with a temperature high enough to damage the natural transparency of the retina. This blanching is typically associated with a temperature rise of 20°to 30°C above baseline body temperature.

Complications that have been associated with the use of conventional lasers for retinal photocoagulation include reductions in visual acuity,1,2 visual field,3,4 color vision,2,3,5,6 night vision,7-9 and contrast sensitivity.5,10 Other complications include choroidal neovascularization (CNV), hemorrhage, epiretinal fibrosis, and serous detachment of the peripheral retina.11 It has, however, been suggested that full thickness retinal damage may not be needed to obtain beneficial effects from laser.12 The benefits might be due to the upand down-regulation of angiogenic growth factors (eg, VEGF)13-16 mediated by the biological reaction of RPE cells that have been only sublethally injured. The RPE plays a significant role in repairing the outer and inner blood-retinal barrier, regardless of the type or location of the laser application. Photothermal elevation that does not produce visible intraretinal damage during or after laser treatment may be termed subthreshold laser treatment.

Emerging evidence suggests that subthreshold laser treatment may be as effective as conventional laser treatment but with less iatrogenic damage to the tissues surrounding the area of the burn in the RPE.17-22 Various optical and thermodynamic principles can be applied to minimize retinal damage.23 Modifying laser parameter eg, decreasing wavelength, spot size, retinal irradiance, and pulse duration—may help limit retinal damage. Changing the clinical endpoint from a visible laser burn to an invisible subthreshold application, achieved with micropulse laser treatment, may also reduce retinal damage. The absence of a visible burn means that fluorescein angiography or indocyanine green might be required to detect lesions, and some lesions may not be detectable at all postoperatively.

Using a micropulse mode, laser energy is delivered with a train of repetitive short pulses (typically 100 to 300 microseconds “on” and 1700 to 1900 microseconds “off”) within an “envelope” whose width is typically 200 to 300 milliseconds.Micropulse power as low as 10% to 25% of the visible threshold power has been demonstrated to be sufficient to show consistent RPE-confined photothermal effect with sparing of the neurosensory retina on light and electron microscopy.25 Tissue-sparing protocols are designed to produce only subtle thermal elevations with effects that are invisible during treatment and remain so thereafter. The inner retinal temperature must remain below the threshold of coagulative damage for the retina to maintain its natural transparency. Instead of delivering the requisite energy with a single, high peak power pulse, a series of repetitive, low-energy pulses are used. A four- to tenfold reduction in the required energy per pulse has been experimentally found with repetitive microsecond pulses, or micropulses, when passing from the visible damage threshold to subthreshold (invisible) damage levels, detectable only with microscopy in histology. Lower energy per pulse reduces peak power, lowers the risk of hemorrhage, decreases the temperature buildup per pulse, and ultimately results in improved confinement of photothermal effects.12 Absence of chorioretinal laser damage may permit high-density therapy with confluent applications over the entire edematous area and retreatment of the same areas. This may be particularly useful for the as-needed treatment of macular edema.