Laser Dosage in MicroPulse TransScleral CycloPhotocoagulation

Glaucoma is a condition where an increase in the accumulation of aqueous humor inside the anterior chamber of the eye results in raised intraocular pressure (IOP), visual field defects and cupping of optic disc. Typically, patients who have glaucoma are treated with topical medications or incisional surgery to lower the IOP. Micropulse transscleral cyclophotocoagulation (MP-TSCPC) is a non-incisional laser surgery used in the treatment of glaucoma. MP-TSCPC powered by the Cyclo G6 (Iridex Corporation, Mountain View, CA, USA) causes mild thermal damage to the ciliary body (gland that produces aqueous fluid in the eye), which, in turn, causes a decrease in intraocular pressure. MP-TSCPC is a variation of a longstanding older treatment for glaucoma, traditional diode TSCPC, which caused a significant amount of tissue damage and therefore was reserved for the treatment of end stage glaucoma. The micropulse laser application includes an on-off cycle. In an on cycle, the micropulse probe administers a short pulse of laser energy followed by an off cycle which is a rest period for the tissue to cool off before the next on-off cycle begins. This corresponds to 0.5 ms duration of "on-time" and 1.1 ms interval of "off-time" during the laser application. This on-off cycle allows for the laser to treat the affected area without evidence of tissue damage, as opposed to the traditional diode TSCPC. Because only 31.3% of the total laser energy is applied to the ciliary body, there is a less tissue damage, less post-operative inflammation, and decreased complication rates. Therefore, MP-TSCPC is currently being utilized for milder cases of glaucoma, and in patients with viable vision. The laser probe is usually applied perpendicular to the limbus in a continuous sweeping motion, for an average of 10 seconds of sweeping time per hemifield, referred to as "dwell time". Therefore, the three parameters that can be varied in the treatment with MP-TSCPC are total laser duration (seconds), laser power (mW) and dwell time (seconds/hemifield). Most of the studies used a laser power of 2000 - 2500 mW. The laser duration also has varied between studies. A duration time of 50 to 180 sec per hemisphere was used in different studies. Most studies have utilized a dwell time of 10 seconds per hemifield, and one cited an application of a "stop and go" method, applying the laser in discrete spots at 10 seconds each. In all of the prior studies, despite the variable applications of power, dwell time, and duration, there have been similar rates of complications such as vision loss, macular edema, and hypotony. Generally, the laser duration is tailor-made and is adjusted based on the iris color and severity of glaucoma. However, there is a gap in knowledge on systematic comparison of different doses of MP-TSCPC for a safe and effective treatment of glaucoma. This study will systematically evaluate the clinical outcome of glaucoma surgery based on a logical variation of MP-TSCPC laser dosing parameters.