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Argon laser trabeculoplasty (ALT) is a well established therapy for lowering intraocular pressure (IOP) in eyes with chronic open angle glaucoma. 1 Whether the exact mechanism of action of ALT is purely mechanical, 2 3 cellular, 4-6 or a combination of both it is widely accepted that ALT causes an enhancement of egress of fluid via the trabecular outflow system resulting in lowered IOP. However, histopathological examination of laser treated meshwork samples obtained from human subjects revealed early necrosis of cells and disruption of trabecular beams. A shrinkage of certain collagenous elements was also associated to treatment. 6-11 Thus, ALT is fundamentally destructive to trabecular meshwork as it focally destroys and alters the biological activity of trabecular cells temporarily ameliorating IOP. ALT uses continuous wave lasers with pulse duration in the millisecond range or greater. With such pulse duration heat is transmitted from pigmented targeted cells to adjacent tissues and subsequent damage of non-pigmented cells and trabecular beams occurs.
Recently Latina and Park 12 used a 532 nm Q-switched Nd:YAG laser to selectively target pigmented trabecular meshwork cells in vitro with low threshold radiant exposures without producing collateral damage to adjacent non-pigmented cells or structures. This laser gives a single pulse of short duration (in the microsecond range) and low fluence (energy/area). At these values, unlike in ALT, thermal diffusion from targeted cells to surrounding non-pigmented trabecular meshwork cells is minimised; therefore, the technique has been named selective laser trabeculoplasty (SLT). A histological study in human cadaver eyes treated with SLT or ALT within 18 hours of death confirmed that selective treatment appears to cause no coagulation damage to the trabecular meshwork and less structural damage than ALT. 13
Following the results of the study by Latina and Park 12 the clinical application of a Q-switched Nd:YAG laser for SLT was proposed as a new laser treatment for chronic open angle glaucoma. 14 The possibility of avoiding relevant thermal or coagulative damage in the trabecular meshwork with SLT represents a major improvement over conventional ALT. As a consequence SLT might be applied many times in the same eye without determining coagulative scarring that focally reduces or prevents aqueous outflow. This study presents preliminary results on the immediate IOP response to SLT in eight eyes...