Energy Dose-Response in Selective Laser Trabeculoplasty: A Review

Abstract

Précis: A literature review of selective laser trabeculoplasty (SLT) energy dose-response found no definitive relationship between intraocular pressure (IOP) reduction with respect to total or pulse energy, race, pigmentation, or application pattern. Purpose: SLT is a safe and effective treatment for lowering IOP. Although evidence is mounting for the advantage of its use as a first-line treatment for IOP reduction, the SLT procedures in use vary widely. The purpose of this literature review was to investigate whether there were any relationships between SLT energy and efficacy for lowering IOP in the published literature. Methods: A literature review was undertaken that included studies in which energy levels required for successful SLT treatment were investigated: in general, with respect to angle pigmentation, race or ethnicity, and treatment arc extent. Results: There was no indication that higher (or lower) energy used in the treatment leads to greater (or less) IOP reduction. Similar results were obtained regarding the level of trabecular meshwork pigmentation. Race was not found to be associated with altered dose response in SLT. There were indications that treating the full 360 degrees, as opposed to smaller arcs, could be beneficial for more IOP reduction. IOP reduction from SLT was found to be similar to that provided by topical medications. Conclusions: The optimal energy level of SLT needed for IOP reduction has not yet been definitively established, with all reported pulse energies resulting in similar IOP reduction. Furthermore, similar lack of conclusive findings exists regarding optimal SLT energy dosage for use in different races and degrees of trabecular meshwork pigmentation. This parameter and each of the abovementioned factors requires further research.

A Proposal for the Use of a Fixed Low-Energy Selective Laser Trabeculoplasty for Open Angle Glaucoma

“…Interestingly, the laser intensity required to show a visible change in the cells occurred at levels 3 to 4 times lower than the authors expected, 27 corresponding to an SLT energy device setting of 0.2 mJ using the mentioned conversion ratios. This comparison supports the concept of the effectiveness of SLT using low energies that have been observed in SLT studies, assuming a correspondence between in vitro and in vivo cellular studies 12 …”

“…This was perhaps reinforced by the observation that melanin granules can be easily seen in histology. However, no correlation between TM pigmentation and IOP reduction has been demonstrated 12 . Furthermore, prospective randomized controlled trials have not been associated with any serious adverse events, and the concern is that post-SLT pressure spikes in heavily pigmented TMs might result in severe long-term side effects.…”

“…The IOP lowering effects of SLT treatments seem to be insensitive to the overall intensity in an SLT laser spot (as evidenced by the results of clinical trials 12 ), insensitive to the variations between commercially available (nanosecond) SLT laser beam properties, and insensitive to variations caused by beam delivery and inherent fluctuations in the laser. In addition, champagne bubble formation has not been shown to be related to IOP-lowering results 21 …”

“…To date, there has been no proven relationship between SLT effectiveness and the laser pulse energy 12 or a relationship between cavitation bubbles and tissue changes 17,21 . Therefore, attempts to precisely adjust the laser energy may not have a clinically significant benefit 12 …”

“…Using lower energies may decrease cumulative tissue damage, which may allow for safer repeat treatments, such as those under investigation in a large trial studying the efficacy of annual re-treatment 7 . A recent SLT review has shown no relation between energy and IOP pressure reduction (IOP energy dose-response), with effectiveness demonstrated even at the lowest energies used (0.2–0.3 mJ/pulse) 12 . Low pulse energies may be desirable, which is a significant study objective of the ongoing COAST trial 7 …”