How to predict outcomes for SLT and ALT

IOP measurements key in decoding the formula for SLT and ALT success.

By Cindy M.L. Hutnik, MD, PhD

The introduction of alternative therapies can cause both excitement and concern. These therapies often create a number of questions for clinicians who contemplate whether they should be offered to patients, as well as the optimal way to provide them. Questions of efficacy, safety and cost arise. Predicting which patients may benefit most prior to offering the options creates both reassurance and understanding.

A patient prepares to undergo the SLT procedure. The greatest predictive factor of SLT success is pre-treatment IOP.
COURTESY OF GLAUCOMA RESEARCH FOUNDATION/LUMENIS

Further understanding the implications of both early and late adoption of alternative therapies is important. The following article outlines factors that predict the success of laser trabeculoplasty in the management of glaucoma and ocular hypertension.

SLT VS. ALT

ALT

Laser trabeculoplasty is an IOP-lowering modality first reported more than 30 years ago.¹ The landmark Glaucoma Laser Trial demonstrated that argon laser trabeculoplasty (ALT) was at least as effective as first-line medical therapy in the initial treatment of primary open angle glaucoma.^2,3

Despite the demonstrated efficacy, ALT was never adopted as a primary therapy and its position after maximally tolerated medical therapy was firmly entrenched in the traditional treatment paradigm in which topical eye drops always proceeded laser trabeculoplasty.

SLT

Nineteen years ago, selective laser trabeculoplasty (SLT) was introduced by Latina and Park.⁴ In contrast to ALT, evidence shows SLT selectively targets melanin-containing cells with a mode of energy delivery that does not cause extensive cytological or histological damage of the trabecular meshwork.⁵ The research provides a theoretical basis to suggest SLT has a greater safety profile than ALT. Further clinical research has demonstrated SLT’s efficacy, the optimal position in the treatment paradigm as first line and predictors of success.

When placed in comparable positions in the treatment paradigm, clinical data demonstrate equivalent efficacy of ALT and SLT.^6-8 Baseline IOP is the greatest predictor of ALT success.⁹ Similar findings have been reported in logistic multivariate regression models that support these conclusions. In addition, the findings provide clinicians with the numerical likelihood of success with SLT, known as prediction rule analyses, which physicians can use to discuss treatment-option discussions with patients.

These studies estimate the probability of success of a procedure using a detailed assessment of patient factors.

PREDICTIVE FACTORS OF SUCCESS

Pre-treatment IOP measurements

Two prediction rule studies allowed the development of a matrix that provides estimates of SLT success based on maximum recorded IOP and IOP just prior to SLT treatment.^10-11 In the studies, a reduction of IOP ≥ 20% from baseline was defined as treatment success. Patients with ocular hypertension, open-angle glaucoma or normal tension glaucoma were included in these studies. Several variables were analyzed including gender, age, diagnosis, pigment of anterior chamber, washout of eye drops, previous ALT, post-SLT drops, pre-SLT IOP at least four weeks prior to SLT treatment, pre-SLT maximum IOP (highest IOP ever recorded for the patient) and SLT energy. The most important factor predicting SLT success was IOP prior to SLT treatment. These findings are key in understanding the optimal positioning of SLT in the treatment paradigm.

Primary therapy

SLT works best as primary therapy, studies show.^12-14 The greatest likelihood of SLT success (20-30% IOP reduction from baseline) occurs when the historical maximum IOP is the pre-SLT IOP in a treatment naïve eye. If, however, the IOP just prior to SLT treatment is lower than the maximum IOP, SLT success will be blunted. This is often the case when SLT is added as an adjunctive therapy to a patient who has already been started on pressure-lowering eye drops. The laser behavior mirrors what is seen when adjunctive eye drops are added to a patient who is already on first- or second-line IOP lowering medications or both.

Several other studies have also demonstrated the importance of elevated IOP prior to SLT. One found that a higher baseline IOP was a predictor of greater IOP response post-SLT.¹⁵ Other factors that were tested, such as age, angle pigmentation, phakic status, gender, or type of glaucoma did not have any effect on SLT success. Similarly, other groups found that high baseline IOP was the greatest predictor of success with SLT.^16,17

SECONDARY PREDICTORS

Influence of previous trabeculoplasty

The repeatability of laser trabeculoplasty is another question not yet convincingly answered. Data are suggestive that ALT should not be repeated in eyes that have already received 360 degrees of ALT. In fact, ALT mediated photocoagulation has been reported as a method to experimentally increase IOP in animal models.¹⁹ A few studies exist to suggest that this is not the case for SLT. SLT has been shown to produce clinically useful decreases in IOP after 360 degrees of either ALT20 or SLT13. Currently there is a large, prospective, multi-center Canadian study in progress that is examining the repeatability of ALT and SLT after 360 degrees of SLT. Hopefully, this study will provide definitive data on the issue.

The SLT spots (400 μm) can be placed in the area of the meshwork while the ALT spots (only 50 μm) must be precisely placed to avoid peripheral anterior synechia.
COURTESY OF CINDY HUTNIK, MD, PHD

Early treatment

The greatest predictive factor of SLT success is pre-treatment IOP. As highest IOP levels are found early in the treatment paradigm, optimal positioning of SLT is as a first-line therapy. SLT is also effective as an adjunctive therapy, but its effectiveness decreases when introduced later in the treatment paradigm. In this latter role and position, the main utility of SLT is to decrease the patient’s dependency on topical medications, particularly when intolerance or compliance are at issue. Many studies have shown comparable efficacy of SLT and ALT. However, the greater risks of ALT likely associated with thermal tissue effects have resulted in a different positioning of ALT and SLT in the management of glaucoma.

Laser trabeculoplasty as an alternative to medical therapy continues to be of significant interest as medical therapeutics transition into a predominantly generic market. The greatest influence on success will be the comfort level of the clinician to use SLT early in the treatment paradigm, rather than an extensive analysis of patient specific factors. OM

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About the Author
	Cindy Hutnik, MD, PhD, is a professor at the Schulich School of Medicine and Dentistry. She is Chair of Research and Director of the Ophthalmology Basic Science Laboratory at the Lawson Health Research Institute in the Center for Clinical Investigation and Therapeutics. Disclosure: Dr. Hutnik has no relevant conflicts to disclose.