Selective Laser Trabeculoplasty (SLT) has been used for over 10 years to lower intraocular pressure (IOP) in patients with open angle glaucoma (OAG) as well as ocular hypertension (OHT). Studies have demonstrated that SLT is a safe and effective means of treatment, and comparable in these respects to Argon Laser Trabeculoplasty (ALT), but how does SLT fare against ALT in the long-term? It was our mission to establish this.

As part of our randomized, controlled trial, which compared SLT with ALT in OAG patients, including pseudoexfoliation and pigmentary glaucoma patients, we wanted to evaluate outcomes over five years.¹

The study begins

There were a total of 176 eyes enrolled in the study and, of those, 120 eyes had reached the five-year time point at the time of analysis. Patients included those who required additional pressure lowering and were already on maximal medical therapy or had previously received argon laser treatment. Patients with advanced visual field defects, those who had undergone previous glaucoma surgery, those on systemic steroids, or those with corneal disease where applanation would have been difficult, were excluded.

Patients were randomly divided into the ALT (87 eyes) and SLT (89 eyes) groups and treated with 50 laser applications over 180° of the trabecular meshwork using standard laser settings (average energy used per spot was 0.8 mJ for SLT and 550 mW for ALT).

Treatment success was defined in two ways: definition 1: IOP reduction of 20% or more with no additional medical, laser or surgical interventions, or; definition 2: IOP reduction of ≥3 mmHg with no additional medical, laser or surgical interventions. Five years post-treatment, data were available for 56 of the ALT-treated eyes and 64 of the SLT-treated eyes.

And the results showed...

Table 1: Mean IOP over time.

The baseline parameters were similar between the two groups; mean baseline IOP was 23.8±4.9 mmHg for the SLT group and 23.5±4.2 mmHg for the ALT group. Five years post-treatment, the mean IOP decrease for the SLT group was 7.4±7.3 mmHg and 6.7±6.6 mmHg for the ALT group. There was no statistically significant difference in IOP reduction over time between the two lasers (Table 1).

Table 2: Additional interventions required during follow-up.

Our data includes pressure changes in patients receiving medication switches or additions, additional laser treatments, and surgical procedures, as would be expected in any group of moderately advanced glaucoma patients over a five-year time period. Therefore, the pressure decrease over time cannot be solely attributed to the laser treatment. Table 2 shows the additional interventional procedures required during follow-up.

Figure 1: Survival over five years: definition 1.

The Kaplan-Meier survival curves (Figures 1 & 2) indicated no significant difference between SLT and ALT over time (p=0.7146 for definition 1 and p=0.6998 for definition 2). According to definition 1, the survival of SLT at five years was 25% and for ALT 27%. When using definition 2 however, the survival was 21% and 32% for SLT and ALT, respectively. The median survival (50% success) time for each group was two years, using either definition of success.

Figure 2: Survival over five years: definition 2.

There are a number of provisos that need to be kept in mind when interpreting our results. The primary outcome of our randomized controlled trial was IOP lowering at one year, i.e. the primary objective was not to follow patients for five years. In addition, our patient population represents those with varying stages of glaucomatous damage who had uncontrolled IOP on maximum tolerated medical therapy and needed another intervention. Hence, the results cannot be generalized to first-line or early adjunctive treatment with either laser. Finally, we did not have enough patients in the pseudoexfoliative or pigmentary glaucoma groups to draw meaningful conclusions about efficacy and safety in these populations. Further study is required to determine which laser is better in both these types of glaucoma.

No difference between the two

In conclusion, we feel that SLT is equally effective as ALT in IOP lowering over five years. The IOP measurements for both the SLT and ALT groups were similar at baseline and at five years and, in terms of success, the two lasers were comparable and had a median effect of two years, using either definition of success. It should be noted that additional IOP lowering medical, laser and surgical interventions were required over the five-year follow-up time, and these were similar in the ALT and SLT groups.

Overall, we can conclude that both SLT and ALT are effective methods of IOP lowering over time in this group of OAG patients with uncontrolled IOP.

Acknowledgements

We wish to thank the patients who participated in this study as well as our colleagues who contributed over the years: William G. Hodge, MD, FRCSC, PhD, William J. Rock, MD, Ralf Buhrmann, MD, FRCSC, PhD, Irene Y. Pan, MSc.

Reference

1. K.F. Damji, et al. Br. J. Ophthalmol. 2006;90(12):1490-1494.