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Editors Selection IGR 9-4

Surgical Therapy: Increased outflow facility after SLT

Comment by Arthur Sit on:

27539 Effect of primary selective laser trabeculoplasty on tonographic outflow facility: a randomised clinical trial, Goyal S; Beltran-Agullo L; Rashid S et al., The British journal of ophthalmology, 2010; 94: 1443-1447

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Selective laser trabeculoplasty (SLT) has been widely adopted as a treatment for open-angle glaucoma. While the reduction in IOP is generally similar to argon laser trabeculoplasty (ALT), the histological damage caused to the trabecular meshwork are far less.¹ Combined with a larger spot size, making treatment easier, and the potential for improved repeatability, ² SLT has largely displaced ALT as the laser therapy of choice for glaucoma. Despite the differences in morphological effects, a general assumption has been that the mechanism of action is similar to ALT, namely the reduction of outflow resistance. However, very few studies have investigated the changes in aqueous humor dynamics following SLT treatment.

Goyal et al. (1970) measured the effect of SLT on outflow facility in untreated glaucoma patients by using constant weight Schiøtz tonography. In addition, the authors randomized patients to treatment over 180°vs 360°. They found that both the treatment groups had significant reduction in IOP and significant increase in outflow facility, with no significant differences between the two groups. What is particularly interesting is that the change in facility was insufficient to explain the reduction in IOP after SLT.

The change in facility was insufficient to explain the reduction in IOP after SLT

As the authors indicate, an increase in facility of 100% would be required to produce the observed IOP change if all other factors remained the same, in contrast to the 37.5% measured. The measured increase in facility was smaller than typically reported for ALT studies.³ This suggests that other changes may be occurring as a result of SLT treatment, including an increase uveoscleral outflow, decrease in episcleral venous pressure or decrease in aqueous production rate. One potential limitation of the study is that the post-procedure assessment was performed only one month after the SLT treatment. Low grade inflammation is common after the procedure. Although inflammation as measured by aqueous flare returns to baseline within 1 week,⁴ it is not clear if transient effects on aqueous humor dynamics last longer. It is possible that over longer periods of time, outflow facility improves further while other parameters return to baseline. A longer term follow-up and assessment of aqueous humor dynamics in these patients would be useful. Nevertheless, this paper highlights the need to better understand the mechanism of action of SLT.

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