Abstract
Aim
Determine if selective laser trabeculoplasty (SLT) affects the fellow eye's intraocular pressure (IOP).
Materials and methods
A retrospective review of 76 pairs of eyes from 76 adult glaucoma patients who underwent SLT in one eye with at least 2 months between treatments were evaluated for IOP and medication reduction in the untreated fellow eye. Success was defined as ≥20% IOP reduction or ≥1 medication reduction without any additional IOP lowering procedures or medication. The primary outcome measures were success, IOP, and medication reduction in the untreated fellow eye at 6 months.
Results
At 6 months after SLT treatment, 48.7% (38/76) treated eyes and 36.8% (28/76) untreated fellow eyes met success criteria. IOP reduction in the treated eye was 2.6 ± 5.8 (14.1%; p < 0.002) and 0.8 ± 4.3 (5.1%, p = 0.122) in the fellow eye. The fellow eye was significantly more likely to meet success criteria if the treated eye was successful [odds ratio (OR): 6.00, 95% confidence interval (CI) (2.11–17.06), p < 0.002].
Conclusion
After a unilateral treatment with SLT, over one-third of the fellow eyes experienced either ≥20% IOP reduction or medication reduction. Additionally, fellow eyes were six times as likely to meet success criteria if this was observed in the treated eye. These findings may support the proposed biochemical mechanism for the therapeutic action of SLT.
Clinical significance
The implication for clinicians is that SLT treatment in one eye may allow the fellow eye to benefit and provide a prediction on the fellow eye's response without subjecting both eyes to the rare but present complications of SLT.
How to cite this article
Hirabayashi M, Mellencamp E, Duong S, et al. Effect of Selective Laser Trabeculoplasty on the Fellow Eye. J Curr Glaucoma Pract 2023;17(4):175–177.
Keywords: Fellow eye, Retrospective chart review, Selective laser trabeculoplasty
Introduction
Glaucoma persists as the leading cause of global irreversible blindness and has been projected to increase in prevalence to 111.8 million people in 2040.1 Laser trabecu1loplasty (LT) has existed since the 1970s with argon laser trabeculoplasty (ALT) representing its first incarnation.2 Technological developments have allowed the application of laser energy to the angle to progress through two subsequent iterations: selective laser trabeculoplasty (SLT) and micropulse laser trabeculoplasty.3–5 Since 1995, SLT has allowed the advantage of equal efficacy to ALT while minimizing damage to the trabecular meshwork (TM) through selective targeting of the pigmented cells by its frequency-doubled short pulse (Q-switched) neodymium-doped yttrium aluminum garnet laser.6 LT plays an important role in the treatment of glaucoma as the American Academy of Ophthalmology proposes it as a first-line treatment for open-angle glaucoma and ocular hypertension, and the recent LiGHT trial has affirmed its value in treatment-naïve eyes for a wide variety of glaucoma types.7–9 The mechanism of action for this therapy, though, is still poorly understood. Since SLT causes no structural damage to the TM while having equal or better efficacy than ALT, the mechanism is thought to be biochemical rather than structural.10 The process of selective photothermolysis results in trabecular and endothelial cell proliferation, cytokine release, and macrophage recruitment (inflammation) in the angle. The net effect is an improved aqueous outflow and reduction of intraocular pressure (IOP). The cytokines released by this process improve transendothelial flow in Schlemm's canal and induce remodeling of the TM's extracellular matrix through metalloproteinase expression.11 Since the mechanism is through these cellular modulators, it is possible for a “crossover effect” in the fellow, untreated eye, reducing IOP through similar mechanisms. Some have proposed the presence of “soluble mediators of IOP” that may access the fellow eye through systemic circulation.
Other studies have observed this phenomenon with reported IOP reductions of around 10%.12,13 In this study, we sought to determine the magnitude of IOP or medication burden reduction of SLT in the fellow eye. We hypothesized that SLT would result in a significant reduction in IOP and medication burden in the untreated fellow eye, with a possible relationship between successful responses between the two.
Materials and Methods
We performed a retrospective chart review of 76 pairs of eyes from 76 glaucoma patients at the University of Missouri who received SLT between 8th May 2016 and 12th January 2017 and had a 6-month follow-up. Cases where treatment was received in both eyes within 6 months were excluded. Patients were evaluated for IOP and the number of medication classes in the fellow eye before and after treatment at the 6-month follow-up. We obtained approval from the Institutional Review Board (IRB) of the University of Missouri (IRB#2040322). The study adhered to all tenets of the Declaration of Helsinki. Our primary outcome measure was surgical success, defined by a >20% reduction in IOP or any reduction in medication classes. Secondary outcome measures included IOP reduction in mm Hg and reduction in medication in the untreated fellow eye at 6 months following treatment, as well as the relationship between success in the treated and fellow eye. We also recorded demographics, baseline data, adverse effects, and additional glaucoma interventions required at the discretion of the treating physician.
Data was collected for both eyes on the day of but prior to SLT and 2 and 6 months after SLT. The data was analyzed using paired t-tests to compare mean baseline 6-month IOPs for the fellow eye and a Wilcoxon signed-rank test to compare baseline and 6-month medication classes. All statistical tests were two-tailed with α = 0.05. The values are reported as mean ± standard deviation (SD) for the means and percentages with proportion of population for categorical variables unless specified otherwise.
Intraocular pressure (IOP) was assessed by Goldmann applanation by an ophthalmologist, and medications were reduced at the discretion of the treating physician based on clinical and patient factors.
Results
Baseline demographics are reported in Table 1. Of note, the treated eyes had significantly higher baseline IOP and were on significantly more medication than the fellow eyes owing to treatment of the more therapeutically resistant eye first.
Table 1:
Baseline demographic and glaucoma status data
| Subject-level parameters | n = 74 | p-value |
|---|---|---|
| Age (year), mean (SD) | 68.7 ± 10.1 | |
| Gender, % (n) | ||
| Male | 55.4 (41) | |
| Female | 44.6 (33) | |
| Ethnicity, % (n) | ||
| Caucasian | 77.0 (57) | |
| African-American | 17.6 (13) | |
| Other* | 5.4 (4) | |
| Eye-level parameters | n = 76 | |
| Glaucoma diagnosis, % (n) | ||
| Primary open-angle/normal tension glaucoma | 80.3 (61) | |
| Pseudoexfoliation | 5.3 (4) | |
| Others* | 14.4 (11) | |
| Glaucoma severity†, % (n) | ||
| Mild | 43.4 (33) | |
| Moderate | 19.7 (15) | |
| Severe | 36.9 (28) | |
| Preoperative (pre-op) IOP treated eye (mm Hg), mean ± SD | 18.4 ± 4.8 | <0.002 |
| Pre-op IOP fellow eye (mm Hg), mean ± SD | 15.5 ± 4.2 | |
| Pre-op medications treated eye (mm Hg), mean ± SD | 2.0 ± 1.3 | <0.002 |
| Pre-op medications fellow eye (mm Hg), mean ± SD | 1.3 ± 1.3 |
*Others, combined mechanism, traumatic, steroid response, and angle recession; †glaucoma severity based on HVF 24-2 SITA standard mean deviation
Success
At 6 months after SLT treatment, 48.7% (38/76) treated eyes and 36.8% (28/76) untreated fellow eyes met success criteria (Table 2).
Table 2:
The 6-month success in the treated and fellow eye
| Success | Failure | |
|---|---|---|
| SLT treated eye | 37 (48.7%) | 39 (51.3%) |
| SLT fellow eye | 28 (36.8%) | 48 (63.2%) |
Table 3 illustrates how many cases in the treated and fellow eyes met the success criteria by IOP reduction vs medication reduction.
Table 3:
Cases meeting success criteria by IOP reduction, medication class reduction, or both
| IOP reduction | Medication reduction | |
|---|---|---|
| Successfully treated eyes | 32/37 (86.5%) | 12/37 (32.4%) |
| Successful fellow eyes | 23/28 (82.1%) | 10/28 (35.7%) |
Intraocular Pressure (IOP) and Medication Reduction
Intraocular pressure (IOP) reduction of the treated eyes was 2.6 ± 5.8 (14.1%; p < 0.002), while that of the fellow eyes was 0.8 ± 4.3 (5.1%, p = 0.122). Medication reduction was not statistically significant for either group (Table 4).
Table 4:
IOP and medication reduction of the fellow eye 6 months after SLT
| Treated eye | Fellow eye | |
|---|---|---|
| 6-month | 6-month | |
| IOP reduction, mean ± SD (%) | 2.6 ± 5.8 (14.1%) | 0.8 ± 4.3 (5.1%) |
| p-value | p < 0.002* | p = 0.218 |
| Medication reduction mean ± SD (%) | −0.2 ± 0.9 (−1.3%) | 0.5 ± 0.9 (3.8%) |
| p-value | p = 0.562 | p = 0.912 |
*Statistically significant when p-value < 0.05
Relationship between Success in the Treated vs Fellow Eye
We found the fellow eye was more likely to meet success criteria if the treated eye was successful [OR: 6.00, 95% confidence interval (CI) (2.11–17.06), p < 0.002] (Table 5).
Table 5:
Success of treated eye compared to fellow eye at 6 months
| Treated eye success | Treated eye failure | |
|---|---|---|
| Fellow eye success | 21 | 7 |
| Fellow eye failure | 16 | 32 |
| p | <0.002* | |
*Statistically significant when p-value < 0.05
Discussion
In this study, we have found that about a third of the fellow untreated eyes responded with some degree of IOP lowering following a unilateral SLT treatment. This, coupled with our finding that fellow eyes were 6 times as likely to meet success criteria if the treated eye met one, provides support for the hypothesis that the effect of laser trabeculoplasty may be the result of more than just physical changes to the TM.
If the treated eye had a positive response to SLT, it is reasonable that the hypothesized “soluble mediator of IOP” in the form of cytokines or some other biochemical modulator would enter the systemic circulation and eventually reach the TM of the fellow eye where it would exert an effect on IOP there.12 This observation is not isolated to laser trabeculoplasty alone. There are reports of IOP reduction and increased aqueous flow in the fellow eye following trabeculectomy, likely due to systemic inflammatory mediators leading to ciliary body shutdown.14 Alternatively, there are also reports of a significant IOP increase in the fellow eye after trabeculectomy.15 In the case of trabeculectomy, both IOP effects may support the role of a possible systemic modulator for IOP just as a response following SLT doses. The modest 4.3% reduction in IOP in the fellow eye is congruent with current literature ranges of 6–20%.16–18
As illustrated above, the fellow eye largely met success criteria by IOP reduction, not medication reduction, which further supports the true effect of IOP lowering experienced in the fellow eye independent of medication reduction.
The major limitation of this study includes a retrospective review of a limited sample of patients. Most of the study patients were of the Caucasian ethnicity with primary open-angle glaucoma, limiting its generalizability. The findings of this study will need to be confirmed by larger prospective studies in the future.
Conclusion
Based on this retrospective study, SLT has a true impact on lowering IOP in the fellow eye. However, clinicians should be aware that treatment with SLT in one eye may not always result in commensurate levels of IOP reduction in the fellow eye. Nevertheless, the fellow eye may still have a high likelihood of having some degree of IOP reduction and more likely so if the treated eye has a successful outcome.
Clinical Significance
The implication for clinicians is that SLT treatment in one eye may both allow the fellow eye to benefit and also provide a prediction on the fellow eye's response without subjecting both eyes to the rare but present complications of SLT.
Footnotes
Source of support: Nil
Conflict of interest: None
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