In a study that was just published, we applied a vigorous, automated statistical matching method, called coarsened exact matching, or CEM, to create a highly balanced comparison. This allowed us to delineate any effect of phacoemulsification on the IOP reduction of NeoMedix Trabectome surgery, a procedure that is performed at our center at a high volume. We found that the IOP decrease is equally good in patients with or without same-session cataract removal.
The background of this study is that patients with glaucoma commonly have cataracts or will develop them twice as fast as the normal population. Cataract surgery itself can lower IOP by 1.5 mm Hg to 3 mm Hg, possibly due to an ultrasound-mediated trabeculoplasty-like effect on the trabecular meshwork. This effect may occasionally be pronounced: In the first randomized controlled trial that assessed the iStent trabecular micro-bypass (Glaukos) at the time of cataract surgery compared with controls with cataract surgery only, there was not a significant difference in IOP between the groups. Unfortunately, it has long been known that IOP reduction from cataract surgery is not reliable and can instead lead to dangerously high postoperative spikes in up to 69% of patients. This can be prevented with same-session Trabectome surgery.
In our previous studies, we had already not observed significant IOP differences between Trabectome alone vs. phaco-Trabectome at 1 year after surgery, but this might have been an effect of group differences because no matching strategies were used. Here, we hypothesized that CEM would be able to remove the effects of confounding variables in these patient groups to make a valid comparison.
We evaluated phakic patients who received Trabectome-mediated ab interno trabeculectomy vs. combined phacoemulsification with Trabectome. Inclusion criteria included a diagnosis of glaucoma with or without a visually significant cataract, at least 12 months of follow-up and no secondary glaucoma surgeries. Subjects who were pseudophakic or had neovascular glaucoma were excluded. Multiple imputation allowed incorporation of data sets with missing values while preserving existing relationships within the data.
From the 1-year results of 753 total cases, every 1 mm Hg increase in baseline IOP correlated to a further 0.80 ± 0.02 mm Hg IOP reduction after surgery. Patients with secondary open-angle glaucoma were found to have an additional IOP reduction of 1.21 ± 0.38 mm Hg more than patients with primary open-angle glaucoma. The 255 patients in the Trabectome-only cohort had a 21% IOP reduction from 21.7 ± 7.0 mm Hg to 15.9 ± 3.5 mm Hg (P < .01), while the 498 combined phacoemulsification and Trabectome patients had an 18% IOP reduction from 19.7 ± 5.8 mm Hg to 15.5 ± 3.6 mm Hg (P < .01). Phacoemulsification was not found to significantly impact IOP when combined with the Trabectome. The number of glaucoma medications also decreased similarly in both groups (P < .01) from 2.4 ± 1.2 to 1.9 ± 1.3 and 2.3 ± 1.1 to 1.7 ± 1.3 mm Hg, respectively.
Several studies have shown that cataract surgery combined with trabecular micro-bypass stents have synergistic effects on IOP reduction. Our results indicate that the up to 3 mm Hg IOP reduction sometimes seen after cataract surgery is most likely attributable to the trabecular meshwork, presumably due to stretch or ultrasound exposure. Ablation of this structure is able to provide a maximal pressure-lowering effect regardless of whether the lens is removed.
This study also highlights the utility of multiple imputation and CEM, a relatively new statistical generalized matching method. Randomized controlled trials are a pure form to test a hypothesis, but they have been criticized for creating a biased environment due to patient selection. In contrast, CEM uses real-world data without avoiding the elimination of data with missing values. This method prunes data to achieve a better balance between treated and control groups, and control the confounding influence of pretreatment variables, thereby allowing a valid comparison of observational data that would otherwise be difficult to match.
The UPMC Eye Center is focused on providing the best evidence-based eye care for its patients while providing a platform of excellent training to the next generation of ophthalmologists. We present data here demonstrating that cataract surgery does not augment the IOP reduction that results from Trabectome surgery. Our current research has now allowed us to develop an effective microincisional glaucoma surgery training model that allows precise quantification of focal outflow.
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- For more information:
- Nils A. Loewen, MD, PhD, an associate professor of ophthalmology and the director of the Glaucoma and Cataract Service at UPMC, can be reached at Eye & Ear Institute, 203 Lothrop St., Pittsburgh, PA 15213; email: email@example.com.
Disclosure: Loewen reports he is a Trabectome trainer.