In a study recently published in PLoS One, we developed a simple glaucoma severity index that combines IOP, number of eye drops and visual field damage to estimate how challenging treatment currently is. We demonstrated in another recent study that cataract surgery contributes almost nothing to IOP reduction when combined with Trabectome and excluded such combined procedures to focus purely on pressure and medication reduction. We hypothesized that more severe glaucoma might have a relatively more reduced outflow facility compared with mild glaucoma and would respond with a larger IOP reduction to trabecular meshwork ablation.
The background of the current study was that microincisional glaucoma surgery has traditionally been reserved for ocular hypertension up to mild glaucoma. In patients who refused to undergo a repeat traditional glaucoma surgery, we were surprised to find that plasma-mediated ablation of the trabecular meshwork with the Trabectome (NeoMedix) works well. It was generally assumed that subendothelial trabecular deposits and amorphous material accumulate in the juxtacanalicular tissue while the collector channels atrophy when bypassed for an extended time. This got us to think about what defines glaucoma severity clinically and how more treatment-resistant glaucoma might respond to simple trabecular meshwork removal because most of the outflow resistance is thought to reside here.
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Only patients with open-angle glaucoma who had undergone ab interno trabeculectomy without any other same-session surgery and without any second-eye surgery during the following 12 months were analyzed in the PLoS One study. Eyes of patients that had less than 12 months of follow-up or were diagnosed with neovascular glaucoma were excluded. To create the glaucoma index (GI), visual field damage was separated into four categories: mild, moderate, advanced and end stage (assigned 1, 2, 3 and 4 points, respectively). Preoperative number of medications was divided into four categories — one or less, two, three, or four or more — and assigned with a value of 1 to 4. Baseline IOP was divided into categories — less than 20 mm Hg, 20 mm Hg to 29 mm Hg, 30 mm Hg to 39 mm Hg, and above — and were assigned 1 to 4 points. GI was defined as IOP × medications × visual field, and patients were separated into four groups: less than 6 (group 1), 6 to 12 (group 2), more than 12 to 18 (group 3) and more than 18 (group 4). Linear regression was used to determine if there was an association between GI group and IOP reduction after 1 year or age, gender, race, diagnosis, cup-to-disc ratio and Shaffer grade.
There were 164 patients in group 1, 202 patients in group 2, 260 patients in group 3 and 216 patients in group 4. Mean IOP reduction after 1 year was 4.0 ± 5.4 mm Hg, 6.4 ± 5.8 mm Hg, 9.0 ± 7.6 mm Hg and 12.0 ± 8.0 mm Hg for groups 1 to 4, respectively. Linear regression showed that IOP reduction was associated with GI group after adjusting for age, gender, race, diagnosis, cup-to-disc ratio and Shaffer grade. Each GI group increase of one was associated with incremental IOP reductions of 2.95 ± 0.29 mm Hg. Success rate at 12 months was 90%, 77%, 77% and 71% for groups 1 to 4.
By creating a simple glaucoma index that captures clinical glaucoma severity and relative resistance to treatment, we found that a higher index was associated with a larger IOP reduction in Trabectome surgery. Postoperative IOPs were highly similar in all GI groups, even when the single contributing variables of the glaucoma index were looked at individually. Such behavior is actually predicted by the Goldmann equation of IOP = F/C + Pv – U (aqueous humor formation/facility + episcleral venous pressure – uveoscleral outflow): When the facility C (ease of outflow) is large, IOP should be mostly limited by the remaining episcleral venous pressure and uveoscleral outflow.
Taken together, the results suggest that there is a role for Trabectome surgery well beyond ocular hypertension and mild glaucoma into more serious stages. Patients will benefit from the expanded range of indications for this microincisional glaucoma surgery and be able to take advantage of its excellent safety record.
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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.