Which works better in adults with glaucoma: fornix-based or limbak-based incisions of drainage surgery?
We reviewed the evidence for the benefits and risks of the two main approaches (fornix-based versus limbal-based incisions) for drainage surgery (trabeculectomy) in adults with glaucoma.
Why is this question important?
Glaucoma is one of the leading largely-preventable causes of blindness in the world. When medical treatment with eyedrops is inadequate to control progression, surgical treatment is performed to enhance flow through the drainage system and lower the pressure in the eye. Trabeculectomy is one surgical technique that creates a passage, allowing drainage of fluid from inside the eye to lower the eye pressure. There are two incision types in this surgery: fornix-based [incision is created at the corneal limbus (between the cornea and conjunctiva) and hinged in the fornix space] and limbal-based [incision is created further away from the limbus under the eyelid]. This review aims to look at whether there are any differences in the surgical outcomes (eye pressure control and complications) between these two different surgical approaches (fornix and limbal-based techniques).
How did we identify and evaluate the evidence?
We searched the medical literature for studies that compared fornix and limbal-based techniques for adults with glaucoma. Next, we compared the results and summarized the evidence from all relevant studies. We also rated our confidence in the evidence based on the factors such as how studies were conducted and how many people were included.
What did we find?
We found six randomised controlled trials (RCTs) with a total of 361 adults with any type of glaucoma. They were followed-up at least 24 months.
- Failure rate at 24 months was not reported in any included studies, and one study reported "late complications" but did not specify a time period, which favoured the fornix-based treatment (our confidence in this evidence was low).
- No difference was noted with respect to lowering eye pressure after 24 months (two studies) and after 12 months (four studies)(moderate confidence).
- The number of medications needed to control eye pressure after surgery was also similar (high confidence).
- Most of the studies reported that the complication rates after the operation were similar except in one complication which was narrowing in the anterior part of the eye after the procedure (more common in the limbal surgery group), but this did not affect the final outcome of the surgery.
What does this mean?
We do not know whether fornix- or limbal-based trabeculectomy surgery is better in terms of benefits and risks due to a lack of robust evidence.
How-up-to date is this review?
The evidence in this Cochrane Review is current to March 2021.
The main result of this review was that there was uncertainty as to the difference between fornix- and limbal-based trabeculectomy surgeries due to the small number of events and confidence intervals that cross the null. This also applied to postoperative complications, but without any impact on long-term failure rate between the two surgical techniques.
Glaucoma is one of the leading largely preventable causes of blindness in the world. It is usually addressed first medically with topical intraocular pressure-lowering drops or by laser trabeculoplasty. In cases where such treatment fails, glaucoma-filtering surgery such as trabeculectomy, is commonly considered. Surgeons can differ in their technique when performing trabeculectomy, for example, the choice of the type of the conjunctival flap (fornix- or limbal-based). In a fornix-based flap, the surgical wound is performed at the corneal limbus; while in a limbal-based flap, the incision is further away. Many studies in the literature compare fornix- and limbal-based trabeculectomy with respect to outcomes and complications.
To assess the comparative effectiveness of fornix- versus limbal-based conjunctival flaps in trabeculectomy for adult glaucoma, with a specific focus on intraocular pressure (IOP) control and complication rates (adverse effects).
We searched the Cochrane Central Register of Controlled Trials (CENTRAL; which contains the Cochrane Eyes and Vision Trials Register; 2021, Issue 3); Ovid MEDLINE; Ovid Embase; the ISRCTN registry; ClinicalTrials.gov and the WHO ICTRP. The date of the search was 23 March 2021. There were no restrictions to language or year of publication.
We included RCTs comparing the benefits and complications of fornix- versus limbal-based trabeculectomy for glaucoma, irrespective of glaucoma type, publication status, and language. We excluded studies on children less than 18 years of age, since wound healing is different in this age group and the rate of bleb scarring postoperatively is high.
We used standard methodological procedures as per Cochrane criteria.
We did not identify any new eligible studies for this review update. As presented in the original review, we included six trials with a total of 361 participants. Two studies were conducted in the USA and one each in Germany, Greece, India, and Saudi Arabia. The participants of four trials had open-angle glaucoma; one study included participants with primary open-angle or primary closed-angle glaucoma, and one study did not specify the type of glaucoma. Three studies used a combined procedure (phacotrabeculectomy). Trabeculectomy with mitomycin C (MMC) was performed in four studies, and trabeculectomy with 5-fluorouracil (5-FU) was performed in only one study.
None of the included trials reported trabeculectomy failure at 24 months. Only one trial reported the failure rate of trabeculectomy as a late complication. Failure was higher among participants randomised to the limbal-based surgery: 1/50 eyes failed trabeculectomy in the fornix group compared with 3/50 in the limbal group (Peto odds ratio 0.36, 95% confidence interval (CI) 0.05 to 2.61)); therefore we are very uncertain as to the relative effect of the two procedures on failure rate.
Four studies including 252 participants provided measures of mean IOP at 12 months. In the fornix-based surgeries, mean IOP ranged from 12.5 to 15.5 mmHg and similar results were noted in limbal-based surgeries with mean IOP ranging from 11.7 to 15.1 mmHg without significant difference. Mean difference was 0.44 mmHg (95% CI −0.45 to 1.33; 247 eyes) and 0.86 mmHg, (95% CI −0.52 to 2.24; 139 eyes) at 12 and 24 months of follow-up, respectively. Neither of these pooled analyses showed a statistically significant difference in IOP between groups (moderate certainty evidence).
One trial reported number of anti-glaucoma medications at 24 months of follow-up with no difference noted between surgical groups. However, three trials reported the mean number of anti-glaucoma medications at 12 months of follow-up without significant difference in the mean number of postoperative IOP-lowering medications between the two surgical techniques. Mean difference was 0.02, (95% CI −0.15 to 0.19) at 12 months of follow-up (high certainty evidence).
Because of the small numbers of events and total participants, the risk of many reported adverse events was uncertain and those that were found to be statistically significant may have been due to chance.
For risk of bias assessment: although all six trials were randomised selection bias was mostly unclear, with unclear random sequence generation in four of the six studies and unclear allocation concealment in five of the six studies. Attrition bias was encountered in only one trial which also suffered from reporting bias. All other trials had an unclear risk of reporting bias as there was no access to study protocols. All included trials were judged to have high risk of detection bias due to lack of masking of the outcomes. Trabeculectomy is quite a standard procedure and unlikely to induce bias due to surgeon 'performance', hence performance bias was not evaluated.