This paper works up data from 6 randomized clinical trials of MIGS (Minimal Invasive Glaucoma Surgery) for open angle glaucoma that were previously evaluated by Cochrane reviews. The authors conducted a network-meta-anlysis (NMA) for 3 outcomes, namely (1) proportion of patients drop-free, (2) mean change in unmedicated IOP, and (3) mean change in number of IOP lowering drops. Use of NMA enabled simultaneous comparisons of multiple interventions. Surgical methods included trabecular bypass with iStent or Hydrus microstents, ab interno trabeculotomy with Trabectome, subconjunctival and supraciliary drainage devices, and endoscopic cyclophotocoagulation. Subconjunctival stents such as XEN and Preserflo were not included as randomized trials were not available at the time of evaluation. These studies focused in particular on MIGS devices that were used in combination with cataract surgery and were compared to IOP lowering of cataract surgery alone. Only a few comparisons included stand-alone MIGS data.
Trabecular bypass devices (Hydrus and iStent) increased the likelihood of remaining dropfree at medium term (6-18 months postoperatively) for Hydrus, and only short-term (6 months) for iStent. Cypass also increased the proportion of drop free cases, but the device has been withdrawn from the market in the meantime. Network meta-analyses supported the direction and magnitude of these results. Neither iStent nor iStent inject lowered IOP more than medical treatment at mid-term (2.3 mmHg in wash-out comparison), while results were minimally better for Hydrus (2.6 mmHg in wash-out comparison). No difference of postoperative drop application was found for ab-interno-trabeculotomy with trabectome combined with cataract surgery compared to trabeculectomy with cataract surgery, but no further details about severity of cases or randomization are given.
The value of the paper is its refined methodology used for reporting relative risk of remaining drop-free after MIGS surgery plus cataract surgery compared to cataract surgery alone as well as the mean IOP change and the mean reduction of drop application in combination with cataract surgery. One concern might be that indications for combined cataract-MIGS procedures randomized to cataract surgeries alone do not represent an average surgical glaucoma population, because patient selection is driven by indication for cataract surgery and not by the need for glaucoma surgery. This introduces a tendency to less severe glaucoma cases and makes comparison to other stages of glaucoma difficult.
Furthermore, other essential parameters, i.e. function and visual field were not considered in these reviewed studies. Up to now, the classical surrogate parameter for effectiveness of glaucoma surgery has been intraocular pressure, preferably without postoperative need of medication (complete success). Surgical success can be defined either by average IOP decrease (mean± standard deviation) or from the percentage of cases that fulfill preset target IOP limits (see EGS and WGA guidelines).
The benefit of these devices in a general glaucoma population remains to be clarified
In this respect, the problem of these studies that were analyzed by Cochrane and NMA methods is the fact that drop application is subjected to multiple biases: Patients might have used drops differently before and after surgery, the prescription of drops cannot be completely standardized, the various compounds have different IOP lowering effects, and drops may be more efficacious after cataract surgery. There is no target IOP limit given that would trigger increase or reduction of drop medication. A decrease of IOP of 2 mmHg is possibly not able to stop visual field progression if present. Therefore, using drop application as a surrogate parameter for success can be considered less reliable than IOP measurements.
Using drop application as a surrogate parameter for success can be considered less reliable than IOP measurement
For cataract patients with ocular hypertension or glaucoma suspect patients the study is of course valuable, but one cannot translate the results to patients with more advanced glaucomatous damage or compare them with classical filtering procedures such as trabeculectomy or glaucoma drainage devices.The benefit of these devices in a general glaucoma population remains to be clarified.