Does placing a tiny tube (microstent) under the surface of the eye relieve long-lasting high pressure inside the eye (glaucoma)?

What is open-angle glaucoma?
Glaucoma is a common eye condition caused by fluid building up in the front part of the eye, which increases pressure inside the eye. The increased pressure damages the nerve that connects the eye to the brain (optic nerve), causing loss of sight. Glaucoma can lead to permanent loss of sight (blindness) if it is not diagnosed and treated early.

Open-angle glaucoma is the most common type of glaucoma and tends to develop slowly over many years. It is caused by drainage channels in the eye gradually becoming blocked over time.

Treatments for glaucoma
Treatment cannot reverse any loss of sight that happened before glaucoma was diagnosed but can slow or stop loss of sight. All treatments for glaucoma aim to reduce the pressure in the eye. These include:

- medicines, given as eye-drops;
- laser treatment to reduce the production of fluid and open up blocked drainage channels; or
- surgery to drain fluid from the eye.

One treatment involves placing a tiny tube (called a microstent) under the surface of the eye to create a drainage channel for excess fluid. Microstents can often be placed during surgery to treat cataracts: cloudy patches that develop on the lens inside the eye and make sight misty and blurred.

Why we did this Cochrane Review
Placing a microstent may lower pressure inside the eye and reduce the need for eye-drop medicines or for other types of surgery that may have greater risks. We wanted to find out if placing a microstent during cataract surgery would lower pressure inside the eye in people with open-angle glaucoma.

We were also interested in how the microstent affected:

- the need for medicines to reduce pressure in the eye; and
- people's well-being.

What did we do?
We searched for studies that tested the effect of placing a microstent during cataract surgery in people with open-angle glaucoma. We looked for randomised controlled studies, in which those people who received a microstent and those who did not, was decided by chance. This type of study usually gives the most reliable evidence about the effects of a treatment.

Search date: we included evidence published up to August 2020.

What we found
We found one study that took place in the USA and involved 505 people (aged 45 years and older) with open angle glaucoma and a cataract.

The study divided patients into two groups. One group had a microstent placed during surgery to treat their cataract; the other group received surgery to treat their cataract only. Patients in the study were assessed for two years.

The study was funded by a company that makes microstents for use in treating glaucoma.

What are the main results of our review?
Two years after having cataract surgery, in people who also had a microstent placed:

- more of them (85% in this group compared to 59% in the other) did not need eye-drop medicines to treat glaucoma (evidence from 448 people);
- they had greater reductions in pressure inside the affected eye, than people not given a microstent (448 people);
- they had greater reductions, on average, in the use of eye-drop medicines, than people not given a microstent (448 people); and 
- fewer people needed further surgery to treat glaucoma (505 people).

However, placing the microstent caused a higher number of unwanted effects (complications) reported in the two years after surgery, compared with cataract surgery alone (evidence from 505 people). On average, for every 1000 people, 390 people given the microstent would have complications, compared with 360 people not given the microstent. There are safety concerns about the microstent used in this study causing long-lasting damage to the clear layer at the front of the eye (cornea).

The study did not measure people's well-being (quality of life) or measure how people's sight was affected over the two years after surgery.

Our confidence in these results
We are confident about the reductions in pressure inside the eye, and about complications after surgery. We do not expect that further evidence will change these results.

We are moderately confident about the reductions in the need for eye-drop medicines to lower pressure inside the eye. Although the patients in the study did not know which treatment group they were in, the people delivering the treatments did know, and this may have affected the results. These results may change if further evidence becomes available.

We are less confident about how many people needed further surgery to treat glaucoma; further evidence is likely to change these results.

Key messages
Placing a microstent in the eye during cataract surgery lowers pressure inside the eye in people with open angle glaucoma, and reduces their need for pressure-lowering medicines, more than cataract surgery alone. But placing of the microstent was linked with an increase in complications after surgery.

Authors' conclusions: 

Data from this single RCT show superiority of supraciliary microstent surgery when combined with phacoemulsification compared to phacoemulsification alone in achieving medication-free control of OAG. However, there are long-term safety concerns with the device used in this trial, relating to the observed significant loss of corneal endothelial cells at five years following device implantation. At the time of this review, this device has been withdrawn from the market.

This review has found that few high-quality studies exist comparing supraciliary microstent surgery to standard medical, laser or surgical glaucoma treatments. This should be addressed by further appropriately designed RCTs with sufficient long-term follow-up to ensure robust safety data are obtained. Consideration of health-related quality of life outcomes should also feature in trial design.

Read the full abstract...
Background: 

Glaucoma is the leading cause of global irreversible blindness, often associated with raised intraocular pressure (IOP). Where medical or laser treatment has failed or is not tolerated, surgery is often required. Minimally-invasive surgical approaches have been developed in recent years to reduce IOP with lower surgical risks. Supraciliary microstent surgery for the treatment of open-angle glaucoma (OAG) is one such approach.

Objectives: 

To evaluate the efficacy and safety of supraciliary microstent surgery for the treatment of OAG, and to compare with standard medical, laser or surgical treatments.

Search strategy: 

We searched the Cochrane Central Register of Controlled Trials (CENTRAL; which contains the Cochrane Eyes and Vision Trials Register; 2020, Issue 8); Ovid MEDLINE; Ovid Embase; the ISRCTN registry; ClinicalTrials.gov and the WHO ICTRP. The date of the search was 27 August 2020.

Selection criteria: 

We searched for randomised controlled trials (RCTs) of supraciliary microstent surgery, alone or with cataract surgery, compared to other surgical treatments (cataract surgery alone, other minimally invasive glaucoma device techniques, trabeculectomy), laser treatment or medical treatment.

Data collection and analysis: 

Two review authors independently screened titles and abstracts from the database search to identify studies that met the selection criteria. Data extraction, analysis, and evaluation of risk of bias from selected studies was performed independently and according to standard Cochrane methodology.

Main results: 

One study met the inclusion criteria of this review, evaluating the efficacy and safety of the Cypass supraciliary microstent surgery for the treatment of OAG, comparing phacoemulsification + supraciliary microstent surgery with phacoemulsification alone over 24 months. This study comprised 505 eyes of 505 participants with both OAG and cataract, 374 randomised to the phacoemulsification + microstent group. 

In this study, the perceived risk of bias from random sequence generation, allocation concealment and selective reporting was low. However, we considered the study to be at high risk of performance bias as surgeons/investigators were unmasked. Attrition bias was unclear, with 448/505 participants contributing to per protocol analysis.

Insertion of a Cypass supraciliary microstent combined with phacoemulsification probably increases the proportion of participants who are medication-free (not using eye-drops) at 24 months compared with phacoemulsification alone (85% versus 59%, risk ratio (RR) 1.27, 95% confidence interval (CI) 1.09 to 1.49, moderate-certainty evidence).

There is high-certainty evidence that a greater improvement in mean IOP occurs in the phacoemulsification + microstent group - mean (SD) change in IOP from baseline of -5.4 (3.9) mmHg in the phacoemulsification group, compared to -7.4 (4.4) mmHg in the phacoemulsification + microstent group at 24 months (mean difference -2.0 mmHg, 95% CI -2.85 to -1.15).

There is moderate-certainty evidence that insertion of a microstent is probably associated with a greater reduction in use of IOP-lowering drops (mean reduction of 0.7 medications in the phacoemulsification group, compared to a mean reduction of 1.2 medications in the phacoemulsification + microstent group).

Insertion of a microstent during phacoemulsification may reduce the requirement for further glaucoma intervention to control IOP at a later stage compared to phacoemulsification alone (RR 0.26, 95% CI 0.07 to 1.04, low-certainty evidence).

There is no evidence relating to the rate of visual field progression, or proportion of participants whose visual field loss progressed in this study.

There is moderate-certainty evidence showing little or no difference in the proportion of participants experiencing postoperative complications over 24 months between participants in the microstent group compared to those who received phacoemulsification alone (RR 1.1, 95% CI 0.8 to 1.4).

Five year post-approval data regarding the safety of the Cypass supraciliary microstent showed increased endothelial cell loss, associated with the position of the microstent in the anterior chamber.

There were no reported health-related quality of life (HRQoL) outcomes in the included study.

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