Minimally Invasive Glaucoma Surgery
– I-stent
– Hydrus
– Viscocanalostomy & the GATT procedure
Traditional (Conventional) Glaucoma Surgery
Minimally Invasive Glaucoma Surgery
Minimally invasive glaucoma surgery or MIGS describes a group of glaucoma operations which when compared to “conventional” glaucoma surgery (trabeculectomy and aqueous shunt/tube surgery) are relatively:
— Quick to perform
— Safe
— Effective
To understand how the majority of MIGS operations work, it is important to understand how the eye drains fluid produced within the eye (aqueous humor). In individuals who do not have glaucoma, about 80% of aqueous humor produced within the eye leaves the eye through the Trabecular meshwork into Schlemn’s canal – a natural drainage canal system which sites in the drainage angle of the eye (figure 3).
Figure 3 – Cross section of the eye showing the natural passage of fluid produced with the eye (aqueous humor) (red arrow) and the drainage angle of the eye – labelled “angle”.
Source: National eye institute media library
The trabecular meshwork which extends 360 degrees around the front portion of the eye, works like an active pump to move the aqueous humour into the drainage Schlemn’s canal from where the fluid drains away into the collector channels & finally the blood circulation of the eye (episcleral veins) ((Figure 4).
Figure 4 – Cross section of the eye showing the drainage system of the eye and the flow of fluid (aqueous humor) in normal conditions (black arrow) and patient with glaucoma (red arrow)
In patients with glaucoma, the Trabecular meshwork either malfunctions or is blocked and this can be accompanied by narrowing or collapse of Schlemn’s canal. Therefore, aqueous humor escape from the eye is impaired resulting in raised eye pressure (intraocular pressure) which can lead to glaucoma. MIGS operations aim to either improve, restore or enhance the natural drainage channel system or bypass it.
The table below summarizes how MIGS work and gives some examples of types of MIGS operations available.
I-stent Implant
The iStent is a microscopic implant measuring less than 1mm in size (figure 5) and is made of titanium.
Figure 5 – Image showing the size of 2 i-Stent implants on a coin. Source: Glaukos
The i-Stent is inserted into the internal drainage canal of the eye (Schlemn’s canal) to enhance or restore the natural drainage of aqueous humor and thereby help to lower the Intraocular pressure.
Normally, two i-Stent implants will be inserted into the eye to ensure adequate pressure reduction.
The I-stent aims to reduce the intraocular pressure and number of glaucoma medications in patient with mild to moderate glaucoma undergoing cataract surgery. It is important to remember that the I-Stent will not cure your glaucoma, reverse any damage already caused by glaucoma, or bring back any lost vision.
In a large clinical trial, two-thirds of i-Stent patients remained medication-free while achieving an eye pressure reduction of 20% compared to only half of patients who underwent cataract surgery alone. In a UK study, eye pressure was 20% lower at 3 years after i-Stent insertion.
However, the eye pressure lowering effect of i-Stent insertion may reduce over time and you may need to resume using glaucoma medications. In a US study, there was no difference in the number of glaucoma medications at 24 months after surgery between patients who had cataract surgery combined with i-Stent insertion and those who had cataract surgery alone.
The operation is usually performed under local anaesthetic whereby medication is administered on the surface of the eye or around the eye to numb the eye.
The I-stent operation is performed at the end of cataract surgery and takes about 5-10 minutes. Following the operation either a pad or a pad plus a clear plastic shield is placed over your eye.
After the operation, your eye may appear red and feel sore for up a few weeks.
Following surgery, your glaucoma drops in the operated eye are stopped and you are given alternative drops. However, if you are using drops in your other eye, these should continue. You will then be seen at about one week following your surgery.
The overall of risk of the I-Stent operation is no greater than cataract surgery alone. Sometimes the I-stent operation may not work to lower your eye pressure and therefore medication may need to be recommenced or increased.
The Hydrus implant or KDB procedure are potential other surgical options which can be performed at the same time as cataract surgery.
Non-surgical options included an increase in the number of glaucoma medications or laser treatment (selective laser trabeculoplasty)
KDB Procedure
The KDB operation is named after the instrument used to perform the operation – Kahook Dual Blade (KDB) (figure 8). The KDB is used to remove about four clock hours (sometimes 3 to 5) of trabecular meshwork tissue to allow aqueous humor to flow freely into the Schlemn’s canal and beyond.
Figure 8 – Image of the Kahook Dual Blade used to perform the KDB procedure. Source: New World Medical
The operation is almost always performed at the same time as cataract surgery.
When combined with cataract surgery, the operation allows us to both remove your cataract and combine this with an operation to lower your eye pressure and/or number of glaucoma medications or both.
A number of studies have now been published showing the success of the KDB procedure at 12 months after the operation. These studies have shown that up to 71% of patients had a 20% reduction in the IOP with up to 81% of patients reducing their total number of glaucoma drops used by one or more.
The operation is usually performed under local anaesthetic whereby medication is administered around the eye to numb the eye.
The KDB operation is performed at the end of cataract surgery and takes about 5-10 minutes. Following the operation either a pad or a pad plus a clear plastic shield is placed over your eye.
After the operation, your eye may appear red and feel sore for up a few weeks.
Following surgery, your glaucoma drops in the operated eye are stopped and you are given alternative drops. You will then be seen at about one week following your surgery.
The KDB operation has the following risks (in addition to those of cataract surgery if this is performed at the same time)
- Hyphema (bleeding into the front chamber of the eye) – has been reported to occur in 17% of eyes within the first week following surgery. The hyphema usually has little bearing on your vision however can cause high IOP and or blurred vision. It has been shown that if you are on blood thinning medications (anti-coagulants) you have a higher risk of hyphema (approximately 24%) compared to not being on this medication (up to 17%). Therefore, please inform you consultant prior to surgery if you are taking ANY blood thinning medication.
- High IOP (defined as a rise greater than 10mmHg above base line pre-operation IOP) – has been reported to occur in about 11% and usually occurs within the first 2 weeks after surgery and is more likely to occur in patients with a hyphema.
- Damage to the iris (coloured part of the eye) or cornea (clear window of the eye) – these are rare complications and can occur during the procedure itself. If they occur it can cause glare which you either adapt to with time (damage to the iris) or settle with time (damage to the cornea)
The I-stent, Hydrus implant, Viscocanalostomy or the GATT procedure are potential other surgical options which can be performed at the same time as cataract surgery.
Non-surgical options included an increase in the number of glaucoma medications or laser treatment (selective laser trabeculoplasty).
Hydrus Microstent
The Hydrus Microstent is a small flexible implant which is inserted into the natural drainage canal of the eye (Schlemn’s canal) to help lower the eye pressure and the need for medications to lower the eye pressure (figure 6)
Figure 6 – Image showing the Hydrus Microstent implant. Source: Ivantis
The Microstent is 8mm in length and made from an elastic alloy of nickel and titanium (nitinol®) which has been used in over a million medical implants (Figure 7). Furthermore, this material will not cause an allergic reaction and is safe in an MRI scanner.
Figure 7 – Image showing the relative size of the Hydrus Microstent implant next to 19mm diameter coin. Source: Ivantis
The implant is suitable for patients with mild-moderate open angle glaucoma on drops who are undergoing cataract surgery. However, it can also be used on it’s own without cataract surgery.
The Microstent implant works by creating an opening and ‘scaffolding’ open the natural drainage canal of the eye (Schelm’s canal) thereby allowing aqueous humor to flow into the canal and lower the eye pressure.
The Microstent helps to reduce the eye pressure within the eye and may reduce the need or dependence on eye drops to lower your eye pressure.
The procedure to insert the Hydrus is faster, safer with quicker recovery than conventional glaucoma surgery.
Following combined cataract and Hydrus implant, 73% of patients were medication free at 5 year after surgery compared to 48% of patient who had cataract surgery alone.
The operation is usually performed under a local ananesthetic – so that your eye is ‘numbed’. In addition, if you would like to feel more relaxed during the surgery, you also have the additional option of light sedation.
If you are having cataract surgery, the Microstent is implanted using a pre-loaded injector at the end of cataract surgery using the same incision.
At the end of the operation, your eye will usually be padded and covered with a shield unless you have poor vision in the other eye, in which case you will only have a shield placed on the eye. You will be able to go home on the same day and are usually seen about one week after the surgery.
After the operation, your eye may appear ‘bloodshot’ and feel ‘swollen’ and your vision may be blurred for up to 2 weeks.
In terms of medication use, your glaucoma drops will be stopped to the operated eye and instead you will be given anti-inflammatory and anti-biotic drops. If you are using glaucoma drops in the un-operated eye, these should continue.
You should avoid strenuous activity for the first month and as a general rule aim for two weeks off work. Sometimes longer time is required off work depending on the nature of your work and your vision in the other eye.
There is a small risk of bleeding within the front chamber of the eye which for the majority of patients resolves within the first week following surgery.
There is a small risk that the Microstent can become blocked or mal-positioned however, this risk is rare.
Sometimes, the Microstent does not lower the eye pressure and therefore you will be advised to restart one or more of your glaucoma medications.
Surgical options include the i-stent implant or the KDB procedure.
Non-surgical options include continuation of your glaucoma drops or Selective Laser trabeculoplasty.
Viscocanalostomy & the Gonioscopy Assisted Transluminal Trabeculotomy (GATT) Procedure
Viscocanalostomy (VC) works by opening up a narrow or blocked Schlemn’s canal (canal where aqueous humor drains into from the front chamber of the eye). The GATT procedure takes VC further, in that the Schlemn’s canal is often opened and then the internal wall of the canal along with the Trabecular meshwork is removed either 180 degrees or 360 degrees – therefore allow aqueous humour direct access to the Schlemn’s canal in 180 degrees or more.
Both VC and the GATT procedure are often performed at the end of cataract surgery however, they have been shown to be effective when performed alone.
Several studies have reported on the outcome of both VC and GATT procedure.
At 12 months following VC, patient had an average of 41% reduction in intraocular pressure with 87% of patient having a 20% or more IOP reduction compared to before the surgery. In terms of glaucoma medication use, 85% of patients required no medication to maintain their intraocular pressure.
At 12 months following the GATT procedure, there was a 44% reduction in the intraocular pressure with 59-63% of patient having a 20% or more reduction in the intraocular pressure. At 24 months following the GATT procedure, there was a 37% and 49% decrease in the intraocular pressure for patients with primary open angle glaucoma and secondary open angle glaucoma, respectively. Furthermore, there was an average decrease of 1.4 to 2 glaucoma medications.
The operation is usually performed under local anaesthesia. If combined with cataract surgery both VC and GATT procedure are usually performed at the end of the operation and use the same incisions used for cataract surgery.
For both VC and the GATT procedure, a microcatheter (fine tube) is inserted into Schlemn’s canal (drainage canal of the eye) and used to open up the canal.
Viscocanalostomy
The main risks of VC (in addition to risks of cataract surgery are)
- Bleeding into the front chamber of the eye – this has been reported occur in up to 13% of patients but resolves for almost all patients by 1 week following surgery.
- It may not work and therefore you will need to restart your glaucoma medications
GATT procedure
As the GATT procedure is more involved, the risks are slightly higher compared to VC
- bleeding into the front chamber of the eye – which has been reported occur in up to 40% of patients. However, this is very mild for almost all patients and for the majority of patients resolves by 1 week following surgery. Rarely the bleeding may be severe enough that it will need to be cleared with another operation
- High Intraocular pressure – requiring additional medication to lower the IOP
- Clouding of the clear window of the eye (cornea) – which has been reported to occur in up to 6% of patients and resolves over time.
The Hydrus implant or KDB procedure are potential other surgical options which can be performed at the same time as cataract surgery.
Non-surgical options included an increase in the number of glaucoma medications or laser treatment (selective laser trabeculoplasty)
XEN Implant
The XEN implant is a small 6mm long tube made out of porcine gelatin which is used to drain fluid out of the front (anterior) chamber to lower the eye pressure (Figure 9).
Figure 9 – Image of relative size of the Xen implant on a 19mm diameter coin. Source: Allergen
The XEN implant is placed into the front chamber of the eye (anterior chamber) and drains fluid from within the eye (aqueous humor) out under the skin of the eye to form a small reservoir called a bleb. The bleb is usually covered by the upper eyelid and is only visible if looking in a mirror when you lift the eyelid and look down.
The XEN implant aims to lower the intraocular pressure and thereby reduce your risk of sight loss from glaucoma. It will not improve any pre-existing sight loss from glaucoma.
In a trial of the XEN implant looking at its effectiveness, the intraocular pressure was found to be reduced by 30% at 12 months after surgery with a 75% reduction in the number of medications needed.
The operation is usually performed under local anaesthetic, whereby either drops are instilled on the eye or a small injection of anaesthetic is given around the eye. The operation can be performed either alone or combined with cataract surgery.
To reduce the risk of the reservoir formed by the XEN implant (bleb) from scarring, an anti-scarring medication (Mitomycin C) is applied to your eye at the start of the operation. A small incision is then made in the eye and a small amount of gel is injected into your eye. The XEN implant is then carefully injected from within your eye to the area marked to form the reservoir or bleb. The gel is then washed out. The whole procedure takes about 10-15 minutes. Following the operation, a pad and plastic shield is placed over your eye.
Initially, your eye will be red and bloodshot for the first 3-4 weeks. Your vision, is likely to be blurred initially for the for 2-3 weeks.
Following the operation, there is a risk that the XEN implant may become blocked. If this occurs, you may require a minor operation to unblock the implant – a procedure called needling. Sometimes, despite the XEN implant working or following needling, your intraocular pressure may not be low enough and therefore you may need to restart your glaucoma drops.
Uncommonly, your eye pressure may drop too low requiring closer monitoring or rarely further surgery.
Rarely, there is a serious risk of bleeding or infection which could affect your vision.
Viscocanalostomy, GATT procedure are potential other surgical options which can be performed at the same time as cataract surgery. Alternatively, conventional glaucoma surgery – trabeculectomy – could be an option.
Non-surgical options included an increase in the number of glaucoma medications or laser treatment (selective laser trabeculoplasty)
Preserflo Implant
The Preserflo is a Microshunt measuring 8mm in length and less than 1mm in diameter, which is made of a synthetic biocompatible material called SIBS. As it is made of biocompatible material, there have been no concerns with rejection of the Microshunt by the body or issues with passing through an airport scanner or having a MRI scan.
The Microshunt works like the Xen implant and conventional glaucoma surgery (trabeculectomy and tube surgery) whereby fluid from within the eye (aqueous humor) is drained out of the eye to under the skin of the eye (conjunctiva) to form a reservoir of fluid called a bleb (Figure 10). This is in contrast to the I-Stent, KDB, Hydrus or GATT procedure, which aim to restore or enhance the normal drainage of fluid (aqueous) from within the eye.
Figure 10 – Image of the Preserflo implant and where it sits within the eye once implanted.
The bypassing of fluid to outside of the eye is generally more effective at lowering the eye pressure than enhancing the normal drainage system (such as with the i-stent, KDB, Hydrus or GATT procedure) and hence the Microshunt is generally used for patient in whom the eye pressure is very high or who have moderate to advanced glaucoma.
The Microshunt will lower your eye pressure to reduce your risk of worsening of your glaucoma. The Microshunt may be as effective as trabeculectomy however, this is currently being investigated by a study comparing the two procedures.
Additionally, the Microshunt has added advantage over trabeculectomy in requiring less time to perform the surgery, causes less tissue damage during the surgery and requires less post-operation visits.
The operation is usually performed under local anesthetic, meaning that the eye is numbed with a small injection of anesthetic given around the eye. If you feel anxious, then sedation can also be administered.
The skin of the eye (conjunctiva) is opened in an area under the upper lid and then mitomycin is applied to reduce your risk of scarring and failure following surgery. The Mitomycin C is then washout out and a small tunnel is created in the wall of the eye. The Microshunt is then inserted from outside in so that a portion of it sits inside the eye to drain fluid out of the eye. The conjunctiva is then closed with stiches (which can later be removed in clinic).
In contrast to trabeculectomy which takes 60 minutes to perform and requires more stitches, the Microshunt operation takes about 30 minutes to perform.
After the operation, your eyelid may be droopy, the eye may appear ‘bloodshot’ and feel ‘swollen’ for the first 1-2 week. Additionally, your vision may be blurred for up to 4 weeks.
In terms of medication use, your glaucoma drops will be stopped to the operated eye and instead you will be given anti-inflammatory and anti-biotic drops. If you are using glaucoma drops in the un-operated eye, these should continue.
You should avoid strenuous activity for the first month and as a general rule aim for two weeks off work. Sometimes longer time is required off work depending on the nature of your work and your vision in the other eye.
As you will need to be seen several times in the first 2 months following surgery, it is advisable to avoid booking travel for this period.
Serious complications after Microshunt surgery are rare. There is a small risk of low pressure which may need further treatment.
Like the Xen implant and conventional glaucoma surgery (trabeculectomy and tube surgery), the eye pressure lowering effect may wear off with time and may require restarting of glaucoma drops and or further glaucoma surgery.
As this operation (like the Xen and conventional glaucoma surgery), creates a reservoir of fluid (bleb) under the skin of the eye (conjunctiva), there is a small lifelong risk of infection.
Like all cataract and conventional glaucoma surgery there is rare risk (1 in 1000 patients) of bleeding in the eye which could permanently affect the vision.
Trabeculectomy and tube surgery (aqueous shunt) are the main alternatives to the Microshunt.