At the completion of this article, the reader should be able to improve their glaucoma management. Including:
• Understand the types of MIGS procedures
• Identify, and refer more confidently, the glaucoma patients suitable for MIGS procedures
• Understand the distinctions between MIGS procedures and MIBS procedures
• Provide pre-operative patient education and set patient expectations for minimally-invasive surgical interventions
• Review post-operative care protocols and complications
With the recent development of novel devices and surgical techniques, approaches to glaucoma management are changing. DR GEOFFREY CHAN and DR JONATHAN CHUNG-WAH-CHEONG discuss why it’s important for optometrists to consider minimally invasive glaucoma surgery, knowing which patients are best suited.
Dr Geoffrey Chan
MBBS (Hons) , MMedSc (UWA), PGCert Med Ed (University of Cambridge), FRANZCO
Lions Eye Institute, Nedlands, Western Australia, Australia
Centre for Ophthalmology and Visual Science,
The University of Western Australia, Perth, Australia
Dr Jonathan Chung-Wah-Cheong
BSc (UWA), MD (UWA)
Lions Eye Institute, Nedlands, Western Australia, Australia
Department of Ophthalmology, Sir Charles Gairdner Hospital,
Nedlands, Western Australia, Australia
As principal primary eyecare providers, optometrists have an important role in glaucoma care as the initiators of a referral to ophthalmologists, flagging any potential requirements for escalation of glaucoma treatments and co-managing patients in shared-cared models of delivery.
Minimally invasive glaucoma surgery (MIGS) has been listed for use in conjunction with cataract surgery on the Medicare Benefits Schedule since November 2018, and as standalone procedures from May 2020. At the same time, the availability of minimally invasive bleb surgery (MIBS) devices has had rapid uptake in Australia. Generally defined, these are the surgeries that bypass the normal trabecular outflow pathway which are more akin in their mechanism of action to trabeculectomy.
As most optometrists know, it’s vital to stay informed of the advances in glaucoma treatment and to understand the differences between the surgical options available and the impact they can have on patient care. In this article, we hope to provide a helpful summary of the glaucoma surgical options currently available. While not exhaustive, our survey focuses instead on the key features of MIGS and MIBS implants in comparison to traditional trabeculectomy and glaucoma drainage devices.
Which surgery for which patients, and when?
For any glaucoma surgery, it’s necessary for the patient to understand that the purpose of any operation is to preserve their vision, and that any vision lost due to glaucoma cannot be restored.
All patients with glaucoma, suspected glaucoma or ocular hypertension require lifelong follow up to monitor for disease onset or progression. Required therapy should be tailored to the patient and their circumstances, with medical and surgical practice patterns for first-line treatment preferentially involving glaucoma medications or selective laser trabeculoplasty.
Historically, glaucoma surgery has been reserved only for cases of advanced or progressive disease, but that is changing. Increasingly, newer glaucoma surgical techniques with higher safety profiles are considered earlier in the patient’s treatment plan.
Trabeculectomy
Trabeculectomy remains a standard of care in the surgical treatment of glaucoma. This procedure reduces intraocular pressure (IOP) by the creation of a new outflow pathway for aqueous fluid to filter from the anterior chamber to the subconjunctival space. The pathway is a hole (sclerostomy) that is created under a partial thickness scleral flap, and hence is considered a ‘guarded fistula’.
One of the main advantages of a trabeculectomy is that it is proven to be a more effective way to lower IOP than both MIGS and MIBS devices.1 However, this does come at significantly increased risk of postoperative complications, as well as increased intraoperative time and postoperative recovery compared to micro-invasive techniques.
MIGS
MIGS offers glaucoma patients the opportunity to have a single procedure that can decrease their medication burden, avoid the ocular surface complications caused by glaucoma eye drops and lower IOP with less risks compared to more invasive surgeries.
In general terms, patients that would most benefit from MIGS are those with stable or mild-to-moderate stages of glaucoma, who may have issues with adherence to medication, concurrent cataracts or who are suffering from ocular surface disease caused by glaucoma medications.
As most MIGS procedures rely on visualisation of the iridocorneal angle structures, it’s more appropriate as a treatment for open angle glaucoma. However in certain circumstances, MIGS can be combined with cataract surgery for treatment of angle closure glaucoma. For these cohorts, MIGS can help reduce the dependency on eye drops and maintain the lower IOP required for disease stabilisation.
With so many surgical options available, the main challenge is deciding which patients will benefit from surgery, what surgery is required and when should they have it. In patients with very high IOPs or advanced glaucoma, MIGS may not be as effective given their effect is limited by normal outflow resistance against episcleral venous pressure, and a more potent surgery that allows aqueous fluid to bypass conventional outflow may be required.
The alternatives to traditional trabeculectomy for these patients include MIBS devices –the PreserFlo MicroShunt and XEN Gel Stent are the main options in Australia – along with tube glaucoma drainage devices.
The ideal MIGS candidates
• Patients who have glaucoma and require cataract surgery
• Patient with glaucoma who are not compliant with medications
• Patients with glaucoma and compliant with medication but have disease progression
• Patients with glaucoma who are suffering from ocular surface disease
• Patients with advanced glaucoma who can’t have more invasive surgery
What is MIGS?
MIGS augments the conventional aqueous outflow pathway through the trabecular meshwork and into Schlemm’s canal, the physiological conduit of fluid that leads to collector channels and aqueous veins.
The key feature of MIGS procedures is that they cause relatively minimal trauma or disruption to surrounding tissue. For patients, the development of MIGS means that they have access to earlier surgical interventions with the benefits of reduced postoperative recovery and perioperative morbidity compared to conventional glaucoma surgery.
A helpful way to approach an understanding of MIGS procedures is to classify them based on their mechanisms of action. That is: devices which either stent, cut or dilate the trabecular meshwork or Schlemm’s canal tissue to achieve a reduction in intraocular pressure.
1. Stent
The trabecular meshwork bypass shunts (iStent Inject W Trabecular Micro-Bypass Stent (Glaukos Corporation), Hydrus Microstent (Ivantis, Inc.)) are micro-bypass devices which increase aqueous outflow by allowing fluid to drain from the anterior chamber to Schlemm’s canal and bypass the trabecular meshwork (Figures 1A and 1B). These devices are inserted via an ab-interno approach, meaning from ‘inside the eye’ through a small corneal incision.
2. Cut
A direct pathway for aqueous flow to Schlemm’s canal is created by cutting through the trabecular meshwork using various devices (Trabectome (NeoMedix Corporation), Kahook Dual Blade (New World Medical), Excimer Laser Trabeculostomy (AIDA, Glautec AG)) or techniques (gonioscopy-assisted transluminal trabeculotomy) via an ab-interno approach.2
3. Dilate
Aqueous outflow through Schlemm’s canal and distal collector channels can be enhanced by cannulation and dilation using certain device platforms (iTrack (Ellex Medical Pty Ltd.) and OMNI (Sight Sciences Inc.)). The iTrack canaloplasty microcatheter is a device used for catheterisation and pressurised viscodilation of Schlemm’s canal and can be performed ab-externo or ab-interno, whereas the OMNI Surgical System is performed via an ab-interno approach only. Both devices allow for treatment up to 360-degrees and may be performed in conjunction with trabeculotomy.
Patients should understand that the IOP-lowering effect of MIGS is relatively modest. The effects are limited by normal physiological outflow channels, and, as such, are generally recommended for patients with stable or mildly-progressive glaucoma.
This surgery is considered safe and can achieve a reduction in medication burden, but there are risks, including a period of postoperative elevated IOP, hyphema (bleeding), endophthalmitis (infection), inflammation and peripheral anterior synechiae which can then lead to stent occlusion. Also, poor positioning of stents can lead to later dislodgement or reduced effectiveness.2
Micro-invasive bleb surgery (MIBS)
The term ‘MIBS’ is used to describe minimally invasive glaucoma devices that work slightly differently to the typical MIGS procedures. These devices target the subconjunctival space and form blebs (a ‘blister-like’ fluid collection), bypassing the conventional outflow pathway altogether which allows drainage of aqueous directly into the subconjunctival space.
MIBS is more aggressive than MIGS, but generally considered less invasive than traditional glaucoma surgeries. There are two MIBS devices currently used in Australia: the XEN Glaucoma Treatment System (Allergan plc) and the PreserFlo MicroShunt (Santen Pharmaceutical Co). In general, the IOP-lowering effect of MIBS is greater than MIGS but lower than trabeculectomy.
The XEN Gel Stent was initially developed at the Lions Eye Institute in Perth. It’s composed of cross-linked porcine gelatin and the standardised size of the device, namely 6 mm length, helps regulate flow and reduces the risk of over drainage and post-operative hypotony.
The device is usually implanted through an ab-interno approach with minimal disruption to the conjunctiva (Figure 3). The aim is for the device to form a posterior, low and diffuse bleb and is generally placed in the superonasal position. Due to its smaller 45 mm lumen size, postoperative needling to maintain stent flow may be a higher requirement for this implant. The IOP-lowering effect and safety profile are similar between the two MIBS devices.
The PreserFlo MicroShunt is composed of a biologically inert material and has been approved for use in Australia since February 2022. It is inserted via an ab-externo approach which requires dissection of the conjunctiva, allowing for the accurate placement of the device beneath the tenons plane.
Due to the 8.5 mm length of the device, the aqueous humour flow and bleb positions tend to be posterior and may be favorable for patient comfort (Figures 4, 5A and 5B).
In the pivotal trial performed by Baker et al which compared MicroShunt surgery to trabeculectomy, significantly less postoperative hypotony was noted with the PreserFlo Microshunt group (<6mmHg) within the early postoperative period (defined as the first three months following surgery), compared to traditional trabeculectomy and so this device may have particular advantages when used for patients at risk of hypotony (myopes, vitrectomised eyes).3
Glaucoma drainage devices
Tube shunt surgery is another type of filtering surgery. These implants are characterised by their larger tube lumen size and connecting plate, which allows for an increased surface area available for aqueous drainage (Figure 6). A scleral patch graft or other material is used to cover the tube and to minimise the risk of postoperative erosion through the conjunctival surface.
Given these devices are larger, require more invasive surgery and/or positioning underneath the recti muscles of the eye, tube shunts are generally reserved for patients who have either failed previous glaucoma surgery or at high risk of postoperative failure with a primary trabeculectomy/MIBS device.
Tube shunt surgery pressure stabilisation relies on the formation of a fibrous capsule, which occurs foiur to six weeks postoperatively, to regulate aqueous fluid outflow and may incorporate internal valve mechanisms to restrict flow. The Molteno tube (Molteno Ophthalmic Limited), Baerveldt tube (Johnson & Johnson Vision) and Ahmed Valve (New World Medical) are the more common devices used in Australia, and newer versions such as the Paul tube (Advanced Ophthalmic Innovations) are now available.
Similar to trabeculectomy, these surgeries require intensive post-operative follow-up to ensure desirable long-term outcomes.4
What should the patient expect following glaucoma surgery?
It’s important that patients are followed up in the immediate post-operative period on a frequent basis after any glaucoma surgery. Patients undergoing less invasive MIGS surgery may be reviewed two-to-four weeks after their surgery, with trabeculectomy and MIBS patients requiring much more frequent and intensive postoperative review. This is generally required the day after the surgery and again at weeks one, two and four following the surgery. This reflects the faster recovery period following less invasive MIGS procedures.
Close monitoring of IOP and bleb function
Each surgery has its own complication profile, however the common complications, especially with MIBS, that the patient may be monitored for are: transient elevated IOP (spikes), low pressure (hypotony), wound leaks, choroidal effusions, shallow anterior chamber, suprachoroidal haemorrhage and infection (endophthalmitis or blebitis).
Antibiotic and steroid drops and/or topical non steroidal anti-inflammatories are typically prescribed following glaucoma surgery to reduce the risk of infection and to control inflammation during the healing phase.5
The patient may be commenced on atropine or equivalent eyedrop to posteriorly rotate the ciliary body if deepening of the anterior chamber is required. For trabeculectomy and MIBS procedures, the patient’s usual glaucoma medications are typically ceased immediately following surgery and can gradually be recommenced depending on intraocular pressure outcomes following the surgery.
Post-operatively, a cause of surgical failure is obstruction of aqueous flow through the glaucoma drainage device with thickened and inflamed tenons tissue. A bleb needing procedure, to cut away fibroblastic scar tissue (typically augmented by an anti-mitotic agent such as 5-fluorouracil) may be performed over the weeks-months or even years downstream to the surgery to unblock the device.
Patients should be counselled about a reduction in best corrected visual acuity temporarily or permanently after glaucoma surgery. This may be largely related to inflammation and hyphaemia, which tends to settle over a few weeks. Patients are told to avoid bending over, lifting or straining after surgery which may increase the risk of haemorrhage.
Refraction changes are typically neutral with the implantation of MIGS devices, but can result in significant with-the-rule astigmatic changes stabilising at three months for glaucoma surgery requiring suture closure, such as trabeculectomy, PreserFlo microshunt and tube surgery. ‘Snuff-out’ of residual optic nerve function is rare but has been described in the literature, especially for patients with very end stage disease and preoperative split fixation.6
Different surgeons have different preferences and may follow up patients at closer or longer intervals, depending on their progress and individual patient risk factors for the development of complications throughout the postoperative phase.
Summary
MIGS and MIBS provides a new opportunity for early surgical intervention for patients with glaucoma which can result in sustained reduction in IOP, delay more invasive surgery, decrease in eye drops dependence and improvement to quality of life.
As adoption of MIGS/MIBS inevitably increases, co-management of glaucoma patients between optometrists and ophthalmologists will be pivotal to delivering safe, appropriate, and effective treatment to patients. Having an understanding of the various glaucoma surgical procedures available will empower optometrists to begin pre-operative patient education, discuss patient concerns with ophthalmologists and be confident in identifying post-operative complications should they arise.
Continued open communication between optometrists and ophthalmologists is important in this ever evolving field to achieve our common goal of decreasing glaucoma-related blindness.
References
1. Marcos Parra MT, Salinas López JA, López Grau NS, Ceausescu AM, Pérez Santonja JJ. XEN implant device versus trabeculectomy, either alone or in combination with phacoemulsification, in open-angle glaucoma patients. Graefe’s Archive for Clinical and Experimental Ophthalmology. 2019 Aug;257(8):1741-50.
2. Wang J, Barton K. Overview of MIGS. In: Sng CCA, Barton K, editors. Minimally Invasive Glaucoma Surgery. Singapore: Springer; 2021. p. 1–10. Available from: https://doi. org/10.1007/978-981-15-5632-6_1
3. Baker ND, Barnebey HS, Moster MR, Stiles MC, Vold SD, Khatana AK, Flowers BE, Grover DS, Strouthidis NG, Panarelli JF, INN005 Study Group. Ab-Externo MicroShunt versus trabeculectomy in primary open-angle glaucoma: one-year results from a 2-year randomized, multicenter study. Ophthalmology. 2021 Dec 1;128(12):1710-21.
4. Gedde SJ, Schiffman JC, Feuer WJ, Herndon LW, Brandt JD, Budenz DL, Tube versus Trabeculectomy Study Group. Treatment outcomes in the Tube Versus Trabeculectomy (TVT) study after five years of follow-up. American journal of ophthalmology. 2012 May 1;153(5):789-803.
5. Almatlouh A, Bach-Holm D, Kessel L. Steroids and nonsteroidal anti-inflammatory drugs in the postoperative regime after trabeculectomy–which provides the better outcome? A systematic review and meta-analysis. Acta ophthalmologica. 2019 Mar;97(2):146-57.
6. Mohammadzadeh V, Galian K, Martinyan J, Nouri-Mahdavi K. Vision Loss After Glaucoma Surgery: Progressive Macular Thinning as a Sign of Snuff-Out Phenomenon. Journal of Glaucoma. 2019 Jun 1;28(6):e99-102.
