The 2014 arrival of MIGS to Australia introduced a whole new field of interventional glaucoma therapies. Queensland ophthalmologist Dr Frank Howes re-defines the term and explores current surgical interventions, explaining where each fits in the glaucoma treatment algorithm.
Despite all the years that minimally invasive glaucoma surgery (MIGS) has been in existence, it’s intriguing that we are still at sea with so many MIGS procedures and often have to return home to the harbour of trabeculectomy.
However, some of these surgical modalities are beginning to prove their worth.
We have been running MIGS programs in Australia for the past six years, with many surgeons now using the MIGS procedures where appropriate. The procedures started on an open and even platform; all initially equal.
Australia is fortunate to have had access to so many procedures on the world stage to use in an attempt to reduce the risks involved in the surgical treatment of glaucoma. Time has allowed these procedures to rank order themselves according to ease of use, efficacy and complication – and not necessarily in that order.
To adequately treat and manage this disease, the ophthalmological world has had to utilise the glaucoma management resources carefully. That means the optimal and judicious use of drugs, laser interventions, minor and major surgical interventions, often in that order, to minimise our patients’ rate of disease progression, while assisting them to maintain an acceptable quality of life.
The choice profile used by surgeons in the glaucoma management field revolves around the risk-benefit ratio. The use of too many drugs can cause systemic and local side effects, related to the drugs themselves and the preservatives present. At the other end of the scale, too early an intervention with a major surgical procedure, places a patient’s vision and eyes at unnecessary risk of loss. The human compliance issues make the situation that much more complex.
Enter, therefore, the ‘middle ground’ procedures. These started in the 1980s with argon laser trabeculoplasty (ALT) and in the 1990s with selective laser trabeculoplasty (SLT). These procedures were of great assistance to ophthalmologists around the world (as was the subsequent entry of preservative-free topical glaucoma drugs). The surgical procedure of SLT well predated the category of procedures we now call MIGS, but it could certainly fit the definition.
The presence of such procedures, and the ever-present need to provide our patients with the ‘less is more’ concept in glaucoma management, pressed entrepreneurial ophthalmologists and scientists both in Australia and around the world to invent procedures and devices to better fill this middle gap.
So, what have we had?
One of the first ‘middle ground’ procedures, pre-MIGS, was the nonpenetrating glaucoma surgery group of interventions (deep sclerectomy and viscocanalostomy), quick to be supplemented by simultaneously implanted tiny devices to maintain their function.
This taught us again that Schlemm and trabecular manipulation, initially done for children many decades ago, was a viable option for managing outflow facility. So, enter canaloplasty – Schlemm intubation and segmental viscoelastic Schlemm dilation, both ab externo and ab interno.
Off shoots from this thinking was the removal or bypassing of the inner wall of Schlemm’s canal and the trabecular meshwork (TM) to better expose the collector channel openings to aqueous humour and enhance outflow. The need for a minimally invasive glaucoma procedure grew from these observations.
Work had begun in the first decade of the 2000s on the trabecular bypass systems, the iStent and the Hydrus groups. Parallel to this was the development of the TM removal procedures – GAT (Gonioscopically Assisted Trabeculotomy), Trab 360, Trabectome, KDB (Kahook Double Blade) trabeculotomy.
However, these TM removal procedures, although relatively safe and effective in the short term (despite the bleeding), proved to be destructive of normal anatomy with unknown long-term effects.
To complicate the view, work was also being done on minimally invasive devices to bypass the full outflow facility, both into the suprachoroidal and subconjunctival spaces (see below).
Of the many surgeons and scientists around the world that contributed to the development of these devices and procedures, Iqbal ‘Ike’ K. Ahmed, MD, FRCSC (Toronto, Canada), was the one who coined the term MIGS in 2009. He did so with the following defining terms, in an attempt to place order into these new surgical approaches:
• Ab interno approach – conjunctival sparing.
• Minimally traumatic – negligible damage to normal anatomy or physiology.
• Micro-incision – clear corneal incision <2.0mm.
• Efficacious.
• Extremely high safety profile – rapid recovery.
As can be seen there are several different procedures that have come to market that come close to fitting the definition, but some perhaps not, so there are some ‘musts’ and some ‘preferables’.
‘Musts’ include an ab interno approach, conjunctiva sparing, and microor minimally-invasive. ‘Preferables’ comprise a blebless outcome, no cyclodialysis cleft, no cautery, and a limited treatment area.
Some of these procedures fit the definitions fully (iStent, Glaukos), some in the epicentre (iStent inject, Glaukos), some close (Hydrus, Ivantis and the ab interno canaloplasty (ABiC) from Ellex), and some further away but still relatively minimally invasive, although invading the subconjunctival space (Xen, Allergan and Preserflo, Santen).
Some thought to be minimally invasive (suprachoroidal space) have proved with time not to be so (Cypass, Alcon), others being possibly more complex than initially thought (Xen), with competitors out there to potentially challenge their position (Preserflo).
The ‘Dart Board’ in Figure 1 demonstrates roughly where the procedures we call MIGS fit. The further from the epicentre, the less the procedures fit the definition of MIGS.
As can be seen, the Glaukos Generation one and Generation two (iStent/iStent inject) within the green bullseye area fit the definition fully, and are beginning to cement their place firmly in the management of glaucoma.
The Ivantis Hydrus device is close to the epicentre, as is the ABiC procedure (inner blue area), but due to the length and extent of intervention of these procedures is by definition not quite as minimally invasive as the microscopic iStent inject.
The other procedures in the blue area but outside the red circle, loosely fit the definition of MIGS as they still fulfil some of the pre-requisites of the definition, but the bleeding and recovery time blow out.
The procedures in the greyed area actually fall out of the definition of MIGS due to conjunctival involvement but knock on the door due to increased safety over full thickness penetrating surgery.
The laser and cryo ciliary destructive procedures have never entered the MIGS arena due to the anatomical change and destructive element, while the suprachoroidal devices have fallen from the MIGS grouping as they have demonstrated significant endothelial cell loss, plus there is the risk of the associated choroidal haemorrhage with manipulation.
This group of devices are still searching for their correct place in the management of glaucoma. Certainly, we know that opening the suprachoroidal space lowers intraocular pressure, but when and how to do this remains to be established. These have fallen out of the active MIGS group for the time being.
The Glaukos iStent devices have fixed their position in the centre of the MIGS group for the safe management of mild to moderate glaucoma and as a safe device for use in the provision of a mild to moderate effect.
This knowledge can also be used to reduce the burden of glaucoma drug management in our patients by allowing a reduction in medication load when used in conjunction with trabecular bypass.
This concept has given birth to yet another new medical acronym, ‘MIST’, coined by leading US ophthalmologist Dr Tom Samuelson, of the Minnesota Eye Consultants, standing for minimally intrusive sustainable therapy.
We started this dissertation with the statement that all MIGS procedures began on a roughly even playing field. But with time it has become apparent that the procedures are in fact not all equal.
Indeed, after rigorous assessment, the Australian Department of Health (MSAC) has recognised this by the introduction of the term ‘trans trabecular drainage device’ for the recently approved MBS code 42705. Sub-categorisation for the differentiation of the devices now appears to have begun. Furthermore, we now welcome the news of the ‘trans trabecular drainage devices’ being usable as standalone devices in phakic or psuedophakic eyes, as from 1 May 2020, on the new MBS item 42504. Australia is the first country in the world to have a standalone device insurance approved. However, this is under strict surveillance circumstances.
Decision making in this field is all about minimising risk and maximising quality of life.
Written by Dr Frank Howes – MBChB MMed FCS FRCS FRCOphth FRANZCO
