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Overview of micro-invasive glaucoma surgery

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Dr Mark Chiang
MBBS MD FRANZCO
City Eye Centre, Sunnyban QLD

 

In the glaucoma management ladder, the most common treatment paradigm for primary open angle glaucoma (POAG) and secondary open angle glaucomas would be initial topical antihypertensive drops, followed by laser therapy such as selective laser trabeculoplasty (SLT), then ultimately, surgery.

Several studies have suggested 360-degree SLT is as effective as initial topical medication such as prostangladin analogues.1 Further larger randomised controlled trials are being undertaken to confirm these findings or support the use of initial SLT therapy; for example, the LiGHT (Laser in Glaucoma and Ocular Hypertension) Study, Moorfields Eye Hospital, UK.

For a number of patients, surgery is still required despite the recent advances in glaucoma medical and laser therapy. Trabeculectomy, introduced by Cairns in 1968, still remains the gold standard surgery for many types of open angle glaucoma (OAG). Although the Tube versus Trabeculectomy study supported the use of glaucoma drainage devices in lower risk cases other than refractory glaucoma,2 glaucoma drainage devices use in primary cases is still being investigated.

Even though the techniques in performing trabeculectomy and glaucoma drainage devices have improved significantly since their introduction, they still carry significant risk profiles to cause patient concerns. For those patients with mild to moderate OAG requiring surgery, micro-invasive glaucoma surgery (MIGS) may be a safer option.

MIGS is a recently-coined acronym to include several modern operations or devices that cause minimal or no disruption to the conjunctiva and use an ab interno approach. Some ab externo incisional surgeries such as canaloplasty have also been included in MIGS.

In Australia, MIGS generally refers to operations that use a clear corneal incision that does not violate the conjunctiva at all, often performed at the time of cataract surgery. Currently, two MIGS devices are approved for use in Australia and they will be discussed below. To insert these devices, a direct intraoperative gonioscopic view of the drainage angle is required, as they are designed to sit in the Schlemm’s canal to bypass the trabecular meshwork. In other words, direct aqueous drainage from the anterior chamber into the Schlemm’s canal and ideally the collector channels is made possible.

The basis for these trabecular bypass stents relies on the fact that in most OAG, the juxtacanalicular trabecular meshwork provided the majority of aqueous outflow resistance.4 Therefore, in theory, it is possible for these trabecular meshwork bypass stents not to work at all if the outflow resistance is located elsewhere beyond the trabecular meshwork/Schlemm’s canal. Unfortunately, at present, it is very difficult to determine the site of outflow resistance clinically in individual glaucoma patients and this remains an area for further investigation.

113. Figure 1. Eye Anatomy _F

Figure 1. Anterior eye, with Canal of Schlemm at lower right3

iStent

The iStent (Glaukos Corp, Figure 2) is an L-shaped heparin-coated titanium device that measures 1 mm in length and 0.3 mm in height. It is a first-generation trabecular bypass stent and one of the smallest implants ever used in humans. It is also the device with the most study data.

113. Figure 2. I Stent _F

Figure 2. iStent (first-generation)

Generally, one iStent is inserted during cataract surgery in Australia. However, some surgeons insert more than one to provide better aqueous drainage into Schlemm’s canal, thereby increasing the likelihood of gaining collector channel access, to provide better IOP control (Figure 3). In fact, the second-generation iStent inject (not yet available in Australia) may come with an injector preloaded with two stents for this exact reason and early results of multiple stenting are encouraging.5

113. Figure 3. Multiple I Stent Insertion _F

Figure 3. Multiple iStent insertion  Image: www.ophthalmicphotography.info

The largest study for first-generation iStent compared one iStent combined with cataract surgery versus cataract surgery alone in 240 eyes with mild to moderate OAG. At one year and two years, more patients in the iStent group achieved IOP = 21 mmHg without medications (72 per cent vs 50 per cent at one year, and 61 per cent vs 50 per cent at two years).6

Hydrus Microstent

The Hydrus Microstent (Ivantis Inc, Figure 4) is a Schlemm’s canal scaffold that bypasses the trabecular meshwork as well as dilating the Schlemm’s canal through its entire length. It spans 8 mm in length or about three clock-hours when inserted into the drainage angle, hence, it targets more potential collector channels (Figure 5). The combined effect of trabecular meshwork bypass with Schlemm’s dilatation may potentially or at least in theory provide more significant IOP lowering.

113. Figure 4. Hydrus Microstent _F

Figure 4. Hydrus Microstent

113. Figure 5. Gonioscopic View _F

Figure 5. Gonioscopic view of Hydrus Microstent in the Schlemm’s canal

The Hydrus Microstent is made of nitinol, a highly biocompatible alloy that contains both nickel and titanium, and has high elasticity and shape-memory. Currently, there are many Hydrus studies near completion but the results are yet to be published; however, preliminary results have very promising. There is no published data comparing the safety and efficacy of Hydrus Microstent to the iStent but a head-to-head trial is currently underway.

The future of MIGS?

MIGS is still relatively new as a treatment. Apart from the aforementioned devices, they are many others being investigated or trialled. There are suprachoroidal space drainage devices such as the iStent Supra from Glaukos Corp, and Cypass from Transcend Medical; as well as those that access the subconjunctival/subTenon’s space through an ab interno approach such as the Xen gel stent from Aquesys Inc.

Currently, we don’t have enough data to indicate the most efficacious device. Perhaps as more study data becomes available, we will be able to choose the best device to tailor for the individual eye and patient. Whether MIGS can have an expanded use in patients with advanced glaucoma remains to be studied. The impact from MIGS in terms of cost-effectiveness, quality of life, ocular surface disease and even IOP fluctuations may become better understood with future trials. 

 

  1. McAlinden C. Selective laser trabeculoplasty (SLT) vs other treatment modalities for glaucoma: systematic review. Eye 2014; 28: 3: 249-258.
  2. Gedde SJ, Singh K, Schiffman JC, Feuer WJ, Tube Versus Trabeculectomy Study G. The Tube Versus Trabeculectomy Study: interpretation of results and application to clinical practice. Current Opinion Ophthalmol 2012; 23: 2: 118-126.
  3. Blausen.com staff. Blausen gallery 2014. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762.
  4. Goel M, Picciani RG, Lee RK, Bhattacharya SK. Aqueous humor dynamics: a review. The Open Ophthalmology Journal 2010; 4: 52-59.
  5. Fea AM, Belda JI, Rekas M, Junemann A, Chang L, Pablo L et al. Prospective unmasked randomized evaluation of the iStent inject (R) versus two ocular hypotensive agents in patients with primary open-angle glaucoma. Clin Ophthalmol 2014; 8: 875-882.
  6. Craven ER, Katz LJ, Wells JM, Giamporcaro JE, iStent Study G. Cataract surgery with trabecular micro-bypass stent implantation in patients with mild-to-moderate open-angle glaucoma and cataract: two-year follow-up. J Cataract Refract Surg 2012; 38: 8: 1339-1345.


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