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New Glaucoma Surgical Alternatives
Summary
There has been, in recent years, a considerable surge in interest in glaucoma surgery; techniques, devices, (implants), and procedures to warrant earnest attention and research.
Several new devices (and implants) are currently being evaluated in clinical trials, all of which aspire to the common goal of achieving acceptable efficacy and limiting the potential for major vision-threatening complications.
Ongoing research is constantly working on ways to lower possible complications and adverse effects aiming to contribute to overall safety and efficacy of procedures. They all follow the common concept of delineating alternative pathways for aqueous drainage, either through Schlemm's canal or the suprachoroidal space, aiming to bypass the area of abnormal resistance in the trabecular meshwork.
Moreover, the interest in novel biomaterials, used in devices and implants, that confer desirable characteristics such as biostability and that invoke minimal inflammation and scar formation, to name a few, is deserved of continuous research.
It remains to be seen whether they will provide adequate levels of IOP reduction, to the levels required by many patients with moderate-to-advanced disc damage from glaucoma, and whether they will be effective and complication-free in the long term.
An essential point to consider is where to place these techniques in the surgical armamentarium for glaucoma. Some of these surgeries, because of their less-invasive nature and minimal, if any, damage to the conjunctiva and Tenon's capsule, may fit into the spectrum of therapies that can be performed prior to trabeculectomy and should not significantly lessen the success of this type of surgery should it be later required.
These devices and techniques have great theoretical advantages over external drainage techniques and the initial results with these new studies are encouraging. Clinical research in the next few years will aim to include multicentric studies with larger numbers of patients and longer follow-up so as to substantiate the long-term safety and efficacy of glaucoma surgical procedures.
Introduction
It is safe to say that there has never been such interest in glaucoma surgery as that which we are witnessing today. Glaucoma surgery that has been relatively stagnant for many decades is now being rejuvenated with the introduction of new technologies. Each of these new surgical methods comes with the promise of improved surgical efficiency and safety compared to the eternal gold standard, trabeculectomy. Claim is one thing though, and scientific reality is another. Most of these technologies are still undergoing the early stages of clinical testing, with evident paucity of published peer-reviewed studies to support an established place for these technologies in the routine practices of glaucoma surgeons and general ophthalmologists. In fact it is extremely hard at this point in time to make accurate evaluations of the utility of most of these technologies and devices. This chapter attempts to list what is commercially available, hitherto, to discuss the current literature and, perhaps, to speculate on future trends and possible positioning of these new methods in the expanding armamentarium of the ophthalmic surgeon.
Terminology
There have been attempts to group most of these devices and technologies under headings like minimally invasive, minimally effective, and conjunctiva-sparing surgery. None of these terms are accurate for a number of reasons. Calling these methods and devices minimally invasive implies that they are less invasive than traditional and well-established glaucoma surgical methods like trabeculectomy, nonpenetrating glaucoma surgery, and tubes. These claims are not supported by evidence-based science, to date. In some of these novel methods the conjunctiva is opened, thus compromising at least one quadrant, which would possibly go against the ‘non-invasive’ terminology. Furthermore each new methodology being introduced since the 19th century has always carried with it the claim of being less invasive, and as such the terminology is abused and exhausted.
Calling it minimally effective would first group the technologies in one single ‘effectiveness’ bracket which would not be unbiased towards or against certain technologies, and would imply that we have enough knowledge on effectiveness of such devices and methods, when we in fact do not possess such knowledge.
Classification
In our opinion, a safe and relatively accurate way of classifying such technologies would be to classify them according to their anatomic approach and thus we can list the technologies as:
- I
Subconjuctival filtration strategy:
- a)
Ab-externo approach
-
Ex-PRESS Implant (see also Chapter 126 )
-
CO 2 laser-assisted sclerectomy surgery (CLASS)
-
InnFocus MicroShunt
-
- b)
Ab-interno approach
-
Aquesys Xen implant
-
- II
Enhanced filtration into Schlemm's canal strategy
- a)
Ab-externo approach
-
Canaloplasty (iScience Catheter) (see Chapter 127 )
-
Stegmann Canal Expander
-
- b)
Ab-interno approach
-
iStent
-
High-frequency deep sclerotomy (HFDS)
-
Ab-interno trabeculotomy (Trabectome or iScience Catheter)
-
Hydrus implant
-
- III
Suprachoroidal filtration strategy
- a)
Ab-interno approach
-
CyPass implant
-
iStent supra implant
-
- b)
Ab-Externo approach
-
Starflo implant
-
Gold Solx Implant
-
I
Subconjunctival Filtration Strategy
Ab-Externo Approach
Ex-PRESS Implant (see also Chapter 126 )
The 2.64-mm stainless steel implant reduces pressure by diverting aqueous humor from the anterior chamber to the subscleral space after a dissection identical to a trabeculectomy with the exception of avoidance of surgical iridectomy ( ). Pressure release is initially the function of the implant's length and inner diameter, until scarring starts in full force, and thus the pressure control is dependent on the same forces as after trabeculectomy. Multiple studies have found superiority for Ex-PRESS in the initial postoperative period where its standardized filter may be a preventing factor for early hypotony with all its associated complications.
One other study also suggested that Ex-PRESS offers a faster visual rehabilitation to operated patients compared with trabeculectomy, which is a primordial factor that should always be taken into consideration ( Fig. 128-1 ).
On the other hand, there are no long-term results reporting on the corneal endothelial effects of Ex-PRESS compared to trabeculectomy, taking into consideration the fact that glaucoma drainage tube studies have shown a progressive long-term reduction of endothelial cells with time.
One additional factor that should always be taken into account is the economical repercussions of the introduction of a new device into everyday surgical practice. Direct cost has a major impact, but is not the only factor that should be taken into consideration. Postoperative care in a procedure that has potential complications and the management of such complications should be factored in. Such studies need to be carried out on national levels as costs vary considerably among different countries and in many cases within the same country itself.
CO 2 Laser-Assisted Sclerectomy Surgery (CLASS)
When manually dissecting the deep corneo-scleral lamellae in deep sclerectomy there is always the potential for either perforation into the anterior chamber or insufficient tissue removal. This is highly dependent on the surgeon's experience and skill. To overcome such challenges, different kinds of lasers ( Fig. 128-2 ) were used to ablate the deep scleral tissue. Experimental and clinical studies using the excimer laser have also been reported with encouraging preliminary results. No comparative studies between laser-assisted deep sclerectomy and trabeculectomy are available as yet.
Surgical Technique (see Chapter 97 , Spotlight 4 ).
A partial-thickness (one-third to one-half) rectangular limbal-based 5 × 5 mm superior scleral flap is dissected at the limbus into the clear cornea ( Fig. 128-3 ).
The desired scanning area and shape are set, the laser beam is focused, and the area to be treated is visually verified using a red laser aiming beam. The CO 2 laser beam is then applied over an area including the Schlemm's canal forming the scleral bed ( Fig. 128-4 ).
The residual charred tissue is wiped away with a sponge and ablation is continued until sufficient percolation is achieved, whereby the fluid absorbs laser's energy and prevents further ablation ( Fig. 128-5 ).
The scleral flap is repositioned and secured with two interrupted 10/0 Nylon sutures and a high-molecular-weight ophthalmic viscosurgical material (Healon 5®) is applied beneath the flap or an intrascleral implant (Aquaflow collagen implant from STAAR, Nidau, Switzerland, for example). The conjunctiva is adequately secured with 9/0 Vicryl continuous suture.
The reported results are sufficiently promising to suggest that the CLASS is an easily learned and simple surgical procedure to perform, which appears to be relatively safe and effective in the short and intermediate term. Randomized controlled trials comparing CLASS to manual technique are needed to improve our knowledge and understanding on how to place this new technology among our existent options.
The CO 2 laser has certain properties that offer significant advantages to facilitate deep sclerectomy ( 
). These include coagulation of bleeding vessels, photo-ablation of dry tissues, and absorption of the laser energy by percolating aqueous humor. As the emitted radiation is readily absorbed by the aqueous humor, the trabecular meshwork is potentially protected from the laser energy. Thus, perforation of the thin trabeculo-Descemet membrane during DS, which is the most frequent intraoperative complication of manual DS, is potentially minimized.
InnFocus MicroShunt
The InnFocus MicroShunt consists of a flexible tube with planar fins, which are located approximately halfway down the length of the tube that prevent the device from migrating into the anterior chamber. The fins also act as a ‘stopper’ to minimize aqueous humor leakage around the tube and to reduce the likelihood of postoperative hypotony. The 70 µm diameter lumen of the device serves as a flow restrictor with the intention of avoiding hypotony yet dampening IOP spikes postoperatively ( Fig. 128-6 ).
A highly biocompatible biomaterial, poly(styrene- block -isobutylene- block -styrene) or SIBS, is possibly a key feature of the InnFocus MicroShunt and is one of three elastomers which have been approved by the U.S. Food and Drug Administration (FDA) for use in the fabrication of medical devices intended for long-term implant applications. The SIBS material is biostable and its inert nature evokes minimal inflammation and scar formation. Initial studies of glaucoma drainage surgery in rabbit eyes compared the tissue response of tubes made from SIBS to silicone rubber tubes. Silicone rubber stimulates inflammation and promotes the development of a fibrotic capsule around the device that quickly becomes non-functional. In contrast, SIBS tubes demonstrated minimal encapsulation with continuous aqueous humor flow after one year.
Implantation of the InnFocus MicroShunt™ is relatively simple and requires only the dissection of a sub-conjunctival/Tenon's capsule flap and the development of a small scleral pocket to permit the passage of a needle tract into which the tube is inserted and the fins on the MicroShunt are wedged. The procedure is performed ab externo and does not require the use of a gonioscope or viscoelastic fluid. Unlike trabeculectomy, neither sclerostomy nor iridectomy are necessary and the only scleral trauma is the needle tract ( Fig. 128-7 and ).
Ab-Interno Approach
The Aquesys Xen Glaucoma Implant
The XEN Implant is a hydrophilic tube composed of a porcine gelatin and cross-linked with glutaraldehyde ( Fig. 128-8 ). It decreases intraocular pressure by creating an outflow pathway from the anterior chamber to the subconjunctival space through which the aqueous humor can flow.
During the implantation procedure, the implant hydrates and swells in place to become a soft non-migrating drainage channel that is tissue-conforming ( Figs 128-9 , 128-10 ).
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