Deep Sclerectomy

Introduction

Trabeculectomy was developed in the late 1960s by Cairns. This filtering surgical procedure was proposed to reduce the elevated intraocular pressure (IOP) in medically uncontrolled glaucoma patients and consisted of bypassing the trabecular meshwork resistance.

Even with the numerous modifications of the original trabeculectomy, the penetrating nature of this procedure presents several serious postoperative complications such as hypotony, hyphema, flat anterior chamber, choroidal detachment, choroidal effusion or hemorrhage, endophthalmitis, and surgery-induced cataract. Even without such complications, the first-day intraocular pressure using this technique is variable and hardly predictable. Moreover, additional procedures may have to be performed (e.g. laser suture lyzes, suture release) to better control the postoperative intraocular pressure. In order to alleviate such drawbacks, various types of nonpenetrating glaucoma surgery have been proposed which are associated with the following potential advantages: increase in the intraocular pressure-lowering effect predictability, reduction in the incidence of the immediate postoperative complications, and creation of new outflow pathways.

In 1962, Kraznov described the sinusotomy technique. The surgery included the removal of a narrow lamellar band of sclera over Schlemm's canal of over 120° from 10 to 2 o'clock. Kraznov believed that the resistance to aqueous humor outflow was located at the level of the scleral aqueous drainage veins and not in the trabecula in most cases of primary open-angle glaucoma. By leaving in place the trabecula and the inner wall of Schlemm's canal he developed the first safe nonpenetrating filtering surgery. Zimmerman described in 1984 a nonpenetrating trabeculectomy (ab externo trabeculectomy) that incorporated the creation of a superficial square scleral flap of about one-third of the full scleral thickness. Unroofing of Schlemm's canal was similar to sinusotomy. The inner wall of Schlemm's canal and the juxtacanalicular trabecula were gently removed while the corneoscleral and uveoscleral trabeculae were left intact. Deep sclerectomy was described later by Fyodorov and Kozlov in 1990. The main distinction between the two previously described methods is that they rely on two different routes for the aqueous humor outflow. In deep sclerectomy the main aqueous humor outflow is located at the level of the anterior trabecula, whereas for sinusotomy and ab externo trabeculectomy the aqueous humor drainage essentially occurred through the posterior pigmented trabecula. The major advantage of deep sclerectomy is that it precludes the sudden hypotony encountered during and after trabeculectomy. The deep sclerectomy allows a progressive filtration of aqueous humor through the thin remaining trabeculo-Descemet's membrane (TDM) into the intrascleral reservoir ( Fig. 97-1 ).

The dissection of deep sclerectomy aims at two distinct goals. First is to create a filtering membrane (TDM) which will ensure a reproducible postoperative intraocular pressure. This will prevent an excessive overfiltration and avoid almost all severe postoperative complications. The second aim is to create an intrascleral filtering space that decreases the reliance on a subconjunctival filtering bleb. The thin remaining scleral bed of the intrascleral bleb also allows redirecting part of the aqueous humor to the subchoroidal space and therefore promotes uveoscleral outflow. In order to maintain intrascleral bleb patency and to prevent a collapse of the superficial flap over the scleral bed, a space maintainer is often used in deep sclerectomy. The most commonly used space maintainer is a device made of degradable porcine collagen. Other implants have been proposed, including viscoelastics, HEMA (hydroxyethylmethacrylate), high reticulated hyaluronic acid and polymethyl methac­rylate (PMMA) ( Fig. 97-2 ).

One of the key steps in performing deep sclerectomy is the removal of the main resistance to aqueous humor outflow, located at the level of the inner wall of Schlemm's canal. This is best performed by peeling both the Schlemm's canal trabecular endothelium and the juxtacanalicular trabecula. This maneuver was also called ‘ab externo trabeculectomy.’ This results in an increased egress of aqueous humor through the posterior trabecula. To perform a correct dissection of the TDM, extension of the filtering surface has to be made by removal of a portion of the corneal stroma anterior to the trabecula. This last part of the deep sclerectomy dissection allows exposing some portion of the Descemet's membrane to the scleral space.

Based on several studies, the mechanism of aqueous humor filtration in deep sclerectomy is the following. Aqueous humor flows from the anterior chamber through the TDM, mainly through the anterior trabecula and into the intrascleral space. From there the aqueous humor is drained through four potential pathways: (1) the subconjunctival bleb, which may present a more diffuse, flat aspect compared to that observed after trabeculectomy; (2) the intrascleral bleb: aqueous humor can also be drained directly from inside the scleral space through newly created transscleral aqueous humor veins interconnected to the episcleral draining system; (3) the uveoscleral outflow: the suprachoroidal space also plays a role in allowing part of the total aqueous humor to flow through the thin remaining scleral bed towards the suprachoroidal space; and (4) the physiological outflow through the two open ostia of Schlemm's canal.

Indications

The two major advantages of deep sclerectomy are: (1) the slow decrease in the intraocular pressure during the entire surgical procedure which will prevent hypotony-related complications; (2) the nonpenetration of the anterior chamber with avoidance of iridectomy reduces the postoperative inflammatory reaction. This reduces the changes in aqueous humor metabolism and will favor the anterior chamber structures, mainly the anterior chamber depth and the integrity of the lens. Deep sclerectomy would therefore be the most suitable surgical technique for young patients with a clear lens in order to avoid any lens opacities and the need for secondary cataract extraction.

The best indications for deep sclerectomy are the following: primary open-angle glaucoma, pseudoexfoliative glaucoma, pigmentary glaucoma, glaucoma associated with myopia, aphakic glaucoma, pseudophakic glaucoma, open-angle uveitic glaucoma, normal-tension glaucoma, and steroid-induced glaucoma. In all types of open-angle glaucoma, the main resistance to aqueous humor outflow lies at the level of the trabecular meshwork. Outflow can be dramatically improved by surgically removing this resistance. Because of the significant reduction in the postoperative complications, deep sclerectomy can be proposed in early primary open-angle glaucoma. Therefore, long periods of medical treatment can be avoided and the conjunctiva is preserved. For the same reasons, deep sclerectomy can be proposed in late-stage glaucoma as long as trabeculectomy is not feared due to the risk of a potentially visually threatening condition. Pseudoexfoliative glaucoma is commonly present with very elevated and irregular levels of intraocular pressure which might be difficult to lower using antiglaucoma medications only. There is also a need for more goniopunctures after deep sclerectomy in pseudoexfoliation glaucoma. Pigmentary glaucoma is also frequently resistant to medical treatment and often diagnosed in young patients who have to take topical medications for many years. Early indication for deep sclerectomy preserves the conjunctiva from the noxious effects of topical medications. These can induce a strong and prolonged postoperative scarring response of the conjunctiva and Tenon's membrane which prevents them from creating an efficient subconjunctival filtering bleb.

Glaucoma associated with high myopia presents a high risk for postoperative complications due to the abnormal globe geometry. The increased eyeball diameter, the lower scleral thickness, and the resulting larger intraocular volume are risk factors for choroidal effusion after a rapid drop in intraocular pressure. The slow preoperative intraocular pressure reduction during deep sclerectomy limits the prevalence of such a complication.

For aphakic and pseudophakic glaucoma, the nonpenetrating nature of the deep sclerectomy represents an advantage. Cataract extraction with or without intraocular lens implantation requires opening of the anterior chamber. The modifications in the intraocular structures involved during cataract surgery, such as the iris, the capsular bag, and the vitreous body, are no longer implicated in the new surgery, thus avoiding secondary complications. For instance, the deep sclerectomy, unlike the trabeculectomy, does not require performing an iridectomy. In the case of aphakia, such iridectomy would be potentially detrimental for vitreous body stability. The base of the vitreous might migrate through the iridectomy and block the filtration site despite an extensive and difficult-to-perform basal vitrectomy, not to mention the increased risk of traction retinal detachment. Early indication for deep sclerectomy instead of prolonged medical therapy would be of benefit for aphakic patients not amenable to trabeculectomy. In patients undergoing ECCE (extracapsular cataract extraction), the point to consider lies at the limbus in the superior quadrant where a large corneoscleral incision is performed to extract the lens. Excessive scarring of the conjunctiva next to the wound could create some problems for the conjunctival filtering bleb. Pseudophakic patients after a clear cornea phacoemulsification should be considered like the nonoperated patients regarding the risks and complications related to deep sclerectomy.

In open-angle uveitic glaucoma the breakdown of the blood–aqueous barrier is detrimental for the filtration survival because the release of inflammatory mediators and cells may stimulate postoperative fibroblast proliferation. The main advantage of deep sclerectomy for this type of glaucoma is the nonpenetrating nature of the technique and the avoidance of the iridectomy. Both these advantages reduce pre- and postoperative inflammation. In normal-tension glaucoma the effect of hypotensive drugs is usually modest and a surgical approach is preferred. Quiet pre- and postoperative periods without severe intraocular pressure variations associated with few bleb manipulations are favorable for such delicate eyes. The early postoperative period following deep sclerectomy may help in protecting the optic nerve head in patients with already compromised ocular blood flow. For steroid-induced glaucoma deep sclerectomy presents the same advantage as for uveitic open-angle glaucoma. The mild postoperative inflammation would generally require only a brief period of corticosteroid treatment. Patients can be switched to nonsteroidal anti-inflammatory drugs to prevent a steroid response.

Preoperative Considerations

Before proceeding to a nonpenetrating deep sclerectomy, the following elements have to be considered. To prevent hemorrhage during the conjunctival and scleral dissection, it is recommended to interrupt any oral anticoagulation medication for several days before surgery.

Pilocarpine and other miotics should be discontinued shortly before surgery because of their negative effect on the blood–aqueous barrier. Similarly, prostaglandin derivatives such as latanoprost, travoprost and bimatoprost should also be suspended because of the proinflammatory reaction they can induce in the early postoperative phase.

The pre- and postoperative inflammation can be best controlled by using antiinflammatory drugs such as corticosteroids or nonsteroidal antiinflammatory topical drugs.

In the preoperative phase, when pilocarpine and prostaglandin analogues have to be discontinued, the intraocular pressure can be controlled with an oral carbonic anhydrase inhibitor. It is recommended to reach the lowest IOP before performing the surgery. This will reduce the risk for decompression retinopathy and choroidal hemorrhage.

Anesthetic Considerations

Several anesthetic techniques might be proposed in performing deep sclerectomy. The procedure can be performed under either local or general anesthesia. Given the surgeon's preferences, the surgical procedures are generally conducted under local anesthesia. This can be further classified as injection or topical anesthesia. A few elements have to be taken into consideration when performing injection in the retrobulbar or the peribulbar space. The anesthesia injection should not exceed 3–4 mL to prevent a block of eyeball mobility. An excess of retrobulbar or peribulbar anesthesia might also induce an increase in the intraocular pressure. This is particularly important in eyes with advanced glaucoma damage, where any increase in the intraocular pressure might induce further ganglion cell damage. Generally, the smallest amount of anesthetic solution giving sufficient akinesia of the extraocular muscles and analgesia should be administered.

General anesthesia should be reserved for uncooperative patients, such as children or extremely anxious or agitated adults, who are unable or unwilling to cooperate during the surgical procedure.