choroid. This is usually evident because the hue of the choroid appears darker as the overlying sclera becomes thinner. Redirection of the diamond blade to a more superficial plane will correct the problem. If the choroid is inadvertently entered, unless there is prolapse of choroid, nothing but redirection of the blade should be done. Only a thin film of sclera is necessary to prevent iris prolapse. If the choroid has a small bulge it can be ignored and the procedure completed in the usual fashion. If however, the bulge is large, the flap must be closed with 10-0 nylon interrupted sutures and a new flap in another location is necessary.

IRIS PROLAPSE

If the scleral flap becomes too thick as dissection proceeds anteriorly near the limbus, then premature entry into the ciliary body, scleral spur, or trabecular meshwork can occur. This isn’t a big problem, especially if the cataract surgery has already been performed at a separate site. However, if the cataract is going to be removed from beneath the scleral flap, iris prolapse is a likely event. The performance of a peripheral iridectomy, prior to phaco, will prevent recurrent iris prolapse, enabling the cataract surgery to proceed. If this is inadequate, a Sheets’ glide may

FIGURE 27–5 Wilson externalized releasable suture. (A) Bite 1 enters the cornea 1 mm from the conjunctival reflection and passes into the corneoscleral limbus and out the sclera adjacent to the trabeculectomy flap. (B) Bite 2 enters the sclera of the trabeculectomy flap near the edge of the flap and into the sclera adjacent to the trabeculectomy flap.

(C) Bite 3 enters the sclera adjacent to the trabeculectomy flap, passes through the corneoscleral limbus, and exits the cornea 1 mm posterior to the conjunctival reflection and 1 to 2 mm nasal to the entrance site for bite 1. (D) Suture is tied with 3-1-1-1 throw pattern. (E) Excess suture is cut on the knot with a 15-degree blade to ensure epithelialization.

be placed such that it will hold the iris in position and phaco can proceed over it. It may be helpful to hold the glide in place with a temporary scleral suture. If the eye is hyperopic, with a crowded anterior chamber, iris prolapse around the phaco tip can still occur. Under these conditions, a 10-0 nylon suture placed radially in the scleral bed to decrease the size of the surgical ostium can help to keep the iris in the eye.

The best way to prevent iris prolapse is to not get too deep when creating the trabeculectomy flap and to enter the anterior chamber 1 mm anterior to the anterior edge of the scleral flap. This puts the entrance into the anterior chamber approximately 1 to 1.5 mm anterior to the Schwalbe’s line (Fig. 27–6).

Trabeculectomy-Sclerectomy Variables

The size and location of the posterior lip sclerectomy, relative to the size of the overlying scleral flap, is a

FIGURE 27–6 Cataract incision made under a trabeculectomy flap should enter the anterior chamber 1 mm anterior to the anterior edge of the flap to avoid iris prolapse.

major determinate of the amount of aqueous egress beneath the flap. If the sclerectomy is large, extending the entire width or length of the overlying flap, then aqueous egress will be greater than if the sclerectomy is smaller. If the sclerectomy is small relative to the overlying scleral flap, then less aqueous egress is usually present and fewer sutures may be required. If the sclerectomy is large, there is a greater chance for postoperative hypotony. If it is small, there is a greater chance that postoperative IOP will be higher initially and possibly more difficult to lower. This is due to significant overlap of sclera between flap and bed. When the sclerectomy is small, and centered, suture release doesn’t necessarily expose the sclerotomy space. Wilson35 has promoted a technique that minimizes the risk of postoperative hypotony, but maximizes the chance of being able to lower the IOP if necessary in the postoperative period. This technique involves making the sclerotomy as wide as the overlying flap on one side rather than both sides. Both sides of a triangular flap are sutured tightly, with interrupted or removable sutures, to minimize aqueous egress and prevent early postoperative hypotony. If IOP needs to be lowered in the postoperative period, laser suture lysis or removal of externalized releasable sutures on the side of the flap adjacent to the sclerectomy site will expose the underlying sclerotomy space (if healing of the flap edges hasn’t occurred) with resultant IOP lowering.

Ultimately it is the healing of the conjunctiva to the underlying episclera/sclera that limits aqueous flow and thereby determines the IOP. Therefore, late suture release (after 4 weeks) is less likely to lower IOP than earlier suture release.

It is important to fill the anterior chamber, through the paracentesis, with balanced salt solution, before closing the conjunctiva. This will allow assessment of aqueous egress beneath the flap. If there is excess egress, additional sutures should be placed to further diminish flow. If there is a paucity of egress, flap sutures should be loosened and sometimes the sclerectomy has to be enlarged in size to increase flow. Although unreported, this surgeon finds that patients who are phakic have a greater tendency to develop a shallow or flat anterior chamber postoperatively than do pseudophakes or aphakes. Consequently, the former group usually gets more tightly tied flaps than either of the latter groups.

NPTS–Superficial Scleral Flap

If the superficial flap in NPTS is made too small in surface area, then the deep scleral flap must also be small, limiting the amount of aqueous egress through a small Descemet’s window.26 This may result in a higher than the desired postoperative IOP. To avoid this problem, the superficial flap should always be at least 4 mm wide at the limbus and 5 mm posterior to the limbus.22,26,27 Some authors prefer larger flaps, 5 5 mm19,21 to 6-mm radius semicircular flaps.28 This allows for an ample deep sclerectomy, in terms of surface area, to provide adequate aqueous egress (3.5 L/min) in most patients.26

Stegman varies the anterior extension of Descemet’s window based on the severity and type of glaucoma, as well as the patient’s race.26 A relatively larger Descemet’s window is made in patients of African or Indochinese descent, in patients who have angle closure, and in patients who have a relatively higher IOP.26 The range for the extension of Descemet’s window anterior to Schwalbe’s line is 200m to 1 mm in whites and 500 m to 1.25 mm in blacks and Indochinese.26 If percolation of aqueous is still minimal, even with an enlarged Descemet’s window, then rarely Stegman will place five or six microperforations in Descemet’s membrane anterior to Schwalbe’s line with fine-tipped minidiamond blade (Greishaber no. 686.02).26 If the goal of surgery is to perform a deep sclerectomy and to rely on subconjunctival drainage of aqueous and bleb formation for IOP control, it is probably safer not to perform intraoperative microperforations, as this may lead to hypotony. Yttrium-aluminum-garnet (YAG) laser goniopuncture19,21 postoperatively, after some wound healing has occurred, would be the preferred technique. If the superficial flap is going to be sutured

tightly as in Sourdille et al’s28 technique of deep sclerectomy, then either intraoperative microperforations or YAG goniopuncture could be performed.

If the superficial flap does not extend far enough posterior to the limbus (5 mm), then Schlemm’s canal may not be found in certain patients (high myopes and those with congenital glaucoma).26 This complication with resultant increase in postoperative IOP is avoided if the flap is made to the correct dimensions. Several scleral markers are made to ensure proper flap size; a 4 5 mm parabolic marker (Duckworth and Kent) and a 5 5 mm square marker (Asico) are available. Alternatively, calipers may be used to correctly size the flap.

A superficial flap that is thin (<200 m) may tear or perforate, resulting in a leak. This may be acceptable in most deep sclerectomies because blebs are created in these operations and the main resistance to outflow is Descemet’s window. It is certainly undesirable in viscocanalostomy and Sourdille’s version of deep sclerectomy. If a superficial flap is recognized to become thin during dissection, redirection of the blade to a deeper plane will result in a thicker flap. However, if the superficial flap is identified to be thin only after its completion, another lamellar scleral dissection of the same dimensions may be attempted but may be difficult to achieve. The surgeon may have to be content with a thin flap and postoperative bleb formation. This occasionally occurs anyway, after viscocanalostomy, usually without having a major impact on IOP. A torn or cut flap should be repaired with sutures, if possible, for a watertight closure.

If the superficial flap is too deep (>300 m) it will decrease the volume of the subscleral lake or reservoir. This is probably not significant unless the space is severely reduced in size. For the surgeon who’s just beginning NPTS, the use of a micrometer diamond blade set at 200 m to outline the superficial flap is a good idea to get consistent depth flaps. Once some experience is acquired, it is relatively easy to create consistent depth flaps free-hand. A 1-mm- width metal spoon blade (Grieshaber no. 681.26) used bevel down is helpful in keeping the surgeon in the correct tissue plane.22,26,27

NPTS–DEEP SCLERAL FLAP

It is necessary for the deep scleral flap to be deep enough to unroof Schlemm’s canal but not so deep that choroid or ciliary body is entered. The deep scleral flap should be two-thirds scleral thickness (450 to 550 m). This author outlines the deep flap with a diamond blade and begins the dissection at the posterior edge of the flap with the same blade. Once the

posterior edge of the flap is raised, the metal spoon blade is utilized bevel down to stay in the correct tissue plane. The landmark for this tissue plane is the underlying dark hue of the choroid, which is seen through the remaining thin (50 to 100 m) translucent sclera.22,26,27 As the dissection proceeds anteriorly, Schlemm’s canal will become unroofed, revealing the granular appearance of the trabecular meshwork.22,26,27 Another landmark, visible on the underside of the deep scleral flap, is the smooth gray white outer wall of Schlemm’s canal, which can be seen to contrast with the rougher collagen of the sclera.22,26,27

Occasionally the deep flap dissection will turn out to be too deep and inadvertently enter the choroid, ciliary body, or anterior chamber. If the choroid or ciliary body is entered, the surgeon should redirect the dissection to a more superficial plane. Suturing of the sclera to close the rent is necessary only if choroid or ciliary body prolapses through the rent. This can be done with interrupted 10–0 nylon sutures.

Sharp dissection of the sides of the deep sclerocorneal flap needs to be performed initially to transect Schlemm’s canal to create “ostia” and then subsequently to allow the flap to be dissected anteriorly to create the window. Prior to the creation of the Descemet’s window, to prevent perforation, any traction sutures should be released and the anterior and posterior chambers decompressed of aqueous through a paracentesis incision.22,26,27 A cellulose sponge is used to place light pressure on the trabecular meshwork while gently pulling up and anteriorly on the deep scleral flap.22,26,27 This maneuver will separate the deep sclerocorneal flap from Schwalbe’s line and Descemet’s membrane. The sharp edge of the diamond blade should not be used to dissect the bed of the window because of the risk of perforation.

Perforation of Descemet’s Window

If inadvertent perforation of Descemet’s membrane does occur during viscocanalostomy, the superficial flap should be closed in a normal watertight manner.26 If the anterior and posterior chambers have been decompressed of aqueous, iris prolapse is unlikely to occur. It is prudent to place several drops of Miochol or Miostat at the perforation site to tighten the iris sphincter and help prevent anterior synechiae to Descemet’s window.26 In addition, in the early postoperative period (3 to 4 weeks) it is advisable to give pilocarpine 2% q.i.d. and 10% phenylephrine (10 minutes later).26 This will generate a maximally taut iris, by simultaneously contracting the iris sphincter and dilator muscles. Thereby this pharmacologic treatment lessens the chance for anterior synechiae formation and IOP elevation.

If perforation of Descemet’s window occurs during a deep sclerectomy, conversion to a standard tra-

beculectomy with basal iridectomy is usually suggested.19 This allows the superficial flap to be sutured more securely than if a collagen sponge were placed in the scleral bed, and therefore hypotony is less likely. The exception to this would be if one were performing Sourdille’s variation of deep sclerectomy in which the superficial flap is closed tightly and therefore conversion to a trabeculectomy would be unnecessary.

Thin Deep Scleral Flap

If the dissection of the deep scleral flap is too shallow, Schlemm’s canal will not be unroofed and the trabeculo-Descement’s window will be too thick to allow for the percolation of aqueous. If this occurs, a deeper plane of dissection must be created to allow both the canal to be unroofed and a thin trabeculoDescemet’s window to be prepared.

To produce well-defined ostia into Schlemm’s canal, it is important to sharply dissect the sides of the deep scleral flap with the diamond or metal blade at an angle perpendicular to the sclera. Sometimes, while creating the ostia into Schlemm’s canal, bleeding will occur due to the episcleral venous pressure. This is a sign that the surgeon is in the correct tissue plane. If stromal fibers overlie the trabecular meshwork or Descemet’s window, aqueous percolation will be decreased. These fibers should be removed with a metal blade or the dull edge of the diamond blade. A scleral planer (Asico AE-2430) that functions like a carpenter’s plane has been designed by Crandall for this purpose. Additionally, an attempt to remove the juxtacanalicular cells and floor of Schlemm’s canal will result in more percolation of aqueous in Caucasians but not in blacks.26 Stegman states that blacks, who frequently have higher initial IOPs than Caucasians, have tightly fused trabecular plates, which prevents aqueous outflow even if the juxtacanalicular cells and the floor of Schlemm’s canal have been removed.

BLEEDING

NPTS

As stated previously, if episcleral bleeding occurs during a viscocanalostomy, minimal cautery should be used because this may compromise the collector channels or the aqueous veins. Vasopressin on a cellulose sponge applied to episcleral bleeders for 1 minute will usually control bleeding. Cautery may be used in both deep sclerectomy and trabeculectomy because these procedures rely on subconjunctival drainage of aqueous and bleb formation for IOP control. Sodium hyaluronate is injected into Schlemm’s canal and the subscleral chamber in the