Chapter 236 – Laser Filtration Procedures
MARTHA MOTUZ LEEN
Laser energy can be delivered either internally (ab interno) or externally (ab externo) to produce a direct opening into the anterior chamber through limbal tissue to achieve filtration. The laser energy may be delivered using a probe (contact) or a gonioscopy lens (noncontact). The rationale for laser sclerostomies is the desire for a glaucoma filtration procedure that is simple, quick, limits the wound-healing response, and minimizes complications.
By using laser energy to disrupt limbal tissue, full-thickness sclerostomies have been created via an ab interno or ab externo approach ( Figs. 236-1 to 236-3 ). Methods for application of laser energy to facilitate dissection in nonpenetrating glaucoma surgery are under development. Currently, most available clinical data are for the contact methods used to create full-thickness sclerostomies—the holmium laser for the ab externo approach     and the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser for the ab interno approach.  These two procedures have emerged as prototypes for laser sclerostomy, and discussion herein is limited to these.
PREOPERATIVE EVALUATION AND DIAGNOSTIC APPROACH
The holmium ab externo sclerostomy is generally most suitable for the creation of blebs in eyes that have heavy conjunctival scarring, as this may severely limit the location of a repeated trabeculectomy but allow the passage of a laser probe. The ideal site for this procedure is over a prior surgical iridectomy, to decrease the risk of postoperative iris incarceration. This technique may offer an advantage over the Nd:YAG ab interno approach in that no intraocular instrumentation is required and the procedure can be performed more safely in eyes that are phakic or have shallow chambers.
Figure 236-1 Noncontact ab interno sclerostomy. Laser energy is directed internally via a goniolens. No conjunctival dissection is required.
The Nd:YAG ab interno sclerostomy is most suitable for the creation of inferior or nasal blebs in eyes that have excessive superior conjunctival scarring. These eyes should be aphakic or pseudophakic to avoid the risk of lens trauma from a probe passing across the anterior chamber. Preoperative laser iridotomy or peripheral iridoplasty in the vicinity of the planned sclerostomy may reduce the risk of postoperative iris incarceration. As an increased risk of infection exists with inferior blebs, this procedure should be avoided in patients who have poor hygiene or chronic blepharitis. This technique may have an advantage over the ab externo method because conjunctival dissection is not required.
Figure 236-2 Contact ab externo sclerostomy. Laser energy is delivered externally via a laser probe. Minimal conjunctival dissection is required.
Figure 236-3 Contact ab interno sclerostomy. Laser energy is delivered internally via a laser probe. No conjunctival dissection is required.
ALTERNATIVES TO SURGERY
In eyes that have not had prior surgery, standard trabeculectomy with or without antifibrotic therapy is currently the favored approach. In eyes with mild to moderate conjunctival scarring from prior surgery, trabeculectomy with an adjunctive antifibrotic agent (see Chapters 239 and 240 ) is an alternative to laser sclerostomy and the preferred approach for eyes in which conjunctival and scleral dissection can be carried out safely. In eyes that have excessive conjunctival scarring, seton placement (see Chapter 241 ) and cyclodestructive procedures are alternative modes of treatment (see Chapter 237 ).
Depending on the individual patient, the holmium and Nd:YAG laser filtration procedures can be accomplished with topical, subconjunctival, peribulbar, or retrobulbar anesthesia.
Ab Externo Technique—Thulium-Holmium-Chromium: Yttrium-Aluminum-Garnet (Holmium) Laser Sclerostomy
Ab externo holmium laser sclerostomy is performed by delivering laser energy through a fiberoptic probe by a subconjunctival approach.
The eye may be rotated inferiorly with a muscle hook to avoid placement of a traction suture. A sclerostomy site is chosen that corresponds to the area with least conjunctival scarring or is adjacent to a prior iridectomy. A 1–2?mm conjunctival incision 8–10?mm away from the intended sclerostomy site and 6?mm from the limbus is made using Vannas scissors. The underlying Tenon’s capsule is also incised until bare sclera is reached. The conjunctiva may be hydrodissected with balanced salt solution or viscoelastic if excessive Tenon’s and/or conjunctival scarring is present.
The probe is then inserted under Tenon’s capsule and advanced carefully toward the limbus. It is manipulated into position at the limbus, as anterior as possible, without folding the conjunctiva under the probe. The aiming beam and laser energy exit 90° from the axis of the probe. With the probe tip held tangential to the limbus, the probe is rotated so that the aiming beam is as parallel as possible to the iris (see Fig. 236-2 ).
The laser is then fired at a repetition rate of 5 pulses per second (80–100?mJ per pulse in previously unoperated eyes; 100–120?mJ per pulse in previously operated eyes). The conjunctiva over the probe tip is irrigated with balanced salt solution while the laser is fired. The surgeon is able to recognize sclerostomy patency from three signs:
• Small bubbles may appear in the anterior chamber;
• The sound of the laser ticking changes from a dull to a sharp noise; and
• A bleb forms upon removal of the probe.
While performing a sclerostomy, care must be taken not to indent the globe, which may result in a sclerostomy with walls that are not parallel to each other, thus creating a smaller internal than external ostium size. If iris incarceration occurs intraoperatively, a peripheral iridotomy can be created with the holmium laser probe. The laser probe is then withdrawn and the conjunctival wound is closed with a running 9-0 Vicryl suture. 5-Fluorouracil can be injected subconjunctivally in an adjacent quadrant.
Ab Interno Technique: Neodymium:Yttrium-Aluminum-Garnet Laser Sclerostomy
Ab interno Nd:YAG laser sclerostomy is performed using laser energy delivered through a fiberoptic probe by a transcameral approach. The laser energy exits from the tip parallel to the long axis of the probe.
Viscoelastic or balanced salt solution is injected subconjunctivally, using a 30-gauge needle, to elevate conjunctiva adjacent to the proposed sclerostomy site. A sharp steel blade or diamond knife is used to create a peripheral corneal paracentesis, approximately 1.5?mm in length, 90–180° away from the proposed sclerostomy site. Viscoelastic is injected intracamerally through the paracentesis site. The laser probe is introduced through the paracentesis and is passed across the anterior chamber until the tip is in contact with the sclera in the region of Schwalbe’s line. A gonioscopy lens can be used to aid visualization.
The aiming beam may transilluminate the sclera in the vicinity of the limbal sulcus. Between 3 and 5 pulses of 200?mJ (10?W, 0.2?sec) are required to achieve filtration. The laser probe is advanced until the probe tip is visualized in the subconjunctival space (see Fig. 236-3 ). Penetration through full-thickness sclera is evident when an adjacent bleb enlarges, and the probe is withdrawn. Additional balanced salt solution or viscoelastic is injected into the anterior chamber to verify the patency of the sclerostomy and further elevate the bleb. The paracentesis is closed with a single 10-0 nylon suture with the knot buried. The conjunctiva that overlies the sclerostomy site is inspected for buttonholes. 5-Fluorouracil can be injected subconjunctivally in an adjacent quadrant.
Attempts can be made to minimize the possibility of iris incarceration. After the probe has been withdrawn from the sclerostomy, an iridoplasty can be performed with several laser applications to prevent iris incarceration. Alternatively, a peripheral iridectomy can be created with a vitrector. If vitreous is present in the anterior chamber, an anterior vitrectomy should be performed through the paracentesis site to minimize the risk of vitreous incarceration.
Iris incarceration is the most frequent complication of full-thickness laser filtration procedures, especially in phakic eyes and those with narrow angles. The risk of this complication can be reduced by performing the sclerostomy over a prior peripheral iridectomy or by the creation of a peripheral iridectomy at the time of the sclerostomy. Often, postoperative iris prolapse can be managed by massage over the sclerostomy site with a Zeiss gonioprism. Alternatively, a peripheral laser iridoplasty or laser iridotomy can be performed. Other complications include hyphema, localized corneal edema, Descemet’s membrane detachment, cyclodialysis cleft formation, conjunctival burn, conjunctival buttonhole, and vitreous hemorrhage.
Laser sclerostomy procedures are associated with overdrainage complications found in other full-thickness procedures, which include hypotony, shallow anterior chamber, choroidal effusion, choroidal hemorrhage, and cataract formation. As minimal conjunctival manipulation helps to preserve episcleral tissue and increase the resistance to aqueous outflow, the occurrence of a shallow or flat anterior chamber may not be as common as expected for a surgical full-thickness procedure.
The success rate of the holmium laser ab externo sclerostomy is 49–55% at 1 year, 36–52% at 2 years, and 26–36% at 4 years.   A success rate of 60% at 2 years for Nd:YAG laser ab interno sclerostomy has been reported. These success figures primarily include eyes at high risk of filtration failure that also have received adjunctive 5-fluorouracil and that may have required supplemental antiglaucoma medication. As a result of these success rates, the role of laser sclerostomy in glaucoma filtration surgery as a primary or secondary procedure is limited. Combination of this procedure with cataract extraction has been described and may be more promising.  Adjunctive administration of mitomycin has been reported to enhance success rates of laser sclerostomy in vivo and in vitro, although methods of application have varied.  
1. O’Donnell FE Jr, Santos BA, Overby J. Laser trabeculodissection with a photopolishing scanning excimer laser. Ophthalmic Surg Lasers. 2000;31:508–11.
2. McAllister JA, Watts PO. Holmium laser sclerostomy: a clinical study. Eye. 1993;7: 656–60.
3. Schuman JS, Stinson WG, Hutchinson BT, et al. Holmium laser sclerectomy: success and complications. Ophthalmology. 1993;100:1060–5.
4. Iwach AG, Hoskins HD, Mora JS, et al. Update on the subconjunctival THC:YAG (holmium) laser sclerostomy ab externo clinical trial: a 4-year report. Ophthalmic Surg Lasers. 1996;27:823–31.
5. Friedman DS, Katz LJ, Leen MM. Holmium laser sclerostomy in glaucomatous eyes with prior surgery: 24 month results. Ophthalmic Surg Lasers. 1998;29:17–22.
6. Wilson RP, Javitt JC. Ab interno laser sclerostomy in aphakic patients with glaucoma and chronic inflammation. Am J Ophthalmol. 1990;110:178–84.
7. Kendrick R, Kollartis CR, Khan N. The results of ab interno thermal laser sclerostomy combined with cataract surgery vs. trabeculectomy combined with cataract surgery 6 to12 months postoperatively. Ophthalmic Surg Lasers. 1996;27:583–6.
8. Iliev ME, Vander Zypen E, Fankhauser F, England C. Spontaneous and pharmacologically modulated wound healing after Nd:YAG laser sclerostomy ab interno in rabbits. Eur J Ophthalmol. 1997;7:24–8.
9. Onada E, Ando H, Jikihara S, Kitizawa Y. Holmium YAG laser sclerostomy ab externo in refractory glaucoma. Int Ophthalmol. 1996–97;20:309–14.
10. Schmidt-Erfurth U, Wetzel W, Drage G, Birngruber R. Mitomycin C in laser sclerostomy: benefit and complications. Ophthalmic Surg Lasers. 1997;28:14–20.