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Complications of Nonpenetrating Glaucoma Surgery
Summary
As with any glaucoma surgery, patients should be told that unlike cataract surgery, nonpenetrating glaucoma surgery is neither a sight-improving operation nor is it a permanent cure for glaucoma. Follow-up continues to be essential. However, balance between safety and efficacy of properly made NPGS undoubtedly makes the learning efforts of this technique worthwhile.
Introduction
Nonpenetrating glaucoma surgery (NPGS) is now recognized as being associated with fewer and less severe complications than trabeculectomy. However, over the years, its efficacy versus trabeculectomy has been greatly debated. Some of the comparative studies show better results with trabeculectomy. However, with careful patient selection, proper surgical technique, adequate postoperative follow-up including use of lasers, needlings and other agents (antimetabolites; anti-VEGF) as routinely accepted for trabeculectomy, NPGS may offer equivalent results even when requesting low intraocular pressures (IOP). NPGS is thus particularly important in eyes with severe glaucomatous damage or with already compromised blood–aqueous barrier (BAB), when marked IOP fluctuations can be detrimental, and in monophthalmic eyes. Indications for trabeculectomy, however, remain wider. NPGS is certainly not suitable for every glaucoma patient and careful preoperative assessment is crucial. Primary and secondary angle-closure glaucoma are contraindicated unless iris apposition to the trabeculum can be relieved and the angle opened by preoperative iridotomy/iridoplasty or cataract extraction done simultaneously ( Fig. 99-1 ) or before. NPGS is also contraindicated in neovascular glaucoma, when the anatomy of the anterior segment is lost or when there is significant scarring. The author would not recommend it either for congenital glaucoma.
To optimize NPGS efficacy, several prerequisites should be considered. The surgical site in association with gonioscopy findings should be decided preoperatively. Any inflammation should be optimally treated, occasionally with oral steroids or immunosuppressants. Routine preoperative use of topical anti-inflammatory drugs as advised by some surgeons in trabeculectomy should also be considered. Control of ocular surface diseases (OSD) and of other ophthalmic, cardiovascular and metabolic disorders should be aimed at. When possible oral antiplatelet and anticoagulant treatments and possibly others (e.g. Gingko biloba) should be interrupted. In some high-risk patients warfarin may need to be replaced by short-acting heparin for a few days. Ideally drops with a negative effect on BAB (pilocarpine, prostaglandin analogs) should be stopped preoperatively. Oral carbonic anhydrase inhibitors may be temporarily required. With very high IOP, intravenous hyperosmotic agents may be used just preoperatively. Preservative-free topical therapy should be given preference, but still remains currently uncommon.
Today basic NPGS can be described as deep sclerectomy (DS) including the variant very deep scerectomy, viscocanalostomy and canaloplasty. Mainly complications specific to DS are discussed in this chapter.
Intraoperative Complications
Traction Suture.
A well-positioned bridle suture is mandatory as good visualization of the surgical site under no tension is required. This can be obtained by preferably passing it through the cornea rather than under one of the rectus muscles, as it can potentially be associated with tendon severing, subconjunctival hemorrhage, increased risks of bleb failure, conjunctival holes and even globe perforation. Nevertheless corneal sutures can also be too superficial and associated with corneal cheese-wiring or can be too deep with subsequent hypotony and more difficult further dissection.
Conjunctival Flap.
In trabeculectomy, fornix-based flaps have been reported to be morphologically better and safer than limbal-based ones with less risk of cystic and avascular blebs and bleb-related infections. Similarly after DS with fornix-based conjunctival flaps, the bleb is more diffuse with less scarring, according to the author's experience. Use of sponges, soaked in antimetabolites, dissection in enophthalmos or in eyes with small palpebral fissures and combined procedures (i.e. with cataract extraction) are also made easier with limbal conjunctival incision. However, watertight closure of the corneo-conjunctival wound is slightly more difficult and the symptom of foreign body sensation is increased with limbal sutures. In all cases, the conjunctiva should be handled with care to avoid hole/tear formation.
Site, Mode of Application, Exposure Time, Concentration and Timing of Antimetabolites/Other Adjunctive Agents.
These should be considered when known risk factors for enhanced fibrosis exist and/or low postoperative IOP are required as they improve the efficacy of DS. Their use, intra- and/or postoperatively, also depends on the surgeon's experience and choice. The optimum concentration and exposure time of antimetabolites are not known although higher concentrations and longer exposure are associated with increased incidence of complications. Their use should therefore be tailored to each patient according to the known risk factors for bleb failure. A 0.02% concentration of mitomycin C for 2 to 3 minutes is most commonly used or recommended. These agents should not come into contact with the edges of the conjunctival incisions. The largest area under the superficial scleral flap (SSF) and between the sclera and Tenon's capsule should be exposed to the drug. If sponges are used as the mode of application, they should all be removed completely and the area should then be thoroughly irrigated with balanced salt solution (BSS). Again care should be given to the conjunctiva when introducing or removing the sponges.
Paracentesis.
This is not mandatory but highly advisable, as it facilitates trabeculo-Descemet's membrane (TDM) dissection by decreasing the IOP. It also allows slow ocular decompression and with TDM perforation diminishes important IOP fluctuations and associated complications in high-risk cases (e.g. high myopia, bleeding tendencies). Anterior chamber (AC) reformation and subsequent filtration testing are then enabled. Paracentesis should be done just before entering Schlemm's canal (SC) to facilitate scleral dissection.
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