CHAPTER Glaucoma outflow procedures

General Considerations

If there is no internal flow block and intraocular pressure (IOP) remains too high despite maximally tolerated medical therapy, surgery to relieve outflow block is needed. Such procedures are designed to increase the flow of aqueous out of the eye, thus reducing IOP.

Laser trabeculoplasty to relieve outflow block is described in detail in Chapter 31. Laser trabeculoplasty is generally attempted before incisional surgery unless the IOP is very high or the optic nerve is severely damaged. Incisional surgery is sometimes the only viable intervention: when subnormal IOPs may be required, as in progressive disease; or when conditions are not amenable to trabecular laser response, such as inflammatory, traumatic, or developmental glaucomas; when the angle is damaged or covered by synechiae; or the cornea is clouded. Several studies have supported using filtration surgery as the initial therapy in routine open-angle glaucoma, citing better mediumand long-term visual outcome as one of the major benefits.1,2 Although this remains an area of active debate,3–5 there is widespread agreement that the individual circumstances of the patient must be evaluated before the appropriate initial therapy can be chosen.

Incisional surgery to relieve outflow block may create external filtration (e.g., trabeculectomy or full-thickness filtering procedures) or internal filtration (e.g., cyclodialysis), or it may essentially disrupt the trabecular meshwork from the outflow pathway (e.g., trabeculotomy ab externo and goniotomy). Regardless of the procedure used, the goal is to reduce the IOP to a level that will prevent further damage to the optic nerve but not reduce it so much as to cause problems from hypotony. The lowest IOP that can be tolerated by the eye is generally above 5 mmHg, although this may depend on the patient’s age: older patients (over 55 years old) can often retain 20/20 acuity with IOPs under 4 mmHg, whereas younger patients (with presumably more elastic and deformable sclera) may develop vision-altering hypotonous maculopathy at similar tensions.6–8 Low single-digit IOPs can predispose to ­cataracts, choroidal effusion, optic nerve swelling, or refractive instability. (Of course the central corneal thickness (CCT) correction for applanation readings needs be considered: some ‘low’ IOPs are, when adjusted for CCT, actually above the ‘hypotony’ range.)

There is no evidence to support the notion that a specific protective effect is conferred on glaucoma patients whose pressure is simply reduced to 20 mmHg or lower, as though this were a magical number. Some long-term studies indicate that more severely damaged nerves may require pressures in the low teens if damage is to be stopped;9–11 with a significant advantage conferred by reducing IOP fluctuations.12,13 Although it is felt that external filtration of aqueous does in fact dampen IOP fluctuations,13b the role of fluctuating pressures per se in contributing to progressive glaucomatous

loss remains controversial. Some authors assert that fluctuating

IOPs contribute neither to the conversion of ocular hypertensives into glaucoma nor to destabilization of visual field function.13c,13d

Long-terms results of the Advanced Glaucoma Intervention Study

(AGIS), however, suggest that IOP fluctuation is a ‘risk factor’ for continued visual field and optic nerve deterioration.13e,13f

There is ample prospective evidence to substantiate that lowering the IOP in glaucoma patients slows the rate of visual field loss, even in normal-tension glaucoma.14,15 Rather than choosing a specific target pressure, most multicenter studies prospectively select an end-point percentage for pressure reduction as the research target goal: e.g., a 30% reduction in the Normal-Tension Glaucoma Study,16,17 25% reduction in the Early Manifest Glaucoma Trial,18 or 20% reduction in the Ocular Hypertensive Treatment Study.19

Full-thickness procedures generally provide lower pressures for a longer time than did guarded filtration procedures viz trabeculectomies before the era of antimetabolite usage.20–22 However, such full-thickness procedures also had a higher complication rate in most surgeons’ hands. Efforts continue to achieve better pressure control with fewer complications using modifications of trabeculectomy technique, pharmacologic modifications of wound healing, and manipulations of flap closure using releasable sutures or laser suture lysis.

External Filtration Surgery

The goal of external filtration is to create a new drainage pathway that allows aqueous to pass from the anterior chamber into the subconjunctival space.There the fluid either is absorbed into the conjunctival blood vessels or lymphatic equivalents or, if the bleb is thin walled, passes directly across the conjunctiva into the tear layer.23–26

Filtering surgery requires an opening through the scleral wall at the limbus. The surgeon makes this opening much larger than the 15- m diameter hole that (theoretically) is adequate for total aqueous flow out of the eye,27 because the healing process works to reduce the ultimate or effective size of the opening. Indeed, the healing process often obliterates the opening entirely. A larger initial opening, however, does not ensure success and may in fact lead to higher failure and complication rates. These rates increase because initial hypotony causes production of secondary aqueous, which apparently contains factors which accelerate wound healing,28 and may eventually reduce the flow of aqueous humor through the sclerostomy. This causes the episcleral surface to scar down around the sclerostomy and close it.

Early postoperative hypotony is to be avoided when possible. The ideal procedure would lower IOP to 8–10 mmHg immediately and keep it there. Ideally the collagenolytic activity of pure aqueous