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Aqueous Shunts after Retinal Surgery


Summary

Elevated IOP is a common occurrence following VR surgery and is more likely with increasing complexity of the repair such as with the use of silicone oil or intravitreal gas with pars plana vitrectomy. The cause of the intraocular pressure rise is multifactorial and intervention should be based on the mechanism of pressure elevation. The rise is usually transient with most patients successfully controlled with topical medications alone. A significant minority develop chronic refractory glaucoma and are difficult to manage. Aqueous shunt implantation often offers the best chance of long-term success but is challenging, requiring significant modifications in technique and, in some cases, modifications to the shunt itself.

Introduction

Glaucoma is a significant complication of vitreoretinal (VR) surgery. Acute intraocular pressure (IOP) elevation of greater than 30 mmHg has been reported in up to 35% of eyes following VR surgery and scleral buckling. In complex retinal detachment repairs that require silicone oil for surgical tamponade, elevated IOP has been reported in up to 48% of cases. However, most patients respond to topical medical treatment and the IOP rise is often transient.

The exact incidence of glaucoma is difficult to ascertain from the literature as glaucoma is often defined as variably elevated IOP for a certain period rather than by progressive disc cupping. A proportion of eyes will have pre-existing glaucoma and are therefore at an increased risk of postoperative IOP elevation. In those who develop de novo IOP elevation, the mechanism may be multi-factorial ( Box 118-1 ). Angle closure from forward movement of the iris–lens diaphragm is an important cause in eyes with intraocular tamponade from expansile gas or silicone oil. Secondary pupil block may occur in aphakic eyes with silicone oil. Chronic synechial angle closure may result if either of the above are not promptly corrected or also if there is neovascularization ( Fig. 118-1 ).

Box 118-1
Mechanisms of Glaucoma after Retinal Surgery

Open Angle (Direct Trabecular Meshwork Obstruction)

  • ▪

    Inflammatory or red blood cells

  • ▪

    Silicone emulsion

  • ▪

    Steroid response

  • ▪

    Retinal pigment epithelial cells?

Closed Angle

  • ▪

    Secondary pupil block

    • Gas-pupil block

    • Fibrin-pupil block

    • Silicone pupil block

  • ▪

    Gas expansion

  • ▪

    Neovascularization

  • ▪

    Ciliary body edema and rotation

Figure 118-1, (A) Types of angle closure after retinal surgery. (AA & AB) Pupil block mechanisms of secondary angle closure after retinal surgery are recognized by convexity of the peripheral iris (AA & AB) (transparent arrow), in the presence of a deep (AB) (solid arrow) or relatively deep (AA) anterior chamber. In AA the patient has a relatively shallow anterior chamber. In AB synechial occlusion of the pupil has resulted in absolute pupil block and dramatic peripheral iris bombe. (AC & AD) represent examples of forward movement of the iris lens diaphragm, which may result from an overtight scleral buckle, an overfill of gas or silicone oil, excessive pan-retinal photocoagulation or aqueous misdirection. In contrast to pupil block, this type of angle closure is characterized by an extremely shallow flat central anterior chamber (solid arrows). (B) Tight scleral buckle and angle closure. A normally positioned scleral buckle indents sclera and retina, but not sufficiently to cause forward movement of the iris lens diaphragm (BA). An overtightened encircling band (BB) may result in sufficient compression to rotate the ciliary processes and iris–lens diaphragm forward (solid arrows) with subsequent angle closure (open arrow). An overfill of silicone oil, expansile gas, or loculation of aqueous in the vitreous (aqueous misdirection) also result in angle closure via this mechanism. (C) Silicone oil pupil block and the inferior iridectomy. Silicone oil and expansile gas may also result in angle closure by causing pupillary block in the aphakic eye (1). This is relieved by an inferior peripheral iridectomy (2).

Many eyes have elevated IOP and an open angle and the mechanisms include inflammation, topical corticosteroids, and a chronic foreign body reaction in the angle from exposure to silicone oil emulsion. Although prompt removal of silicone oil is thought to minimize the potential side effects, this may not always be advisable in some eyes with complex vitreoretinal disease. Furthermore, inflammation of the trabecular meshwork is a likely cause of elevated intraocular pressure in some and removal of oil will not necessarily reverse the IOP elevation. Determining the cause of elevated IOP is important as therapeutic intervention should be directed to the mechanism of pressure elevation.

The management of patients who develop medically uncontrolled glaucoma after VR surgery is often challenging. Filtering surgery has a poor prognosis largely due to conjunctival scarring. There is also a risk that residual silicone oil within the bleb will result in progressive scarring in the subconjunctival space, even with the use of antimetabolites. Cyclodestructive procedures such as trans-scleral diode laser cyclophotocoagulation (cyclodiode), can be used to temporize but this often has to be repeated with an increasing risk of hypotony with repeat treatments. Consequently, aqueous shunt implantation is an important part of the therapeutic repertoire for the refractory glaucoma that occurs following retinal surgery.

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