Although osmotic drugs are not useful in the long-term medical management of chronic glaucoma, they may be invaluable in the therapy of acutely elevated IOP and in the perioperative treatment of certain glaucoma patients.15,21-23

8.7.1 Angle-Closure Glaucoma. The therapy of acute angle-closure glaucoma is directed at lowering IOP and opening the anterior chamber angle. Osmotic drugs are one of the mainstays in the treatment of angle-closure glaucoma because they not only lower IOP but also facilitate opening of the angle. Vitreous dehydration caused by osmotic drugs allows the lens and iris to move posteriorly, thereby deepening the anterior chamber. In addition, the iris sphincter is often nonreactive due to relative ischemia when IOP is elevated. Rapid reduction of IOP by osmotic drugs may relieve this ischemia, permitting sphincter function and miosis and facilitating opening of the anterior chamber angle.

In many instances, therapy with osmotic drugs may be adequate to open the anterior chamber angle, thereby terminating the attack of acute glaucoma. Laser iri-dectomy may then be performed immediately or several days later, when the eye is less congested. If the angle remains closed after treatment with an osmotic drug, IOP will likely be lowered for a period of time, which may reduce corneal edema and facilitate iridectomy.

Oral isosorbide or glycerol is easier to administer in an office setting compared with intravenous mannitol. Isosorbide may cause less nausea and vomiting than does glycerol, and isosorbide is not metabolized to glucose, which is an advantage in diabetic patients. However, isosorbide is no longer commercially available. When nausea and vomiting or blood sugar considerations preclude the use of an oral osmotic drug, intravenous mannitol may be administered.

8.7.2 Secondary Glaucomas. In the secondary glaucomas, osmotic drugs are useful in the treatment of disorders characterized by transient but highly elevated IOP, glaucomas requiring control of IOP until the underlying problem is corrected, and disorders requiring glaucoma surgery that would benefit from preoperative reduction of IOP. Oral osmotic drugs have been given daily or even up to two or three times daily for up to several weeks without complications. In this situation, use of isosorbide avoids the large caloric load that would be ingested with glycerol therapy.

Patients with certain uveitic and posttraumatic glaucomas may be treated with osmotic drugs when they present with markedly elevated IOP that is expected to improve, as is the case in eyes with inflammation or blood in the anterior segment. The transiently and highly elevated IOP sometimes observed after cataract surgery or penetrating keratoplasty may be treated with osmotic drugs.32

These drugs are also helpful when reduction of IOP is beneficial prior to correction of the underlying cause, such as a tight scleral buckle or an intumescent lens. Surgery for lens-induced glaucoma is safer when performed at normal IOP following osmotic therapy. Dehydration and shrinkage of the vitreous may facilitate opening the angle and normalizing IOP prior to lens removal for phacomorphic glaucoma.

Osmotic drugs may be used preoperatively in eyes with extremely high IOP that require glaucoma surgery, such as neovascular glaucoma, to minimize the degree of intraoperative decompression. These drugs may delay the need for surgery, which can allow, for example, reduction of inflammation with corticosteroid therapy. In markedly inflamed eyes, glycerol or mannitol may be preferable because they penetrate the eye poorly.

8.7.3 Aqueous Misdirection. Although initial medical therapy for aqueous misdirection is with mydriatic-cycloplegic drugs and aqueous suppressants, the influence of osmotic drugs on the vitreous may be helpful in the therapy of this disorder.33-35 These drugs at least temporarily dehydrate the vitreous and reduce its volume, which may facilitate correction of aqueous misdirection.

8.7.4 Perioperative Use. Some glaucoma patients may benefit from osmotic therapy during the perioperative period. In combined cataract and filtration surgery, for example, softening of the eye may be desirable, especially when IOP is even mildly elevated. Osmotic drugs may be used preoperatively or intraoperatively to reduce positive vitreous pressure, or postoperatively to treat transient elevation of IOP. This situation may be encountered, for example, in the open-angle glaucoma patient who undergoes cataract surgery without concomitant filtration surgery. Some glaucoma patients may be treated with osmotic drugs prior to surgical therapy for corneal, retinal, or other disorders.

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