Drugs and Biological Agents Used in Ophthalmic Surgery

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adjuncts in anterior segment surgery Viscoelastic substances assist in ocular surgery by maintaining spaces, moving tissue, and protecting surfaces. These substances are prepared from hyaluronate, chondroitin sulfate, or hydroxypropylmethylcellulose, share the following important physical characteristics: viscosity, shear flow, elasticity, cohesiveness, and coata-bility, and are broadly characterized as dispersive or cohesive. They are used almost exclusively in anterior segment surgery. Complications associated with viscoelastic substances are related to transient elevation of Iop after the procedure.

ophthalmic glue Cyanoacrylate tissue adhesive (isodent, dermabond, histoacryl), while not FDA approved for the eye, is widely used in the management of corneal ulcerations and perforations. It is applied in liquid form and polymerized into a solid plug. Fibrinogen glue (tisseel) is increasingly being used on the ocular surface to secure tissue such as conjunctiva, amniotic membrane, and lamellar corneal grafts.

corneal band keratopathy Edetate disodium (disodium EDTA; endrate) is a chelating agent that can be used to treat band keratopathy (i.e., a calcium deposit at the level of Bowman's membrane on the cornea). After the overlying corneal epithelium is removed, it is applied topically to chelate the calcium deposits from the cornea.

anterior segment gases Sulfur hexafluoride (SF6) and perfluoropropane gases have long been used as vitreous substitutes during retinal surgery. In the anterior segment, they are used in nonexpansile concentrations to treat Descemet's detachments, typically after cataract surgery. These detachments can cause mild-to-severe corneal edema. The gas is injected into the anterior chamber to push Descemet's membrane up against the stroma, where ideally it reattaches and clears the corneal edema.

vitreous substitutes The primary use of vitreous substitutes is reattachment of the retina following vitrectomy and membrane-peeling procedures for complicated proliferative vit-reoretinopathy and traction retinal detachments. several compounds are available, including gases, perfluorocarbon liquids, and silicone oil (Table 63-6). With the exception of air, the gases expand because of interaction with systemic oxygen, carbon dioxide, and nitrogen, and this property makes them desirable to temporarily tamponade areas of the retina. However, use of these expansile gases carries the risk of complications from elevated IOP, subretinal gas, corneal edema, and cataract formation. The gases are absorbed over a time period of days (for air) to 2 months (for perfluoropropane).

The liquid perfluorocarbons, with specific gravities between 1.76 and 1.94, are denser than vitreous and are helpful in flattening the retina when vitreous is present. If a lens becomes dislocated into the vitreous, a perfluorocarbon liquid injection posteriorly will float the lens anteriorly, facilitating surgical retrieval. This liquid can be an important tool for flattening and unrolling severely detached and contorted retinas such as those found in giant retinal tears and proliferative vitreo-retinopathy but are potentially toxic if it remains in chronic contact with the retina.

Silicone oil has had extensive use for long-term tamponade of the retina. Complications from silicone oil use include glaucoma, cataract formation, corneal edema, corneal band keratopathy, and retinal toxicity.

surgical hemostasis and thrombolytic agents Hemostasis has an important role in most surgical procedures and usually is achieved by temperature-mediated coagulation. In some intraocular surgeries, thrombin has a valuable role in hemostasis. Intravitreal administration of thrombin can assist in controlling intraocular hemorrhage during vitrectomy. When used intraocularly, a potentially significant inflammatory response may occur, but this reaction can be minimized by thorough irrigation after hemostasis is achieved. This coagulation factor also may be applied topically via soaked sponges to exposed conjunctiva and sclera, where hemo-stasis may be a challenge due to the rich vascular supply. Topical aminocaproic acid (caprogel) has been advocated to prevent rebleeding after traumatic hyphema (blood in the anterior chamber), but recent clinical trials report mixed success. Depending on the intraocular location of a clot, there may be significant problems relating to IoP, retinal degeneration, and persistent poor vision. Tissue plasminogen activator (t-PA) (see Chapter 54) has been used during intraocular surgeries to assist evacuation of a hyphema, subretinal clot, or nonclearing vitreous hemorrhage. t-PA also has been administered subconjunctivally and intracamerally (i.e., controlled intraocular administration into the anterior segment) to lyse blood clots obstructing a glaucoma filtration site. The main complication related to the use of t-PA is bleeding.

botulinum toxin type a in the treatment of strabismus, ble-pharospasm, and related disorders Botulinum toxin type A (botox) is used to treat strabismus, blepharospasm, Meige's syndrome, spasmodic torticollis hemifacial spasm, facial wrinkles, and certain migraine headaches (see also Chapter 9). By preventing acetylcholine release at the neuromuscular junction, botulinum toxin A usually causes a temporary paralysis of the locally injected muscles. The variability in duration of paralysis may be related to the rate of developing antibodies to the toxin, upregulation of nicotinic cholinergic postsynaptic receptors, and aberrant regeneration of motor nerve fibers at the neuromuscular junction. Complications related to this toxin include double vision (diplopia) and lid droop (ptosis).

Table 63-6

Vitreous Substitutes

Vitreous Substitute

Chemical Structure

Characteristics (Duration of Viscosity)

Nonexpansile gases Air, Ar, CO2, He, Kr, N2, O2, Xe

Expansile gases Sulfur hexafluoride

Octafluorocyclobutane

Perfluoromethane

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