Chapter 9 Surgical procedures of the anterior chamber and anterior uvea
Introduction
Small isolated iridal neoplasms in dogs and horses may be treated by iridectomy or iridocyclectomy combined with excision of the adjacent sclera, and more recently by diode laser photocoagulation (Fig. 9.3a,b). Diffuse iridal melanomas in cats are managed differently, because the potential for local infiltration and metastasis is greater. Cats with diffuse iridal melanomas are usually presented with a progressive brown to black pigmentation of the iris (Fig. 9.3c). The iridal mass increases in thickness late in the disease. Pupillary changes, secondary glaucoma, hyphema, and retinal detachments indicate that the iridal neoplasm is advanced, and an enucleation should be performed. Controversy exists among veterinary ophthalmologists and veterinary pathologists as to early clinical management of these neoplasms when the only clinical sign is the iridal pigmentation, which is progressing slowly. Diffuse iridal melanomas in cats usually involve the majority of the iris and are not amenable to sectional iridectomy or iridocyclectomy.
Pathophysiology
Box 9.1 Benefits of iridocycloplegia during iridocyclitis
• Administration of a mydriatic or combination of mydriatics to constantly change the pupil size, produce iridal movement, and discourage iridal attachments to the lens or posterior cornea.
• Dilatation of the pupil to position the majority of the iris near the peripheral lens and away from the closer central and visually important axis.
• Dilatation of the pupil to prevent obstruction with inflammatory materials or formation of annular (360°) posterior synechiae.
• Suppress iridociliary inflammation and tissue swelling.
• Paralyze the iridal sphincter and ciliary body musculature to minimize the pain from iridocyclitis.
• Restore the blood–aqueous barrier to decrease as much as possible the cellular and protein (fibrin) content of the secondary or plasmoid aqueous humor, and reduce the possibility of formation of fibropupillary membranes.
Surgery of the anterior chamber
Keratocentesis/anterior chamber paracentesis
In keratocentesis a small gauge (25–30) hypodermic needle is inserted into the peripheral clear cornea or limbus to enter the anterior chamber and aspirate a small amount (0.1–0.2 mL) of aqueous humor. Alternatively, this technique may also be used for intracameral injection of materials such as tPA, adrenaline (epinephrine) or antibiotics. The indications for keratocentesis are summarized in Box 9.2. The aqueous humor sample has a limited volume, usually 0.1–0.3 mL. The value of each diagnostic procedure (cytology, culture, protein analyses, antibody titers) may require prioritization and only the most important tests performed (Fig. 9.11).
Box 9.2 Indications for keratocentesis in animals
• Aqueous humor cytology may assist in the diagnosis of the anterior uveal inflammation.
• Aqueous humor culture may determine the infectious organism.
• Aqueous humor titers to selected diseases.
• Aspiration of small iris/ciliary body cysts.
• To abruptly decrease intraocular pressure in patients with medically non-responsive glaucoma.
For keratocentesis, a 25–30 g hypodermic needle and 1 mL syringe are used (Fig. 9.12a). A thumb forceps is used to grasp the bulbar conjunctiva and stabilize the eye. The hypodermic needle is directed through the peripheral cornea into the anterior chamber at an angle that avoids contact with the posterior cornea and the anterior iris (Fig. 9.12b). A small volume (0.1–0.3 mL) of aqueous humor is withdrawn. The needle bevel may be positioned down (or toward the iris) to avoiding snagging the iris and to provide a self-sealing needle puncture.
As a variation of the clear corneal method, the hypodermic needle is inserted in the bulbar conjunctiva 2–4 mm posterior to the limbus (Fig. 9.13a). It is then carefully moved forward subconjunctivally to the limbus, and then inserted through the limbus at an angle between the posterior cornea and anterior iris (Fig. 9.13b). After aqueous humor sampling, the hypodermic needle is carefully and slowly retracted. If some aqueous humor leaks from the limbal penetration, it remains trapped in the bulbar subconjunctival space.