| The glaucomas represent a group of diseases characterized by increased intraocular pressure with resultant retinal and optic disk destruction. Low-tension glaucoma, characterized in humans by normal levels of intraocular pressure and progressive optic disk damage, has not been documented in domestic animals. In dogs, the glaucomas occur in ~1.7% of the canine population in North America. The frequency of bilateral breed-predisposed glaucomas in purebred dogs is the highest of any
animal species, except humans (0.9%). In cats, the glaucomas are predominately secondary to anterior uveitis and neoplasms; however, primary open-angle glaucoma occurs. In horses, the glaucomas appear underdiagnosed because applanation tonometry is not routinely done; they appear most frequently in older animals, Appaloosas, and with concurrent anterior uveitis. In cattle, the glaucomas have been associated with congenital iridocorneal anomalies and anterior uveitis. |
| Diagnostic procedures essential to manage the glaucomas include tonometry, ophthalmoscopy (direct and indirect), and gonioscopy (visualization of the iridocorneal angle and anterior ciliary cleft). Newer techniques, such as pattern electroretinograms and visual evoked potentials, estimate damage to the retinal ganglion cells and their axons, and appear to be sensitive indicators of glaucoma-related destruction of these cells. In small animals, both Schiotz indentation and
applanation tonometers are used to estimate intraocular pressure; in horses and cattle, only applanation-type tonometers can be used. Intraocular pressure is reasonably consistent in most species (see
Table:
Intraocular Pressure (IOP) by Applanation Tonometry ) and diurnal variations have been documented in the dog, cat, and rabbit. Ophthalmoscopy permits detection of the intraocular pressure-related damage to the retina and optic disk. Gonioscopy is the basis for classification of all glaucomas; it detects iridocorneal outflow changes as the glaucoma progresses and helps determine the most appropriate medical and surgical treatments. Ultrasound biomicroscopy permits further examination of the anterior
chamber angle and the entire ciliary cleft.
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| Clinical signs are traditionally divided into acute and chronic; in reality, most cases of acute glaucoma are superimposed on chronic glaucoma rather than occurring as singular events. Most dogs with early to moderate chronic glaucoma are not taken to the veterinarian because the early clinical signs—sluggish to slightly dilated pupils, mild bulbar conjunctival venous congestion, and early enlargement of the eye (buphthalmia or megaloglobus)—are so subtle. To detect
early glaucoma, tonometry should be routinely performed on high-risk breeds of dogs as part of the general physical examination. The clinical signs of acute and often markedly increased levels of intraocular pressure are a dilated, fixed, or sluggish pupil; bulbar conjunctival venous congestion; corneal edema; and a firm globe. With prolonged increases of intraocular pressure, secondary enlargement of the globe, lens displacement, and breaks in Descemet’s membrane of the cornea
result. Pain usually is manifest by behavioral changes and occasional periorbital pain rather than by blepharospasm. |
| Classification of the glaucomas assists in the optimal plan for clinical management and preservation of vision. The choice of medical or surgical treatment, or most frequently a combination of both, is based on the progressive iridocorneal angle closure that occurs in most of the canine glaucomas. For open-angle glaucoma in dogs, short- and longterm management is by treatment with miotics, topical and systemic carbonic anhydrase inhibitors, prostaglandins, osmotics, and β-blocking
adrenergics. These same treatments are used for initial control of narrow and closed-angle glaucoma, but short- and longterm management often requires surgery, eg, filtering procedures, anterior chamber shunts, cyclocryotherapy, or laser transcleral cyclophotocoagulation. Short- and longterm management of end-stage glaucoma with buphthalmia and blindness in dogs also requires surgery, eg, intrascleral prosthesis, enucleation, cyclocryothermy, or intravitreal gentamycin (10-25 mg)
combined with 1 mg dexamethasone. Surgical procedures in dogs have traditionally provided only short-term resolution because the filtering fistulas eventually scar over and fail. More recently, anterior chamber shunts, with and without valves, offer improved results. Antifibrotic drugs, such as mitomycin C and 5-fluorouracil, may delay or prevent scarring of the alternate aqueous outflow channels and prolong their function. |
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