Chapter 137 Glaucoma
Glaucoma is an increase in the intraocular pressure (IOP) beyond that compatible with maintenance of normal ocular physiology and function.
ETIOLOGY
Primary Glaucoma
Primary glaucoma is not associated with any other event or problem within the eye. It is usually breed related and often hereditary in nature. A partial list of predisposed breeds is given in Table 137-1.
Secondary Glaucoma
• Anterior uveitis can cause glaucoma through obstruction of the flow of aqueous humor. This results from peripheral anterior or posterior synechiae, inflammatory debris in the iridocorneal angle, or the formation of a pre-iridal fibrovascular membrane. Anterior uveitis is discussed in Chapter 136.
• Anterior lens luxation can obstruct the flow of aqueous humor from the posterior to the anterior chamber or to the iridocorneal angle. The etiologies of anterior lens luxation include hereditary (in terriers), intraocular neoplasia, anterior uveitis, and trauma (see Chapter 135). Anterior lens luxation can also result from chronic glaucoma with enlargement of the globe (buphthalmos) and tearing of the lens zonules.
• Intraocular neoplasia, whether primary or secondary, can block the flow of aqueous humor through shedding of neoplastic cells into the iridocorneal angle or displacing of normal intraocular structures (see Chapter 136). In addition, intraocular neoplasia can result in the formation of a pre-iridal fibrovascular membrane.
• Hyphema can result from trauma, systemic hypertension, congenital ocular anomalies, vascular disorders, or bleeding disorders (see Chapter 136). Red blood cells can obstruct the trabecular meshwork of the iridocorneal angle resulting in ghost cell glaucoma.
• Trauma can result in anterior uveitis, anterior lens luxation, hyphema, or damage to the iridocorneal angle, all of which may lead to secondary glaucoma.
• Retinal detachment, when chronic, may be associated with proliferation of a pre-iridal fibrovascular membrane arising from the anterior iris stroma and resulting in anterior and posterior synechiae and iridocorneal angle obstruction. Chronic uveitis and intraocular neoplasia can result in a similar change.
CLINICAL SIGNS
• Acute glaucoma is a true medical and ultimately a surgical emergency. The diagnosis is made based on history and clinical signs (Table 137-2). If in doubt, err on the side of a diagnosis of acute rather than chronic glaucoma.
• Chronic glaucoma implies ophthalmic changes that cause the irreversible loss of vision (Table 137-3). The clinical changes most indicative of chronic glaucoma are retinal and optic nerve degeneration and buphthalmos.
Table 137-2 CLINICAL SIGNS OF ACUTE GLAUCOMA
Slow to absent pupillary light response
Weak to absent menace response
DIAGNOSIS
• IOP measurement requires a tonometer. Digital tonometry is an inaccurate and unreliable method of IOP determination. While previous recommendations suggested that the Schiφtz tonometer was the most cost-effective tonometer for private practice, the Tonopen (Mentor O&O, Inc.) is a more portable, accurate, and user-friendly tonometer and will result in more frequent and more accurate intraocular pressure readings. This in turn will improve the veterinarian’s ability to make an early diagnosis and more accurately assess response to treatment (see Chapter 131). The Tonopen is lightweight, portable, accurate, and self-calibrating, and it averages several readings and gives a percentage error to ensure accuracy. In addition, the small foot plate allows this tonometer to be used on painful eyes in less cooperative patients because only a small area of cornea is required to obtain a reading, and the position of the patient’s head is not related to obtaining the reading. Normal IOP in the dog and cat is 15 to 25 mm Hg, although most ophthalmologists are concerned with when IOP exceeds 20 mm Hg.
TREATMENT
In order to treat glaucoma appropriately, answer two specific questions (Fig. 137-1). Is the glaucoma primary or secondary? Is the glaucoma acute or chronic?
• Treatment can be grouped into medical and surgical approaches. If the glaucoma is acute, a return of a portion of, or all of, the animal’s vision is then possible. In acute glaucoma, immediate aggressive medical therapy is required to reduce the IOP to within normal range. Failure of medical therapy to lower and maintain IOP at a normal level indicates the need for surgical intervention and the possible referral to a veterinary ophthalmologist.
• The most appropriate treatment for chronic glaucoma is surgery, the goal of which is to reduce the IOP in order to relieve discomfort. Medical therapy of chronic glaucoma is neither effective nor cost-effective over time. The surgery of choice is either placement of an intraocular silicone prosthesis or an enucleation.
• Intravitreal injection of gentamicin or lidofovir (Vistide) designed to destroy the intraocular tissues is used by some to reduce intraocular pressure. This results in uveitis, cataract, and often phthisis bulbi and is cosmetically unappealing and unpredictable.
Medical
Medical therapy of acute glaucoma includes some, or possibly all, of the following agents (Table 137-4). The choice of medication varies according to the severity of the glaucoma, the etiology, and the response to the initial therapy.
Table 137-4 MEDICAL TREATMENT OF ACUTE GLAUCOMA
| Drug/Concentration | Trade Name | Dose |
|---|---|---|
| Systemic | ||
| Osmotic Drugs | ||
| Mannitol | 0.5–1.0 g/kg, IV | |
| Carbonic Anhydrase Inhibitors | ||
| Dichlorphenamide | Daranide | 2–4 mg/kg, q8–12h, PO |
| Methazolamide | Neptazane | 2–4 mg/kg, q8–12h, PO |
| Topical | ||
| Carbonic Anhydrase Inhibitors | ||
| Dorzolamide 2% | Trusopt | q8–12h |
| Dorzolamide 2%:timolol 0.5% | Cosopt | q8–12h |
| Parasympathomimetics | ||
| Pilocarpine 2% | Isopto Carpine | q8–12h |
| Sympathomimetics | ||
| Epinephrine 1% | q8–12h | |
| Dipivefrin HCl 0.1% | Propine | q8–12h |
| Combination | ||
| Pilocarpine 2%:epinephrine 1% | E-Pilo-2 | q8–12h |
| Sympatholytics | ||
| Timolol maleate 0.5% | Timoptic | q8–12h |
| Prostaglandins | ||
| Latanoprost 0.005% | Xalatan | q12–24h |
Osmotic Drugs
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