Chapter 252

Glaucoma is one of the most frustrating conditions to treat in veterinary ophthalmology. In most cases, despite intensive therapy, the disease process continues and eventually leads to loss of vision. Glaucoma is a group of diseases that have an abnormally elevated intraocular pressure (IOP) as a common feature. The elevated IOP may cause irreversible damage to the retina and optic nerve and lead to blindness. Clinical symptoms in cats often are subtle, and many cats are not brought for treatment until late in the course of the disease when the eye is already permanently blind.

The normal IOP range in cats has been reported to be between 15 and 25 mm Hg (mean, ~20 mm Hg). IOP is generated by aqueous humor production and outflow, which normally are in equilibrium. The elevated IOP in glaucoma is due to reduced outflow of aqueous humor rather than overproduction of aqueous humor. Glaucoma can be categorized as congenital, primary, or secondary.

Congenital glaucoma is very uncommon in cats. It is due to abnormalities in the aqueous humor outflow pathway and generally occurs in very young patients (<6 months of age). Most kittens with congenital glaucoma have an acute onset of buphthalmia (enlarged globe) and corneal edema (Figure 252-1). Congenital glaucoma can be either unilateral or bilateral and may occur along with other ocular abnormalities.

In cats primary glaucoma also is very rare. Primary glaucoma is not associated with any other ocular disease and in veterinary medicine is classified as open, narrow, or closed angle, depending on the appearance of the drainage angle. It has been reported in certain breeds including Burmese, Persian, European shorthair, and Siamese. The increased incidence of primary glaucoma in purebred cats likely is due to inbreeding. Pectinate ligament dysplasia is another form of primary glaucoma that has been reported in a group of related Siamese cats. Primary glaucoma is a bilateral, heritable condition in dogs, and although the mode of inheritance has not been established in cats, breeding of cats with primary glaucoma is not recommended.

Secondary glaucoma occurs when another disease condition causes a decrease in the outflow of aqueous humor from the eye. In cats secondary glaucoma is the most common form of glaucoma. Several conditions can cause an increase in IOP, including uveitis, hyphema, intraocular neoplasia, and aqueous misdirection. The two most common causes of secondary glaucoma in cats are chronic uveitis (intraocular inflammation) and neoplasia.

Uveitis is one of the most common and clinically important ophthalmic disorders in domestic cats (see Chapter 250). Uveitis in cats has been associated with trauma, primary or metastatic neoplasia, abnormalities of the lens, infectious agents, and idiopathic causes. In a retrospective histopathologic study of 158 eviscerated or enucleated feline globes, idiopathic lymphocytic-plasmacytic uveitis was found to be the most common type of uveitis (Peiffer and Wilcock, 1991). Infectious agents that have been associated with feline uveitis include Toxoplasma gondii, feline immunodeficiency virus, feline leukemia virus, feline infectious peritonitis virus, and fungi causing systemic infections (Blastomyces dermatitidis, Cryptococcus neoformans, and Histoplasma capsulatum).

Although it is important to establish a cause for uveitis, standard diagnostic studies such as cytologic analysis of aqueous humor and serologic testing often are unsuccessful, and the majority of cases are classified as idiopathic. This may be related to the sensitivity and specificity of the tests as well as the stage of the disease process when the cat shows signs of uveitis. In general a complete blood count (CBC), chemistry panel, urinalysis, feline leukemia virus assay, feline immunodeficiency virus assay, and possibly feline infectious peritonitis tests should be performed in cats with intraocular inflammation. Other diagnostic tests can be performed depending on the geographic location of the cat and the general health of the patient. Treatment of uveitis is aimed at decreasing intraocular inflammation and relieving signs of discomfort. Antiinflammatory medications in addition to a specific antimicrobial agent when an infectious cause has been identified are recommended for treatment of uveitis. In general, topical antiinflammatory medications, such as prednisolone acetate 1% suspension every 6 to 12 hours, are recommended to treat anterior uveitis (depending on the severity of the inflammation). Topical nonsteroidal antiinflammatory drugs, such as flurbiprofen 0.03% and diclofenac 0.1%, may increase IOP and should be avoided in feline patients with an elevated IOP.

Intraocular neoplasia is the second most common cause of secondary glaucoma in cats. Although any primary or metastatic intraocular tumor may lead to the development of increased IOP, diffuse iris melanoma is the most common tumor to cause secondary glaucoma in cats. This tumor generally starts as a small focal area of iris hyperpigmentation. These tumors progress slowly, over the course of several months to even years. As the tumor progresses, infiltration of the iridocorneal angle with neoplastic cells leads to the development of glaucoma. In a study evaluating the survival rate of cats in which enucleation was performed to treat diffuse iris melanoma, cats with secondary glaucoma due to the tumor had shorter survival times than cats with diffuse iris melanomas that did not have secondary glaucoma. Lymphosarcoma is the second most common intraocular tumor to cause secondary glaucoma in cats. Typically it manifests as pinkish-whitish nodular lesions on the iris.

Hyphema (blood in the anterior chamber) is another cause of secondary glaucoma, which results from obstruction of the iridocorneal angle with blood. Hyphema can be caused by numerous disorders, such as trauma, coagulopathies, infectious diseases, and neoplasia, but in older cats systemic hypertension probably is the most common cause. In cats with systemic hypertension (systolic blood pressure of ≥160 mm Hg) a CBC, chemistry panel, urinalysis, and thyroxine level measurement should be performed to rule out chronic kidney disease or hyperthyroidism as the cause of systemic hypertension.

Aqueous misdirection, also known as malignant glaucoma, is another form of glaucoma that has been described in cats. In this condition the aqueous humor is misdirected into the vitreal space and the lens and iris are shifted anteriorly, which occludes the iridocorneal angle. Cats with aqueous misdirection may show mydriasis (dilated pupil), decreased pupillary light reflexes, decreased menace response, an abnormally shallow anterior chamber, elevated IOP, and glaucomatous changes in the optic nerve head. In a study of 32 cats with aqueous misdirection, the mean age at the time of diagnosis was found to be approximately 12 years, and female cats were affected significantly more often than males. This condition has been reported to occur more often unilaterally, but it can also occur bilaterally. Medical treatment of aqueous misdirection includes the use of topical carbonic anhydrase inhibitors such as dorzolamide 2% every 8 to 12 hours to reduce the production of aqueous humor. Drugs that cause miosis (constriction of the pupil) such as prostaglandin analogs, pilocarpine, and timolol should not be used to treat aqueous misdirection syndrome because these medications may exacerbate glaucoma by increasing contact between the iris and the lens and further decreasing the outflow of aqueous humor through the pupil. If the IOP cannot be controlled sufficiently with medical therapy, referral to an ophthalmologist for a lensectomy and anterior vitrectomy may be indicated. Ideally the IOP should be less than 25 mm Hg to prevent the discomfort associated with elevated IOP and also to prevent damage to the retina and optic nerve, which could lead to loss of vision. With early detection and medical intervention many cats with this condition can remain comfortable and visual.

Clinical signs of glaucoma in cats usually are subtle, and most cats are not brought for treatment until late in the course of disease. Owners may notice mydriasis, a cloudy eye (corneal edema), or buphthalmos. In most cases of feline glaucoma conjunctival hyperemia is absent to minimal. On examination of the fundus retinal degeneration may be present, which is characterized by tapetal hyperreflectivity and vascular attenuation. Optic nerve cupping is difficult to appreciate in cats due to the lack of myelin on the optic nerve head. An optic nerve that is atrophied appears darker than a normal optic nerve.

Two commonly used instruments to measure IOP are the Tono-Pen and the TonoVet tonometers (see Chapter 242). The Tono-Pen is an applanation tonometer and estimates IOP by flattening the surface of the cornea. A topical anesthetic such as ophthalmic proparacaine must be used to obtain readings with this instrument. The instrument is held perpendicular to the corneal surface and is tapped lightly on the central portion of the cornea. Brief beeps indicate when an individual reading has been recorded, and a longer tone indicates that the mean of those IOP readings has been calculated. The mean IOP is displayed along with the coefficient of variation obtained from the readings (i.e., 5%, 10%, 20%, or >20%, calculated as standard deviation/mean). The Tono-Pen is very accurate in the normal range of IOPs but tends to overestimate IOP in the low range and underestimate IOP in the high range. Ideally readings with a coefficient of variation of 5% or less should be recorded as the IOP for that patient.

The TonoVet measures the rebound action of a magnetic probe as it contacts the corneal surface and bounces back. One of the advantages of the TonoVet tonometer is that it does not require the use of a topical anesthetic. The instrument should be held upright while taking measurements. The mean IOP reading obtained with the rebound tonometer (TonoVet) has been found to be 2 to 3 mm Hg higher than that measured with the applanation tonometer (Tono-Pen VET) in cats. Although a difference of 2 to 3 mm Hg is not clinically significant, it highlights the importance of using the same tonometer for follow-up examinations in the same patient.

IOP can be elevated artificially in cats without any ophthalmic abnormalities, which indicates that a single abnormal IOP measurement in the absence of other ophthalmic abnormalities may be insufficient for a diagnosis of glaucoma in cats. Artificially high IOP readings can occur in fractious or nervous cats. Excessive pressure on the globe, eyelids, or neck also can increase IOP readings artificially. Cats that have IOP readings of 25 mm Hg or more or a difference in IOP between the two eyes of 12 mm Hg or more but that do not show any ophthalmic abnormalities or changes in the optic nerve head may have a falsely high IOP. Tonometry should be repeated in these cats within a few weeks, and if the elevation in IOP persists, referral to an ophthalmologist for a more thorough examination may be warranted before antiglaucoma therapy is initiated.

Because secondary glaucoma is much more common than primary glaucoma it is important to diagnose the cause of glaucoma (e.g., uveitis or neoplasia) and address the underlying issue. In cases of uveitis, topical antiinflammatory therapy and specific antimicrobial agents when an infectious cause has been identified are recommended (see Chapter 250). Antiglaucoma medications can be used at the same time as topical antiinflammatories. In cases of intraocular neoplasia, enucleation and histopathologic examination of the eye is recommended. If an intraocular tumor is suspected, thoracic and abdominal radiography and abdominal ultrasonography should be discussed with the owner before surgery is undertaken to remove the eye.

The goals of glaucoma therapy are to preserve vision and alleviate discomfort. The therapeutic plan depends on the cause of the glaucoma and the patient’s visual status.