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In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews glaucoma in dogs and cats. Make sure to check out medical options for glaucoma and surgical options for glaucoma in Part 2 and 3 of these VETgirl blogs! Classification of glaucoma is important to allow for proper diagnostic and therapeutic recommendations. Glaucoma should be classified based upon:
- The etiology of the glaucoma: congenital, primary, or secondary
- The duration of the glaucoma: acute or chronic
Congenital glaucoma is rare in both dogs and cats and is caused by developmental abnormalities in the aqueous humor outflow pathways. Affected animals generally present young (3 – 6 months) with an acute onset of buphthalmia and corneal edema. The disease may be unilateral or bilateral and may be associated with other ocular anomalies such as cataract, persistent pupillary membranes, and retinal dysplasia.
Primary glaucoma is the most common type of glaucoma in the dog and is due to an inherited malformation in the iridocorneal angle that progresses over time to lead to complete collapse of the drainage system. Primary glaucoma is strongly breed-related, and the most commonly affected breeds include the Beagle, Basset Hound, Boston terrier, Cocker Spaniel, and Shar Pei. Mixed breed dogs are also affected with primary glaucoma. Although primary glaucoma is less commonly seen in cats, breeds at risk include Siamese, Burmese, Persian, and domestic shorthairs.
Secondary glaucoma results from a physical obstruction to aqueous humor drainage pathway which was otherwise previously normal. The flow can also be obstructed prior to the iridocorneal angle if the pupil becomes blocked by posterior synechia (iris bombe), a luxated or swollen lens, or vitreous. Reduced aqueous humor drainage is seen commonly with uveitis, intraocular neoplasia, and hyphema. Secondary glaucoma is the most common form of glaucoma in cats.
Duration of Glaucoma
Acute glaucoma is defined as an elevation in IOP of < 24-48 hours duration. Intermittent spikes in IOP often occur prior to the development of a sustained elevation but are not usually detected clinically. Once the IOP elevates persistently, clinical signs of glaucoma develop. If patients are treated during this phase, vision may be salvageable. Unfortunately, the subclinical spikes in IOP are quite detrimental, thus only 50% of patients regain sight even when treated in the acute phase.
Chronic glaucoma develops after the IOP elevation is sustained for days or longer. Medical therapy may be effective at reducing the IOP, however vision cannot be regained. With time, many of the ocular structures undergo both physiologic and morphologic changes in response to the persistently high IOP. Many dogs and almost all cats present with chronic glaucoma, as the acute phase is either misdiagnosed or overlooked completely by the owners.
Tune in for a future blog to learn more about medical and surgical management of glaucoma.
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