Clinical Vignette 1—Initial Presentation
A 6-year-old male neutered mixed-breed dog presents with the owner complaint of sudden vision loss. When questioned further the owner mentions that the dog has been losing weight over the last few weeks and has had occasional urinary accidents in the house. Ophthalmic examination reveals bilateral negative menace reflex, positive direct and consensual pupillary light reflexes, moderate hyperemia of the bulbar conjunctiva, mild aqueous flare, and diffuse opacity of the lens with no tapetal reflection visible. Physical examination is unremarkable except for a 2/6 body score. The temperature, pulse, and respiration are normal.
Clinical Vignette 2—Initial Presentation
A 5-year-old female spayed Jack Russell terrier presents with the history of holding the right eye closed, tearing, and a cloudy appearance to the eye since last night. She refused food this morning and has been lethargic. Ophthalmic examination of the right eye reveals marked blepharospasm, congested scleral and bulbar conjunctival vessels, and diffuse corneal edema. Menace reflex appears negative and is positive in the left eye. A pupil outline in the right eye can just be visualized by viewing the tapetal reflection through the corneal edema. There is no direct pupillary light reflex in the right eye and no consensual reflex to the left eye; there is a positive pupillary light reflex in the left eye but negative consensual response to the right eye. Using the slit beam on the direct ophthalmoscope and full light intensity, careful examination of the anterior chamber in the right eye shows a luxated lens and in the left anterior chamber a filmy opacity that resembles puffs of smoke.
Problem Definition and Recognition
The lens is a clear, avascular structure like the cornea and similar to the cornea loss of transparency results in loss of vision. Cataract, which is defined as an opacification of the lens, is the most common disease of the lens. Luxation of the lens occurs with rupture or degeneration of the zonules. This can be a vision-threatening and painful condition if the lens falls into the anterior chamber, resulting in acute, severe glaucoma.
Pathophysiology
The lens is composed of perfectly aligned lens fibers that create a transparent structure capable of transmitting and focusing light onto the retina. Any derangement of the lens fibers due to alterations in lens nutrition, energy metabolism, protein metabolism, or osmotic balance will create a cataract (Ofri 2008). The most common cause of cataract development in the dog is genetic followed by diabetes mellitus. Common etiologies for cataracts in the dog are listed in Table 51-1.
Cataracts are rare in the cat and when present are usually caused by chronic uveitis. Lenticular nuclear sclerosis is a normal, physiologic aging change in the lens that can be mistaken for a cataract especially in geriatric animals. Lens fibers are continually produced at the lens equator throughout life. As the animal ages lens fibers in the center (nucleus) of the lens become compressed and appear more prominent. Nuclear sclerosis can clearly be identified by dilating the pupils with tropicamide (1%). It appears as a bilaterally symmetrical, perfectly round haze (or clear marble effect) in the center of the lens. Unlike a cataract nuclear sclerosis will not obstruct the tapetal reflection, interfere with viewing the fundus, or interfere with vision.
The lens is supported by fine fibers called zonular fibers or zonules. These are anchored between the ciliary processes of the ciliary body and attach at the lens equator around the circumference of the lens. Zonules are not visible on the ophthalmic examination. Zonules can undergo degeneration due to chronic inflammation in the eye or due to a genetic predisposition. Zonules can also break due to stretching caused by buphthalmia or lens resorption in a hypermature cataract.
TABLE 51-1. Common etiologies of cataracts in dogs
| Etiology | Presentation | Prognosis |
| Genetic/inherited | Bilateral although the onset between eyes can vary from weeks to years; onset is often very rapid in young dogs (< 2 years of age) and more gradual in older dogs | Very good with early surgical intervention |
| Diabetes mellitus |
