Veterinary Ophthalmology

Corneal Diseases in Dogs and Cats

Pet owners spend time with their pets every day: feeding, playing, and walking with them. Sometimes you might not notice changes in your pet’s behavior and overall well-being immediately, but it is unlikely that you will miss changes in your pet’s eyes if they have lost clarity and the cornea is cloudy, or if the conjunctiva is swollen and red. It will also be concerning if your pet is squinting one or both eyes or keeping them closed. Of course, you will notice a constricted pupil, sensitivity to light, tearing, and eyelid spasms (blepharospasm). All these clinical signs indicate corneal inflammation, which is quite common in animals. The cornea is part of the outer layer of the eye, typically transparent and devoid of blood vessels. The cornea of dogs and cats consists of four layers. The superficial epithelial layer is made up of several rows of cells that regenerate quickly, nourishes, and maintains optimal corneal moisture. Next is the corneal stroma, the thickest layer containing collagen fibers and many nerve endings, making stromal ulcers painful for animals and causing blepharospasm and tearing. Behind the stroma is the strongest layer of the cornea: Descemet’s membrane. If exposed due to trauma, there is a risk of globe perforation, making surgical treatment advisable. The innermost layer is the endothelium, which removes excess fluid from the cornea, ensuring its transparency. Overall, the cornea helps protect the eye’s inner structures and plays a crucial role in focusing light onto the retina. Thus, the cornea is key to good vision. Here is a brief overview of the main corneal diseases in dogs and cats commonly encountered in veterinary practice. Mechanical injuries to the cornea might be the most common. Minor trauma results in corneal erosion, where only the epithelial layer is damaged, typically healing within 3-5 days without special medical therapy. In contrast, deep corneal damage, involving all layers and leading to leakage of aqueous humor from the anterior chamber, hyphema (bleeding in the anterior chamber), and damage to the iris and lens, is the most dangerous. Clinically, you will observe eyelid spasms, tearing, and significant pupil constriction in the animal. Immediate surgical intervention is required in such cases. Often, deep corneal wounds are complicated by the presence of collagen enzymes and bacterial toxins, causing ulcer formation and tissue lysis. Within three days, the corneal syndrome develops, with blepharospasm, light sensitivity, and pericorneal vessel growth. The cornea loses its transparency, shine, and sensitivity. These injuries tend to progress rapidly, necessitating urgent medical and sometimes surgical intervention to prevent panophthalmitis (inflammation of all eye tissues and membranes), leukoma (opaque corneal scar), or a fistula discharging fluid. It is important to note that ulcers can be caused not only by infected corneal injuries but also by endogenous factors such as infections (herpes virus), vitamin deficiencies (B1, B2, PP hypovitaminosis), reduced tear production (dry keratoconjunctivitis in dogs), eyelid pathologies (entropion, eyelid tumors), eyelash ectopia, and distichiasis. Successful ulcer treatment depends on correctly diagnosing and eliminating the cause. A specific type of ulcer, primarily found in dogs, is the so-called boxer ulcer or recurrent erosion or refractory corneal ulcer, difficult to treat due to the disruption of the connection between corneal stromal and epithelial cells. Here, medical treatment is combined with surgical intervention, performing a punctate keratotomy to disrupt the stromal collagen, providing a foundation for the migrating epithelium to adhere. The next two corneal conditions to differentiate are corneal dystrophies and degenerations. Corneal dystrophy is a non-inflammatory disease presenting as a focal, well-defined white or metallic-like lesion, often in the subepithelial or anterior stromal part of the cornea (all layers can be affected in Siberian Huskies, Airedale Terriers, and Pointers). Corneal dystrophies can be:

• bilateral (one eye may be affected before the other);
• symmetric;
• congenital (breeds prone to dystrophy include Alaskan Malamutes, English Toy Spaniels, Beagles, Bichon Frises, Bearded Collies, Weimaraners, Golden Retrievers, Mastiffs, German Shepherds, Poodles, Samoyeds, Whippets, Chihuahuas, Cavalier King Charles Spaniels, Miniature Pinschers, Irish Wolfhounds, etc.).

The infiltrate in the cornea consists of cholesterol, phospholipids, and neutral fats. Dogs do not feel discomfort if the epithelial layer is intact. Breeding animals with malignant stromal dystrophies is not recommended. Corneal degeneration, on the other hand, can affect one or both eyes, is often asymmetrical, and is associated with corneal ulcers and vascularization. Particularly dangerous is endothelial degeneration, characterized by the loss of corneal endothelial cells and secondary corneal edema (commonly seen in Boston Terriers, Dachshunds, and Chihuahuas). In addition to lipids and cholesterol, calcium can also deposit during corneal degeneration. The cause is systemic diseases (hypothyroidism, pancreatitis, diabetes mellitus, hypervitaminosis D, hypercalcemia, hyperlipoproteinemia, Cushing’s disease, etc.), requiring a general examination of the animal. Another condition to mention is corneal melanocyte infiltration in dogs, known as corneal melanosis or pigmentation, or pigmentary keratitis (pigmentation forms when normal melanocytes migrate from the limbus to the corneal epithelial or stromal surface layers during inflammation). This is not a separate disease but a result of chronic corneal irritation or inflammation. It is common in brachycephalic breeds like Pugs and Pekingese due to nasal fold irritation or entropion of the lower eyelid’s inner corner. Other causes of pigmentation include pannus or superficial immune-mediated keratitis (related to chronic conjunctival inflammation, often seen as an autoimmune response in shepherd breeds, their crosses, and Greyhounds, Airedale Terriers) or dry keratoconjunctivitis (a condition where primary tear glands have ceased functioning for various reasons, more common in English Bulldogs, West Highland White Terriers, Yorkshire Terriers, etc.). Pannus presents with vascularization, edema, and infiltrate on the cornea, while dry keratoconjunctivitis shows mucous or purulent discharge that is hard to treat with antibiotics. In cats, conjunctivitis is mainly viral (herpes virus infection FHV-1). The virus causes corneal necrosis or sequestration, a dark brown to black spot on the cornea (more common in Burmese, Himalayan, and Persian cats). Clinically, mild blepharospasm and small mucous, purulent discharge are observed. Corneal sequestra are treated surgically. Another syndrome in cats is eosinophilic keratoconjunctivitis, characterized by irregularly shaped, pink to white corneal lesions with an uneven surface. It is treated medically with corticosteroids and immunomodulators. In summary, corneal diseases in dogs and cats vary, and their causes can be exogenous, endogenous, immune-mediated, congenital, or acquired. Clinical manifestations are often similar: blepharospasm, light sensitivity, tearing, corneal clouding, possible miosis (pupil constriction), conjunctival and scleral edema and hyperemia, and superficial and deep corneal vascularization. It is crucial to seek veterinary assistance at the first signs of corneal inflammation rather than self-medicating, as some medications can cause serious complications. For example, corticosteroids are not permissible for corneal ulcers as they hinder healing and can lead to superinfections with fungi and other bacteria. The most you can do at home is to moisten the eye with artificial tear drops. Be attentive and take care of the vision of our little friends!