Feline Corneal Diseases Study Guide

Overview and Clinical Importance

Corneal diseases represent a significant proportion of feline ophthalmic conditions encountered in clinical practice. The feline cornea has unique characteristics that predispose cats to specific pathologies not commonly seen in other species. Understanding these conditions is essential for the NAVLE, as corneal diseases directly affect vision, comfort, and quality of life in affected patients.

Unlike dogs, cats have primary conjunctival and corneal pathogens, particularly feline herpesvirus-1 (FHV-1), which causes approximately 80-90% of infectious keratitis cases. This virus is ubiquitous in the cat population, with an estimated 80-95% of cats being latently infected. Understanding the relationship between FHV-1 and corneal disease is fundamental to feline ophthalmology.

Feline Corneal Anatomy

The feline cornea is an avascular, transparent structure composed of four main layers: epithelium, stroma, Descemet's membrane, and endothelium. Understanding corneal layer anatomy is critical for assessing ulcer depth and determining appropriate treatment.

Corneal Layer Characteristics

Feline Herpesvirus-1 Keratitis

Pathophysiology

Feline herpesvirus-1 (FHV-1) is an alphaherpesvirus with worldwide distribution. Following primary infection (usually in kittens 2-6 weeks old), the virus establishes lifelong latency in the trigeminal ganglion. Approximately 80% of infected cats become latent carriers, and 40-50% experience recrudescent disease triggered by stress, immunosuppression, or corticosteroid administration.

FHV-1 keratitis can manifest as: ulcerative keratitis (direct viral cytopathic effect) or stromal keratitis (immune-mediated response to viral antigen). Clinical signs develop 2-6 days after infection, with corneal epithelial damage from viral replication causing cell lysis.

Clinical Signs and Diagnosis

Treatment of FHV-1 Keratitis

Feline Corneal Sequestrum

Pathophysiology and Etiology

Corneal sequestrum (corneal mummification, corneal nigrum) is a condition UNIQUE to cats characterized by focal necrosis and brown-black pigmentation of the corneal stroma. The sequestrum represents devitalized corneal tissue that is gradually rejected by surrounding healthy cornea, acting as a foreign body and inducing inflammation.

Predisposing Factors

Clinical Signs and Diagnosis

The clinical appearance is pathognomonic: a focal, tan to dark brown-black plaque in the central or paracentral cornea. The lesion color ranges from translucent amber (early) to opaque black (advanced). Surrounding corneal vascularization and loose epithelial edges are common. The condition is typically extremely painful due to chronic epithelial erosion.

Diagnostic approach: Visual examination is typically sufficient for diagnosis. Fluorescein uptake may occur at the lesion periphery where epithelium is eroded. Schirmer tear test and tear break-up time should be performed to assess tear film. Corneal cytology can rule out concurrent bacterial infection.

Treatment Options

Exam Focus - Sequestrum Memory Aid: "PERSIAN PROBLEMS" P = Persian/brachycephalic predisposition E = Entropion/eyelid abnormalities R = Recurring ulceration S = Stromal necrosis with pigmentation I = Irritation from chronic exposure A = Antiviral therapy for FHV-1 component N = No grid keratotomy in cats!

Feline Eosinophilic Keratitis

Pathophysiology

Eosinophilic keratitis (EK) is a chronic, progressive, inflammatory corneal disease characterized by infiltration of eosinophils into the corneal stroma. The exact etiology is unknown, but it is believed to be an immune-mediated hypersensitivity reaction. Up to 75% of affected cats test positive for FHV-1, suggesting a possible link, though the causal mechanism remains unclear.

Clinical Signs and Diagnosis

The clinical appearance is nearly pathognomonic: raised, white to pinkish plaques with corneal vascularization, typically starting at the superotemporal limbus (76.8% of cases). A white, friable, gritty exudate often adheres to the surface. The condition is usually unilateral initially (75.6%), though bilateral involvement can occur.

Definitive diagnosis requires corneal cytology: Use a cytobrush, Kimura spatula, or blunt scalpel blade to collect samples from the plaque after topical anesthesia. The presence of even ONE eosinophil is considered diagnostic, as eosinophils are not normally present on healthy feline corneas.

Treatment

Feline Corneal Ulceration

Ulcer Classification and Assessment

In feline patients, superficial corneal ulceration should be presumed to result from FHV-1 until proven otherwise. Other causes (trauma, eyelid abnormalities, tear film deficiencies) are less common in cats compared to dogs. Corneal ulcers are classified by depth, which determines treatment approach.

Feline Acute Bullous Keratopathy

Feline acute bullous keratopathy (acute corneal hydrops) is a unique feline condition characterized by sudden onset of focal or diffuse stromal edema with corneal protrusion. The edema is caused by acute Descemet's membrane rupture, allowing aqueous humor to flood the stroma.

Clinical features: Acute onset; marked corneal edema and bulging visible from lateral view; may be focal, geographic, or complete. Unlike endothelial disease, bullous keratopathy often resolves spontaneously with supportive care. There is a reported association with systemic cyclosporine administration. Left untreated, corneal rupture and loss of the eye can occur.

Summary: Differential Diagnosis of Feline Corneal Diseases