Diagnostic Ophthalmology

History and clinical signs

A 6-year-old chocolate point Himalayan cat was examined by the ophthalmology service at the Western College of Veterinary Medicine (Saskatoon, Saskatchewan). This cat was presented with a discolored spot on the left cornea, squinting, and tearing. The menace responses and palpebral, oculocephalic, direct, and consensual pupillary light reflexes were normal bilaterally. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were 10 and 19 mm/min in the right and left eyes, respectively. Intraocular pressures were estimated with a rebound tonometer (Tonovet; Tiolat, Helsinki, Finland) and were 19 and 20 mmHg in the right and left eyes respectively. Fluorescein staining (Fluorets; Bausch & Lomb Canada, Markham, Ontario) of the corneas was negative in the right eye and positive in the left eye. Examination of both eyes using a transilluminator (Welch Allyn Finoff Transilluminator; Welch Allyn, Mississauga, Ontario) and handheld biomicroscope (Kowa SL-17 Portable Slit Lamp; Kowa, Tokyo, Japan) revealed mild medial ventral entropion. The left eye had blepharospasm, epiphora, and superficial corneal vascularization from 12 to 3 o’clock, extending toward a poorly demarcated, brown lesion within the axial cornea. A photograph of the left eye at presentation is provided for your assessment (Figure 1). Following application of 0.5% tropicamide (Mydriacyl; Alcon Canada), examination of both eyes using a transilluminator (Welch Allyn Finoff Transilluminator; Welch Allyn) and handheld biomicroscope (Kowa SL-17 Portable Slit Lamp; Kowa) revealed no other abnormalities. Indirect ophthalmoscopic (Heine Omega 500; Heine Instruments Canada, Kitchener, Ontario) examination was within normal limits in both eyes.

Figure 1.

Anterior segment photograph of the left eye of a 6-year-old Himalayan cat.

What are your clinical diagnoses, differential etiologic diagnoses, therapeutic plan, and prognosis?

Discussion

The ophthalmic diagnoses are feline corneal sequestrum of the left eye, superficial corneal ulcer of the left eye, and medial ventral entropion of both eyes. Corneal sequestrum most typically presents as a circular-to-ovoid, amber-to-light-brown discoloration of the central cornea. With chronicity, the discolored area can become denser and appear as a well-demarcated, dark-brown-to-black lesion. Chronic lesions tend to be associated with corneal vascularization and ulceration. Although the clinical presentation of corneal sequestrum is characteristic for the condition, another, less-likely differential diagnosis to consider is a corneal foreign body.

Feline corneal sequestrum, also known as corneal nigrum or corneal mummification, is a condition characterized by necrosis of the cornea, appearing clinically as a focal area of amber-to-brown/black discoloration of the stroma and varying in diameter and depth. It most commonly presents unilaterally but presented bilaterally in up to 29% of cats in one study (1). The mean age at diagnosis is between 3 and 8 y of age, and there is a brachycephalic breed predilection (1–4). Approximately 50% of cats will present with concurrent corneal ulceration that causes discomfort manifesting as blepharospasm, epiphora, and photophobia (5).

The pathogenesis of corneal sequestrum is not fully understood but is thought to be associated with chronic irritation owing to brachycephalic skull conformation, which is associated with a wide palpebral fissure, shallow orbit, trichiasis secondary to medial ventral entropion, lagophthalmos, and reduced corneal sensitivity. Persian, Himalayan, Burmese, sphynx, and exotic shorthair breeds are overrepresented (1–4,6,7). It was recently shown that corneal sequestrum is highly heritable and likely has a multifactorial, complex pattern of inheritance (8). The association of feline herpesvirus Type-1 with corneal sequestrum remains unclear and inconsistent across studies (4,9–12). Histologic and ultrastructural findings of affected cornea reveal a reduced number of keratocytes, necrotic keratocytes between disarranged collagen layers, stromal necrosis, inflammatory cells such as lymphocytes and plasma cells, and varying degrees of overlying corneal epithelial loss (10,13).

Therapy for feline corneal sequestrum is surgical and consists of referral for a lamellar keratectomy, alone or in combination with a graft to provide tectonic support. Lamellar keratectomy alone can be done when less than 50% of the stromal depth is involved. Several procedures have been described for deeper lesions (> 50%), such as conjunctival grafts; corneoconjunctival transpositions; autologous, homologous, and heterologous corneal grafts; and the placement of biomaterials such as bovine pericardium, equine amniotic membrane, and porcine urinary bladder to graft the keratectomy site (1–5,14–17). With surgical therapy, the prognosis for vision and the eye is good. Recurrences are uncommon with surgery but are possible, particularly when there is incomplete excision, or in brachycephalic breeds. It is currently unknown whether surgical correction of anatomical features such as medial ventral entropion is indicated in preventing recurrences. Medical therapy alone is almost always unsuccessful and is geared toward preventing infection of concurrent corneal ulceration using topical antimicrobials, improving comfort, and reducing irritation with topical lacrimomimetics. Over several months to years, some corneal sequestra may progress to extrusion from the cornea; and depending on the depth of the lesion, sloughing could result in a deep defect or corneal perforation.

In the present case, the cat was placed under general anesthesia and a superficial lamellar keratectomy was completed using the operating microscope. A drop of atropine was administered in hospital to reduce discomfort associated with ciliary body muscle spasm triggered by corneal surgery. The cat was discharged wearing an E-collar and with instructions for administering ciprofloxacin q6h, diclofenac q12h, and Optixcare (Aventix) q6h. A reevaluation was done 2 wk postoperatively to confirm complete healing of the keratectomy site; examination at that time revealed a corneal scar and receding corneal vessels. A follow-up 1 y later confirmed the absence of recurrence and the cornea appeared clear.