Corneal Diseases in Clinical Practice
As the outer and most exposed structure of the globe, the cornea is not only at risk for trauma, but is also a structure that receives close attention from clients. Therefore, corneal changes are a common presenting complaint in veterinary practice. Correct interpretation of corneal changes is critical for diagnosing corneal disease. Since corneal disease can alter the corneal structure permanently and potentially lead to blindness appropriate management of corneal disease is important. Proper diagnostic tests and timely initiation of optimal therapy are required to increase the chance of saving vision and retaining a comfortable globe.
Selected Corneal Diseases and Management
Pigmentary Keratitis (Superficial Pigmentary Keratitis)
Chronic corneal irritation can lead to pigmentary keratitis in the dog. It is typically a nonspecific response to chronic keratitis and can occur in any canine breed although some brachycephalic breeds (such as Pug, Shih Tzu, Lhasa Apso, and Pekingese) are to marked and sometimes rapidly occurring corneal pigmentation. In brachycephalic dogs this often begins as focal (often in the nasal cornea) corneal pigmentation and this can progress at a variable rate over the entire ocular surface and eventually lead to blindness. The clinician should investigate if distichiasis, districhiasis, nasal fold trichiasis, entropion, ectropion, keratoconjunctivitis sicca (which should be monitored regularly), and chronic exposure resulting from macropalpebral fissure is present. The latter occurs in many brachycephalic breeds. It is important to note that pigmentary keratitis is often a multifactorial condition. Treatment is generally aimed at treating the causes of the chronic irritation and halting the progression of pigmentation. Correction of entropion or ectropion, removal of hair touching the cornea, and in dogs with a macropalpebral fissure lateral or medial canthoplasty combined with medical therapy as needed will often halt the progression of the pigmentation. Surgical removal of the pigmented cornea alone will often lead to recurrence of the pigmentation as well as fibrosis. Other therapies described in the literature include cryotherapy and beta radiation. Topical cyclosporine or tacrolimus can be used to treat non-ulcerative keratitis and may decrease corneal pigmentation.
Chronic Superficial Keratitis (“Pannus’’)
Another corneal disease in which the eye can be blinded due to superficial corneal pigmentation is chronic superficial keratitis or “pannus”. This condition, which is an immune-mediated disease thought to be associated with UV damage, is seen most commonly in the German Shepherds, but does occur in other breeds. The condition is progressive and worsens in areas of higher elevation as well as during summer months where the UV exposure is higher. The corneal changes consisting of neovascularization and pigmentation usually begins in the temporal cornea. Treatment consists of reducing UV exposure as well as suppressing the immune reaction with topical steroid medication and cyclosporine. A related condition, where a similar immune medicated reaction is confined to the conjunctiva and the third eyelid, is called plasmoma. Plasmoma may be present in conjunction with pannus. The treatment is the same as for pannus. Since pannus is an immune-mediated disease lifelong management is required.
A corneal dermoid is a benign congenital growth of normal tissue (skin) in an abnormal position (the cornea) (choristoma). Ocular dermoids are often hairy and these hairs can cause eye irritation. Ocular dermoids can be seen in cornea, conjunctiva, and rarely in the nictitans or eyelids and they are most often unilateral. The primary cause of ocular dermoid formation is not fully understood, but they can occur as a result of abnormal differentiation of an isolated group of cells early in development or abnormal invagination of ectodermal tissue late in development. The skin may be pigmented, contain sebaceous and sweat glands, fat, and/or grow hair. Any dog breed can be affected, but they occur more frequently in breeds such as German Shepherds, Dachshunds, Dalmatians, Doberman Pinschers, and St. Bernards. Dermoids have also been reported in the Birman and Burmese cat breeds. Ocular dermoids can affect the patient’s normal ability to blink, they can irritate the cornea and conjunctiva, and they can be vision obstructing. Diagnosis can be made with an ophthalmic exam. Surgical removal (keratectomy) is the treatment of choice and is curative if the dermoid is fully removed. The surgery is ideally performed under a surgical microscope and depending on how much corneal thickness is removed, an amniotic membrane or conjunctival graft can be placed. Until surgery can be performed, it is recommended to use lubricating eye ointment frequently to reduce mechanical irritation from trichiasis and/or incomplete blinking.
Canine Limbal Melanomas
Although limbal or epibulbar melanomas are typically benign neoplasms, they may invade the cornea or intraocular structures. Limbal Melanomas are typically smooth and pigmented lesions, but can be also be amelanotic. These tumors occur in two age groups. The tumor tends to be more rapidly growing and invasive in the younger group (2–4 years) whereas they are more likely to be slowly growing in the older group (>8 years). They are usually located in the dorsal arc between the dorsomedial and ventrolateral limbus. German Shepherds, Golden Retrievers, and Labrador Retrievers appear predisposed and limbal melanomas may be associated with heavily pigmented dogs. They must be differentiated from intraocular or conjunctival melanomas. In older dogs with nonprogressive limbal tumors monitoring may be adequate, but treatment is often required in younger dogs with fast growing tumors. Treatments include lamellar or full-thickness excision and grafting procedures, cryotherapy, and laser photocoagulation. Complete removal or removal with adjunctive cryotherapy is usually curative, while recurrence has been reported following laser therapy.
Feline Corneal Sequestra
This condition is characterized by an area of corneal degeneration with an amber (early phases) or brown to-black (late phases) discoloration. Corneal sequestra can occur after chronic corneal ulcers or keratitis caused by infection with FHV-1 or by corneal irritation from entropion or trichiasis or iatrogenic damage (grid keratotomy). It can also appear as a primary stromal disease in Persians and Himalayans. The sequestra consists of necrotic cornea, but the cause of the corneal discoloration is currently undefined. The necrotic cornea can be surgically removed by keratectomy and a graft is most often placed. For superficial sequestra, a graft may not be required, but covering the defect with a soft contact lens and/or tarsorrhaphy during healing is usually recommended to reduce chances of recurrence. Conjunctival grafts may help to prevent recurrence. Corneoconjunctival transposition, porcine small intestinal submucosa graft or amniotic membrane or heterologous corneal graft transplantation can also be performed following the keratectomy. It is important to treat the underlying disease process in order to prevent recurrence following surgery.