Read the German translation: Nicht-Ulzerative Hornhauterkrankungen
Introduction
The cornea is a rather simple structure and therefore the pathologic response of the cornea to any insult is limited and can lead to opacification of the tissue and loss of vision. The cornea changes its appearance from clear and transparent to white/yellow/grey/blue, red or brown/black. The treatment is aimed at the underlying cause and the goal is to restore transparency.
The White/Yellow/Grey/Blue Cornea
Lipid and mineral deposition in the cornea create a sparkly white, refractile appearance.
The deposition of lipid in the cornea has been divided into three clinical types: 1) Crystalline stromal dystrophy--bilateral, axial or paraxial crystalline appearance without inflammation or vascularisation. The deposition of lipid in the cornea without the presence of another disease. Cavalier King Charles Spaniel, Husky Beagle, etc. 2) Lipid keratopathy (corneal lipidosis)--Arcus lipoides corneae--peripheral lesion bilateral ocular manifestation of systemic disease (hypothyroidism, systemic lipid abnormalities). 3) Corneal degeneration--primary or secondary to anterior segment disease (ulcerative keratitis, pannus).
Treatment: May include the reduction of fat intake, the control of the underlying systemic diseases and control of ocular disease.
Mineral deposition (mostly calcium) in the cornea occurs spontaneously or secondary to systemic or local disease, usually in the older patient. Ulcers may form in the affected area and are slow to heal.
Treatment: Topical EDTA drops may help reduce the mineral deposits; in severe cases a superficial keratectomy is indicated.
Corneal cellular infiltrates and bacterial invasion create a whitish/yellowish appearance. Possible cells to be found in the cornea include: neutrophils in ulcerative keratitis; fibroblasts, plasma cells in chronic superficial keratoconjunctivitis (pannus); eosinophils in feline eosinophilic keratitis; and neoplastic cells in squamous cell carcinoma.
Treatment: Topical medications may include antibiotics, steroids, and cyclosporine depending on the condition. Surgical removal of neoplastic cells may be indicated depending on the size of the lesion.
Fibrosis of the cornea creates a dull white to grey appearance. The cornea may be thinner in the affected area and blood vessels may still be present in the area. Corneal repair involves keratocytes and invading macrophages and fibroblasts. The new collagen is laid down irregularly like in the sclera and causes the loss of transparency. The scarring becomes denser if granulation tissue forms.
Treatment: Topical anti-inflammatory medications may be used to reduce scar formation.
The accumulation of fluid in the cornea creates a bluish white appearance with indistinct borders. Various clinical situations may result in corneal edema including: 1) Loss of corneal epithelium--corneal ulceration, 2) Loss of corneal endothelium--corneal dystrophy, inflammation, glaucoma, trauma, 3) Deficient precorneal tear film (reduced O2).
Treatment: Topical antibiotics to treat the ulcer, 5% NaCl to reduce corneal edema, lubrication to improve the tear film.
The Red Cornea
Normally there are no blood vessels in the cornea. Vascularisation occurs as part of an inflammatory and healing response. Superficial vascularisation is present in the anterior third of the stroma and below the epithelium and continuous with conjunctival vessels at the limbus. Vessels branch frequently at the advancing border and are of bright red colour. Deep vascularisation is continuous with the ciliary limbal vascular arcades. The vessels are short, straight, less branched, of dull red colour, often associated with uveitis or glaucoma. If the stimulus to angiogenesis persists, the vessels persist.
Treatment: depends on the cause of the vascularisation. Vessels are helpful during the healing of a complicated corneal ulcer and not treated specifically until the ulcer is well healed. Underlying problems like uveitis and glaucoma require specific treatments. Topical steroids and non-steroidal anti-inflammatories reduce the numbers and size of blood vessels.
The Brown/Black Cornea
Corneal pigmentation is a nonspecific response to chronic corneal inflammation or irritation and found in the epithelium or the stroma. Epithelial pigment arises from the basal epithelial cell layer because of chronic irritation. Stromal pigment arises from proliferation of limbal melanocytes. They migrate into the stroma either alone or with blood vessels. Stromal pigmentation may be seen as extension of an anterior uveal melanoma or with a limbal melanoma. Endothelial pigmentation may result from uveitis, corneal rupture (anterior synechiae), persistent pupillary membranes or uveal cysts.
Treatment: Aimed at the underlying disease, topical steroids and cyclosporine can stabilize superficial corneal pigmentation. A surgical treatment may be chosen for a melanoma.
A specific type of corneal degeneration, the corneal sequestrum, is seen in the domestic cat. Most often in Persians and Himalayans, but can occur in any breed. It may be a flat or slightly raised, lightly to darkly pigmented lesion and corneal vascularisation may be present with it. It develops usually secondary to chronic irritation (feline herpes virus, entropion, corneal exposure, ulcerative keratitis).
Treatment: Medical management or correction of the underlying disease, surgical removal of the sequestrum.
The Multicolored Cornea
In some diseases the cornea can assume all of the colours listed above because of a more complicated disease process. Typical diseases in this category are the chronic superficial keratoconjunctivitis (CSK)/ pannus and keratoconjunctivitis sicca.
Treatment: Medical management of the underlying disease.