The cornea Flashcards
What is the anatomy of the cornea
The cornea and sclera together comprise the dense fibrous tunic of the globe
The cornea is composed of four layers:
- an outer stratified squamous epithelium with a basement membrane
- a thick stromal layer
- Descemet’s membrane
- an inner cuboidal epithelium (corneal “endothelium”)
How is corneal transparency obtained
The stroma consists of multiple parallel lamellae of collagen fibers that extend accross the entire diameter of the cornea from limbus to limbus
The fibers are supported by glycosaminoglycans, which control stromal hydration and assist in maintaining the exact parallel arrangement of the collagen lamellae necessary for corneal transpparency
What are the main congenital and developmental conditions in cats? Give some specific features for each
Dermoid
- a dermoid is a congenital displacement of skin tissue
- it uncommonly affects the cornea in cats, but more usually affects the eyelids and the conjunctiva
- excessive laccrimation +/- corneal ulceration are the presenting signs
- surgical removal by superficial lamellar keratectomy and conjunctival resection is generally required
Microcornea and megalocornea
- microcornea occurs with microphtalmos
- microcornea has been associated with griseofulvin use in the pregnant queen
- cataracts and persistent pupillary membrane remnants are frequent accompanying defects
- both conditions are congenital, bilateral and non-progressive, treatment is unwarranted
Leukoma associated with persistent pupillary membranes
- adhesions of strands arising from the surface of the iris with the posterior cornea
- non-progressive congenital condition
- no treatment warranted
Lysosomal storage disease
- rare in cats
- inherited inborn errors of metabolism result in the accumulation of by-products of metabolism within lysosomes due to the lack of a specific enzyme
- the most common ocular manifestation is progressive bilateral diffuse corneal clouding due to structural alterations within the stroma
- other systemic signs may be present depending on the type and severity of disorder, including neurological signs, skeletal abnormalities and stunted growth
- prognosis is poor
- no treatment
Corneal dystrophy
- primarily inherited condition that manifests in early life
- typically bilateral
- stromal corneal dystrophy occurs in the Manx cat, presenting as progressive stromal oedema that can lead to bullous keratopathy
- possible treatment: keratoplasty
What are the corneal disease syndromes associated with FHV-1 infection
Stromal keratitis
Eosinophilic keratitis
Corneal sequestration
Keratoconjunctivitis sicca
Corneal ulceration
What are the key features of eosinophilic keratitis
Epidemiology:
- relatively common
- average age at presentation: 5 years
Clinical signs:
- unilateral in 75% of cases
- characterized by inflammation extending across the limbus (affecting both the conjunctiva and cornea) and the dorsolateral quadrant is most commonly involved
- typically, there is vascularisation associated with superficial white corneal plaques with a very irregular surface
Diagnosis:
- typical clinical appearance
- cytological examination of surface brushings (PN, mast cells)
Treatment:
- topical corticosteroids are effective
- concurrent use with topical or systemic antiviral is recommended
- topical cyclosporin is also effective
What are the key features for corneal sequestrum
Epidemiology:
- corneal sequestration is a relatively common condition
- breed predisposition in the Persian, Himalayan, Siamese for example
Pathomechanism:
- a sequestrum is a focal area of stromal collagen necrosis associated with accumulation of pigment
- sequestrum development appears to be a nonspecific response to chronic stromal injury
Clinical signs:
- most commonly unilateral
- there is always a characteristic focal amber, brown or black oval to circular corneal plaque often with a surrounding corneal reaction
Treatment:
- may be treated medically or surgically based on the comfort of the patient
- medical treatment consists of topical antibiotics and lubrication
- surgery is usually recommended as it lessens the time course of the condition and lessens the risk of complications (i.e., keratectomy +/- graft)
What are the key features for acute bullous keratopathy
Epidemiology:
- usually affects younger cats
- uncommon condition with a very rapid onset and rapid progression
Clinical signs:
- characterized by pronounced corneal edema, with dense coalescing bullae that grossly distort the contour of the cornea
- typically, there is epiphora, blepharospasm and conjunctival hyperemia
Treatment:
- prompt surgical support with nictitans flap, 360° conjunctival graft, conjunctival pedicle graft or temporary tarsorrhaphy (released after 2 weeks)
What is corneal degeneration
Corneal degeneration may occur:
- spontaneously
- secondary to corneal injury or chronic inflammation
- as a response to corneal dystrophy or lipid keratopathy
It manifests as calcium or lipid deposition within the corneal stroma
- typically, it is unilateral
- it is always accompanied by vascularisation
Corneal calcium and lipid can be difficult to distinguish clinically:
- calcium is generally denser and thicker
- lipid has a more delicate, lacy appearance
Treatment:
- treatment of calcium accumulation can be attempted with topical 0.5% EDTA eye drops 2 to 4 times daily
- keratectomy might be considered
- topical antiinflammatory treatment is generally avoided as it may promote further calcium or lipid deposition
Why topical corticosteroids are contraindicated for all types of corneal ulcer
Topical corticosteroids are contraindicated for all types of corneal ulcer because:
- they may potentiate collagenolysis by destructive corneal bacterial and inflammatory cell enzymes
- they may allow reactivation of latent FHV-1
What is the pathomechanism of reflex uveitis
Reflex uveitis occurs due to stimulation of sensory nerve endings of the ophtalmic branch of the trigeminal nerve xithin the cornea
Signs of reflex uveitis include:
- miosis
- aqueous flare
- hypotony
