◊ 7-Cornea ◊
CORNEAL AFFECTIONS
I-CONGENITAL DISEASES OF THE CORNEA: -
1-Microcornea: -
Definition:
It is abnormally small cornea. Unilateral cases can be easily diagnosed by comparing it with the normal eye. Bilateral conditions can be compared with the standard dimensions in literatures.
Small cornea usually present with microphthalmia.
2-Macrocornea (Megalocornea): -
Definition:
It means large cornea in a globe. Megalocornea is reported is association with congenital buphthalmos due to increased intraocular pressure.
3-Dermoids: -
Definition:
It is skin-like mass present on the cornea and/or conjunctiva.
Treatment:
Superficial keratectomy is the successful method
4-Coloboma of the cornea: -
Definition:
It means absence of a segment of the cornea. It is a very rare condition.
II-ACQUIRED DISEASES OF THE CORNEA: -
1-Keratitis: -
Definition:
It means inflammation of the cornea. In general keratitis is accompanied by subjective and objective symptoms.
A-Subjective symptoms: -
These signs are slight or severe depending on the type of keratitis.
1-Pain of the cornea. It is exhibited by photophobia and blepharospasm
2- Lacrimation (epiphora)
B-Objective symptoms: -
1-Conjunctival and ciliary injection of the blood vessels:
The conjunctival and ciliary blood vessels around the cornea become injected and engorged with blood. The conjunctival blood vessels appear bright red, dilated, tortuous and movable while ciliary blood vessels are darker in color, straight and not movable.
2-Loss of corneal transparency:
Usually results either from corneal edema or due to inflammatory exudates which is usually leucocytic infiltration. Edema results when excess water accumulates within the stroma and forces the collagen lamellae apart.
3-Vascularization of the cornea:
The normally avascular cornea may be invaded by different blood vessels as a defensive mechanism against disease or injury. New vessels invade the cornea from the limbus at the level of the pathological lesion. Vascularization of the cornea either superficial or deep.
*In superficial Vascularization of the cornea the new blood vessels are present at the anterior third of the stroma under the epithelium. They originate from the conjunctival blood vessels at the limbus. These vessels are bright red in color and tortuous with numerous branching. It may be confined to a segment of the cornea or may extend around the limbus and readily seen making transition from the conjunctiva to the cornea.
*Deep vascularization is derived from the anterior ciliary branches. They are usually short, straight, non-anastomosing and dark red in color. They occur at the level of the lesion and usually exhibited about the entire peripheral cornea.
4-Ulceration of the cornea:
Corneal ulceration may be superficial or deep and spreading or localized. The extent of ulceration is easily determined by fluorescein dye that stains green all ulcerated and abraded areas.
5-Pigmentation of the cornea:
It is a nonspecific response to inflammation of the cornea, either mild or severe. The pigments are deposited in the epithelium or stroma. It originates from lirnbal melanoblasts that migrate into the stroma directly or with the blood vessels in neovascularization of the cornea.
6-Hypopyon:
It means accumulation of inflammatory exudates at the anterior chamber of the eyeball.
Classification of keratitis:
Keratitis may be classified according to the etiological agents or according to the anatomic structures involved. However, the later classification is easier
A-Superficial keratitis: -
The process of inflammation is limited to the epithelium and superficial part of the stroma.
i-Superficial punctate keratitis: -
Definition:
It is an epithelial and sub-epithelial lesions range in size between fine circular to large white opacities. Evidence of ulceration is present and thus it may or may not stain faintly with fluorescein. There are usually no clinical signs or ocular inflammation. The cause of such condition is not yet clear but it may be associated with long standing keratoconjunctivitis or due to virus infection
Treatment:
No specific treatment is recommended since the lesions leave the cornea within several months to a year. Topical corticosteroids, antibiotics and/or yellow mercuric oxide may be helpful.
ii-Superficial corneal abscess: -
Abscess formation at the superficial layers of the cornea usually due to trauma, scratches or presence of a foreign body. A green yellowish swelling is usually observed and vary in size from few millimeters to 20 mm
Treatment:
Opening of the abscess and suction of its content followed by curettage will correct the conditon. Superficial keratectomy has provide a satisfactory results in most cases.
iii-Pannus or uberreiter's syndrome: -
It is a diffuse inflammatory condition affecting the superficial layers of the cornea. It is a primary condition in German Shephard breed and termed degenerative pannus. Pannus is a subepithelial connective tissue infiltration and vascularization of the cornea.
Causes:
Unknown. Breed predisposition is suggestive.
Symptoms:
It is usually bilateral condition begins as a grayish haze at the temporal limbus and then starts at the nasal limbus and grow to cover the whole cornea. Superficial vascularization derived from the conjunctiva. The blood vessels are wavy and much branched. Pigmentation continues to spread over the cornea. If the condition is untreated, blindness may result in many cases.
Treatment:
Pannus is a chronic progressive corneal disorder that cannot be cured. It can be controlled by a variety of medical and surgical means, so that blindness rarely occurs. Treatment must be continued for animal life.
1-Early cases treated by topical application of corticosteroid 4-6 times daily until improvement, then 2-3 times daily. Dexamethasone solution is used 4 times a day then the dose is decreased gradually if response has been observed.
2-In moderately advanced cases subconjunctival corticosteroids are indicated.
3-In advanced cases with minimal scarring Beta radiation is per- formed.
4-Chemical cauterization with pure carbolic acid (phenol) may be applied to the pannus area.
5-Superficial keratectomy is performed in advanced cases with severe scarring. Antibiotic ophthalmic ointment is applied until re-epithelialization have been completed.
6-Periotomy: The conjunctival blood vessels are destroyed at the limbus at the base of the pannus by heat or electrocauterization.
4- Keratoconjunctivitis sicca: -
It is a diffuse superficial keratitis secondary to lacrimal gland insufficiency.
5-Exuberent granulation tissue
Builds up on the surface of the cornea as a result of injury. Superficial vascularization can be observed crossing the cornea to the area of granulation.
Treatment:
1- Periotomy
2- Surgical removal of the granulation tissue.
B-Pigmentary keratitis: -
Definition:
It means deposition of pigments in the cornea as a response to an irritating factors or a form of stress. Deposition of pigments may be superficial in the epithelium and superficial stroma or deep at the deeper layers of stroma.
Causes:
1-Trichiasis and distichiasis 2-Exophthalmos and large palpebral fissure
3-Entropion and prominent nasal folds 4-Keratoconjunctivitis sicca
5-Diffuse superficial keratitis (pannus) 6- Following corneal injuries
7-Anterior synechia (congenital or acquired) 8-Persistent pupillary membrane
Symptoms:
Pigments may be present superficially at the epithelium and superfic- ial layers of stroma or deep at the deeper parts of the strorna. It may accompanied by opacities of the cornea and vascularization according to the cause.
Treatment:
1-Elimination of the cause, as correction of distichiasis or trichiasis, removal of nasal folds, reduction in the size of palpebral fissure and etc.
2-Removal of the pigments '' If the pigments not interfere with the animal vision, treatment is not indicated, but if the pigments has already resulted in visual impairment, it can be removed by superficial keratectomy.
C-Interstitial or deep keratitis: -
Interstitial keratitis indicates inflammation of stroma, Descemet's membrane and endothelium.
Causes:
1-Extension of bacterial infection from a focus in the animal body such as; diseased tooth, gingivitis tonsillitis, cellulites, or prostatitis
2-Systemic disease such as canine distemer and ifectious hepatitis in dogs leptospirisis in horses and pinkeye in cattle.
3-Extension of infection either from superficial layer of the cornea or from the sclera or secondary to anterior uveitis.
4-Traumatic injuries. 5- Neoplasia.
Symptoms:
1-Corneal opacity due to edema and cellular infiltration. The opacity appears as ground glass-like appearance and occupies the entire cornea or may be localized.
2-Deep vascularization of the cornea specially if there is a concurrent iridocyclitis. The blood vessels are usually present at the limbus circumferentially in a form of short vessels parallel to each other and perpendicular to the limbus. They are directed towards the center of the cornea. In later stages they become prominent and progress in a brush-like fashion.
3-Conjunctival and ciliary injection of blood vessels is usually evident.
4-Hypopyon may be present.
Treatment:
1-Efforts are directed to define the cause and correct it.
2-Topical application of atropine sulphate 1% solution to reduce the chances of anterior synechia and to reduce pain relieving the ciliary spasms.
3-Corticosteroids locally and systematically. Be sure that ulceration of the cornea is absent.
4-Broad spectrum antibiotics locally and systematically especially when bacterial infection is suspected. Select the antibiotic having the power of penetrating the blood-aqueous barrier and intact corneal epithelium to be more effective on the deeper layers of the cornea.
D-Ulcerative keratitis or corneal ulcer: -
Definition:
Corneal ulcer is a lesion in which the epithelium and a variable amount of stroma have been lost. Corneal ulceration usually accompanied by infection and of the cornea and the ulcer is chronic and heals slowly or do not heal.
Causes:
1- Mechanical causes:
a-Abrasions b-Trichiasis, distichiasis and ectopic cilia c-Foreign body injury
d-Exophthalmos and exposure keratitis e-Entropion f-Tumour of the lid margin
2-Infectious causes:
a-Bacterial-strept., Staph., E.coli, and Moraxella b-Mycotic
c-Viral (Canine distemper and herpes virus in dog and cat) d-Chlamydial
3-Metabolic causes:
a-Vitamin A difficiency b-Senility c-Keratoconjunctivitis sicca
4-Neurotrophic causes:
Paralysis of the ophthalmic branch of the trigerninal
5-Allergic causes:
Very rare and the ulcer usually present near the limbus.
6-Exposure factors:
Exophthalmos and lagophthalmos.
7-Hereditary causes:
Certain breeds of dogs specially those having prominent eyes has a hereditary predisposition for corneal ulceration.
8-Burns
Symptoms:
1-Severe pain is manifested by btepharospasm and photophobia with partial or complete closure of the eyelids. Rubbing of the affected eye against objects is evident.
2-Serous, mucopurulent or purulent discharge
3-Loss of transparency of the cornea. Corneal opacity due to edema or cellular infiltration is usually observed around the ulcer or affects the entire cornea.
4-Vascularization of the cornea is the natural response to corneal damage. Superficial and deep vascularization is observed according to the type of the ulcer.
5-The presence of the ulcer its self is diagnostic. The contour of the cornea is changed. Deep ulcer is easily seen but superficial one needs fluorescein dye for staining.
Diagnosis:
1-Clinical symptoms
2-Staining with nonvital stain as fluorescein. Impregnated papers are used or 0.5 - 2% solution. Fluorescein is water-soluble and will not penetrate the intact corneal epithelium. If the epithelium is disrupted, fluorescein will stain the corneal stroma with green color. The strip is moistened with sterile saline and is placed in contact with the superior bulbar conjunctiva. The stain is distributed over the surface of the cornea by closing and opening of the eyelids. The dye is flushed from the corneal surface adequately. The stain will penetrate the stroma and stains it green. Source of light and magnification is used for thorough examination.
Treatment:
The main object of the treatment is to control infection and hastening the repair process.
1-Elimination of the possible cause
2-Control of infection: In bacterial infection, try broad-spectrum antibiotics as gentamycin, neomycin or polymyxin and bacitracin. Viral infection is treated by idoxuridine drops 0.1% every 1-2 hours or 0.5% ointment 3-4 times daily. Mycotic infection is corrected by pimaracin. In chlamydial infection use erythromycin and gentamycin.
3- Atropine 1% solution 3-4 times daily to relieve pain
4-Cauterization by using tincture of iodine or phenol for sterilization of the ulcer. It is important to cauterize the ulcer and 1 mm area of surrounding margin.
5-Third eyelid flap and/or conjunctival flap
6-Superficial keratectomy
Sequelae of corneal ulceration:
I-Scar formation:
When corneal stroma is destroyed, regeneration is made by keratocyte and fibroblasts cells. Collagen fibrils produced by these cells are not laid down in a regular manner and do not transmit light. With time scars tend to clear optically. The tendency of clearing is greater in young animals. The deeper the initial injury, the more dense and permanent the scar. Corneal scars are termed nebula, macula, and leucoma according to its size and density.
Treatment:
In most cases no treatment has been recommended, as the scar not interfere with vision. In large leucomas, corneal graft is the treatment of choice.
2-Descemetocele (Keratocele):
Definition:
Means protrusion of the Descemet's membrane through the floor of a deep corneal ulcer forming a small transparent vesicle. The membrane is protruded due to the pressure of the aqueous humour and not rupture due to its elasticity. Sometimes the protruded membrane is covered fibrinous exudates and corneal epithelium.
Treatment:
1-The treatment is performed by reposition of the protruded membrane back with an iris spatula or blunt instrument then the wound is sutured by one or two interrupted stitches.
2-When the cause is a deep ulcer, paracentesis of the anterior chamber is required to relieve pressure over membrane. A small needle is introduced at the limbus to reduce the pressure
3-Nictitating membrane flap is placed over the cornea for 10-14 days.
4-Medical therapy consists of topical antibiotic and 1% atropine ointments.
3-Iris prolapse:
Definition:
Means protrusion of iris through penetrating corneal wound or rupture corneal ulcer. The iris is carried forward into the corneal defect by escaping aqueous humour.
Treatment:
The wound is flushed with boric acid solution and the iris is redeposited into the anterior chamber or amputated if it appears unhealthy. The corneal wound can be sutured after trimming of the edges of the wound. The eyelid flap is applied for 10-14 days, and topical antibiotics and atropine ointments are applied for several successive days.
4-Iris staphyloma:
Definition:
Means protrusion of a part of the iris through the ruptured corneal ulcer and the protruded part is covered with fibrin and layer of epithelium. Adhesion usually present between the protruded part of iris and the edges of corneal wound.
Treatment:
The protruded part of iris is excised and iris spatula is used to free the adhesions between the iris and edges of the ulcer. The wound edges are trimmed with care and then are apposed with simple interrupted sutures. Sterile saline solution and air is injected into the anterior chamber to restore it. A nictitating membrane flap is then performed and topical antibiotic and atropine is applied.
5-Mypopyon:
Definition:
Means accumulation of inflammatory exudate at the ventral part of the anterior chamber. It usually occurs in cases of severe corneal ulceration with secondary iridocyclitis.
Treatment:
Paracentesis and aspiration of the inflammatory exudates or injection of Alfapsin enzyme intracameral
6-Anterior synechia:
Definition:
Means adhesions between the iris and corneal endothelium (posterior surface of the cornea).
Treatment:
Application of atropine 1% may relieve the condition. Surgery may be recommended to remove the adhesions.
7-Panophthalmitis:
Definition:
Means severe purulent inflammation of the eyeball (pus in the eyeball).
Treatment:
Enucleation of the eyeball
Conjunctival flaps:
Advantages:
1-An excellent means for supporting the cornea. The conjunctiva provides tissue to fill in strdmal defect. They provide a little more support to the cornea than does a nictitating membrane flap.
2-The conjunctival flap provides a vascularized tissue in intimate contact with the corneal defect.
3-Small and thin conjunctival flaps allow partial vision.
4-Partial conjunctival flap allows observation of the defect and direct application of medication on the cornea. The conjunctival flap should consists of conjunctiva only. It is very elastic, thin and nearly transparent. If the Tenon's capsule is incorporated, it reduces the elasticity of the flap making it more difficult to position. The flap should be transparent enough to permit some vision. Thicker flaps cannot be left permanently as they will interfere with sight. In some severe corneal degeneration the flap may be left permanently. Thin flaps can be dissected after subconjunctival injection of B.S.S. Solution. After transportation, the deep conjunctival surface will adhere to any area where corneal epithelium is missing. It will not adhere to intact surface of the cornea. Most conjunctival flaps have served their purpose and can be removed back in 2 - 3 weeks after transportation.
Bulbar conjunctival flaps are classified into:
1-Partial or Hood bulbar conjunctival flap
It is employed in case of corneal ulcers with stromal loss near the limbus in the following manners