Cornea Flashcards
Function of the cornea
It functions as a refractive surface (bends light). It is transparent by the absence of blood vessels, pigment, lymphatics and myelinated nerves, and by maintaining a relative state of dehydration (deturgescence).
thickness of the corneal epithelium
3-5 layers, lipophilic, and has a basement membrane.
Innervation of superficial cornea
The superficial cornea is highly innervated by the trigeminal nerve (CrN 5). Ulcers in the superficial cornea are more painful than deeper ulcers
thickest layer of the corena
stroma
Layer that takes up stain
stroma NOT basement membrane
Corneal epithelium function
It is an active Na-K-ATPase water pump to maintain corneal deturgescence (dehydration). Once lost, endothelial cells do not regenerate in most species.
corneal edema
is a result of disruption of the epithelium or damage to the endothelium. Edema has a blue-white mottled or cobblestone appearance and causes a loss of corneal transparency.
Focal edema
epithelial loss (ulcer) allows tear film to overhydrate the stroma
Generalized edema
endothelial pump failure
Mechanisms of generalized edema
a- increased IOP – ‘disables’ pump
b- uveitis-toxic to pump
c- endothelial dystrophy- hereditary or senile change, loss of endothelial cells
d- endothelial degeneration (inflammatory disease), blood vessels and/or lipid, calcium deposition
e- anterior lens luxation disabling the endothelial cells
f- immune complex: CAV1
Endothelial dystrophy - breeds affected
This is commonly seen in middle aged to older dogs, but primarily in Boston Terriers and Chihuahuas.
endothelial dystrophy pathogenesis
Usually starting at the temporal limbus, as endothelial cells fail, adjacent cells try to compensate for their loss and they in turn fail, causing the edema to progress across the cornea. As water bullae form near the epithelial surface and rupture, the eye becomes painful. These ulcers are difficult to heal and frequently recur.
Treatment of endothelial dystrophy
Medical treatment is with 5% NaCl ointment to dehydrate the epithelial surface. This is symptomatic treatment and may require 3-4 applications per day. The disease will progress.
surgical treatment of endothelial dystrophy
A surgical alternative is to perform a superficial keratectomy and place a permanent graft over the corneal stroma allowing the vessels to drain the edema and preventing painful ulcerations and progression of the disease.
Lipid dystrophy
subepithelial deposition of lipids due to a hereditary predisposition (bilateral and paracentral), corneal trauma, hyperlipidemia or other metabolic disorders. It has a white, crystalline appearance.
sequelae of lipid dystrophy
This is a non-painful, progressive disorder that is worsened with topical steroid use. Over time the cornea may degenerate with superficial calcium deposition, become friable and create painful ulcerations.
management of lipid dystrophy
A low fat diet is recommended to slow lipid deposition progression.
Corneal ulcer
defect or break in the epithelium that exposes the stroma. The stroma is hydrophilic and takes up fluorescein stain.
Causes of corneal ulceration
Trauma, KCS, Exposure, Corneal Degeneration, Viral
Corneal degeneration:
This may be the sequela to chronic lipid dystrophy or trauma. Calcium in the superficial epithelium is brittle and prevents normal corneal epithelial cells from being confluent across the cornea.
Uncomplicated superficial corneal ulcers:
These are superficial, non-infected ulcers that resolve with appropriate treatment within 3-5 days.
Superficial ulcers can cause what other abnormality?
Superficial ulcers can cause reflex uveitis. This is a consistent occurrence in horses and needs to be treated to prevent intraocular damage. Treatment should be directed at both the ulcer and uveitis.
Monitoring superficial ulcers- when should healing occur?
Corneal epithelial healing begins within an hour of injury and an epithelial defect can be covered in 6 hours. Superficial ulcers with proper treatment should be healed within 5 days. If a superficial ulcer has not healed in 7 days, changing the antibiotic is not the solution- look again for an underlying cause.
Treatment for superficial ulcers dogs
- remove underlying cause
- Neomycin, polymyxin, bacitracin (BNP) good 1st choice for dogs & horses (not cats)
- atropine
- e-collar
- pain management
antibiotics for superficial ulcers in cats
terramycin or erythromycin for cats
antibiotics for superficial ulcers in exotics
ofloxacin for exotics or when corneal penetration is needed.
atropine contraindicated for treatment of ulcers when?
use with caution in patient with KCS as it decreases tear production. It is contraindicated with glaucoma.
Non-healing ulcer, Indolent ulcer, Boxer ulcer, SCCED
Spontaneous Chronic Corneal Epithelial Defect
Any superficial ulcer that has not healed or improved in over 5 days with no underlying cause identified. These ulcers are superficial (only epithelial defect, no stromal loss), painful, and do not involve the stroma.
Signalment of indolent ulcers
Indolent ulcers are seen in any age boxer, or any breed dog over 6 years old.
etiology of indolent ulcer
This is due to failure of the attachment apparatus of the basal epithelial cells to the stroma resulting in loose, unattached epithelium (hang nail).
indolent ulcers and corneal vascularization
In some patients corneal vascularization is robust (beginning 7 days after ulceration), and in others corneal vascularization is not provoked even after prolonged ulceration.
Why does corneal edema occur with indolent ulcers?
may occur in the ulcer site due to stromal absorption of the tear film.
Treatment of non-healing, indolent ulcers in dogs
debride the corneal epithelium with a sterile cotton-tipped swab removing all loose epithelium leaving a sharp edge of firmly attached epithelium. This can be performed using a topical anesthetic and sedation.
treatment to decrease discomfort following debridement?
contact lens, morphine eyedrops
prevention of ciliary muscle contraction and spasm in patients not at risk of glaucoma
atropine 1 percent ointment
medication to control corneal edema
Topical NaCl 5% ointment
what medication may shorten corneal epithelium healing time
doxycycline
why should you not use grid keratomy on cats?
corneal sequestrum may develop
Complicated corneal ulcer:
Any ulcer that has not healed or improved in 5-7 days, or is infected, deep stromal or melting.
Deep Stromal Ulcers
These are deep or progressive ulcers that are usually the result of trauma, inappropriate use of steroids in the face of an ulcer, infected ulcers, or KCS; Stromal ulcers take longer to heal and will leave a scar.
What color are infected stromal ulcers?
yellow
Treatment consideration if stromal ulcers are deep?
sugery
Descemetocele
deep ulcer that has breached the entire stromal thickness and exposes Descemet’s membrane.
What do descemetoceles look like?
It may appear as a deep defect that only takes up fluorescein stain around the rim, or it may bulge due to intraocular pressure.
Treatment for descmetoceles
surgery (graft)
Melting Ulcers (corneal malacia):
The corneal stroma is made of collagen bundles. Proteinase and collagenase destroy the collagen and cause it to ‘liquify’ or melt.
Proteinase is found in:
1- Steroids
2- Neutrophils, macrophages
3- Some bacteria and fungi
melting ulcers are
an emergency
These eyes can perforate quickly (hours).
Melting ulcers are more common and more severe in horses than in dogs or cats.
how often should anti-proteinase therapy be applied to melting ulcers
hourly
What antiprotease medications should be used for melting ulcers
EDTA is easily compounded by adding sterile water to purple top tube to fill line
Tetracycline-topical and/or oral
Serum – sterile collection, refrigerated (better than plasma) keeps for 5-7 days
Acetylcystein
Surgical treatment of melting ulcers
If the melting does not respond to medical therapy or worsens in the face of treatment, surgery is indicated. This involves a surgical keratectomy to remove malacic tissue and a conjunctival graft.
Surgical repair uses what techniques?
corneal graft, conjunctival graft, amnion graft, biosist or A-cell graft.
conjunctival graft
from the dorso-temporal bulbar conjunctiva and sutured onto the cornea over the debrided corneal defect. Adequate magnification and expertise are required.
Infectious causes of ulcerative keratitis in the cat
the herp
Infectious causes of ulcerative keratitis in the horse
the herp & 5, pseudomonas, staph, strep & fungal keratitis)
& 5, pseudomonas, staph, strep & fungal keratitis) - dog
less common but staph, strep, pseudomonas
Stromal abscess
sterile or septic, either bacterial or fungal. They may be associated with a corneal ulcer or non-ulcerated within the stroma.
distinguishment of stromal abscesses from lipid and edema
They are very painful and can be distinguished from edema and lipid their creamy, solid appearance.
treatment of stromal abscess
Abscesses generally require surgical excision (keratectomy) and a graft to heal.
Pannus - what breed most predisposed
GSD (and others (Greyhound, Border Collie and others).)
appearance of a pannus
This is an immune mediated disease of the superficial cornea manifested by corneal inflammatory cell infiltrate (gray haze), pigmentation (brown), vascularization (red), and edema (white). The dogs are not painful but often have a gray ocular discharge.
NON ULCERATIVE
pannus sequelae
but punctate ulcers may occur as part of the immune process, or indolent ulcers may occur secondary to the corneal edema.
What happens if you do not manage the pannus?
Pannus is a chronic, progressive disease that will lead to vison loss due to corneal pigmentation if not controlled. Pannus is not ‘cured’ but requires life-long management. The cornea can be cleared of inflammatory cells, edema and blood, but pigment is irreversible.
What triggers the pannus and what is the clinical presentation?
It is triggered and exacerbated by UV light. It often starts at the inferior lateral limbus and progresses across the cornea in GSD. It usually starts from the superior limbus in Greyhounds. It is a bilateral disease.
Plasmoma
depigmentation and thickening of the third eyelid due to inflammatory cell (plasma cells) infiltrate that is often seen with pannus. In some cases medial or lateral canthal ulceration occurs
Diagnosis of pannus
Breed, location of lesion, presence of blood vessels, inflammatory cells, edema and pigment.
Treatment of pannus
Cyclosporine (CSA): Optimmune is an IL2 blocker causing immune suppression (of T-lymphocytes).
NPDex: (neomycin, polymyxin, dexamethasone) - Dexamethasone reduces inflammation and reduces corneal vascularization.
Avoid UV light:
other treatments for pannus
Doxycycline can be used as an immune modulator as part of the initial treatment plan for 2 weeks
Hypoallergenic diet such as Z/D may be beneficial in helping to control the disease in some dogs.
Terramycin- may have immune modulating benefit used topically.
Pigmentary Keratitis:
Pigmentation of the cornea may occur secondary to corneal exposure, irritation, low tear flow, aberrant hairs or growths rubbing the cornea.
Breed disposition to pigmentary keratitis
It is most commonly seen in brachycephalic dogs and may occur without obvious underlying problem in Pugs, Boston Terriers and Shih Tzu.
Surgical repair of pigmentary keratitis
medial or lateral canthoplasty, removal of cilia and lid abnormality correction
Medical management of pigmetnary keratitis
treatment with cyclosporine or tacrolimus may help to decrease pigmentation and improve tear film quality
Limbal melanomas
arise from the melanocytes of the limbus and are distinct from uveal melanomas.
Treatment of limbal melanoma
In some cases there is no progression of the tumor, but in patients with evident progression or invasion into the cornea, full thickness excision with a tectonic graft, or partial thickness excision with diode laser of the remaining melanoma are options.
