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What is the function of the cornea?
- transparency - lets light into the eye
- helps to focus light on the retina
How is nourishment provided to the cornea?
by tears, aqueous humor
Other tissues are provided nourishment through blood vessels, but not the cornea. Why?
blood vessels would reduce the transparency of the cornea
How many layers does the cornea have?
What are the layers of the cornea from superficial to deep?
- Bowman's membrane
- Descemet's membrane
What is the epithelium of the cornea?
outer layer - covers the surface of the cornea
How many cell layers thick is the epithelium?
5 - 6 cells
What is the epithelium of the cornea filled with?
tiny nerve endings (unmyelinated)
Is the cornea very sensitive to pain?
What is the function of the epithelium of the cornea?
- blocks the passage of dust and microorganisms
- smooth surface absorbs oxygen and cell nutrients from tears, then distributes these nutrients to the rest of the cornea
Does the epithelium of the corena regenerate well after an injury?
Where is the Bowman's membrane?
lies beneath the epithelium
Is the Bowman's membrane tough and difficult to penetrate?
What is the function of the Bowman's membrane?
- anchors the epithelium
- protects the deeper layers
- regenerates well after injury
- injuries deeper than the Bowman's membrane can leave an opaque scar
Where is the stroma?
lies beneath the Bowman's membrane
Which layer is the thickest layer?
What is the stroma composed of?
tiny collagen fibrils that run parallel to each other
What is the function of the stroma?
precision formation gives the cornea clarity and strength
What are the two ways to go blind?
- injury to the stroma can leave an opaque scar
- injury to the endothelial layer can result in fluid accumulation in the stroma, causing opacity and blindness
Where is the Descemet's membrane?
lies beneath the stroma
What is the Descemet's membrane?
thin but strong sheet of tissue
What is the Descemet's membrane composed of?
composed of collagen fibers (different from those of the stroma)
What is the Descemet's membrane made?
made by the endothelial cells that lie below it
What is the function of the Descemet's membrane?
protects against infection and injuries
Does the Descemet's membrane regenerate well after injury?
Where is the endothelium?
innermost layer of the cornea
What is the endothelium?
extremely thin - only one cell layer thick
What is the function of the endothelium?
endothelial cells pump excess fluid out of the stroma, keeping it clear
Does the endothelium regenerate after injury?
What is the Fluorescein dye test?
is a way to test for corneal injury or nasolacrimal duct patency by placing fluorescein dye onto the surface of the cornea
What is fluorescein dye?
yellow water soluble dye - looks orange on the paper strip, looks green in the tear film, and stains the corneal stroma green
What does fluorescein dye do under black light?
What is the main purpose of the fluorescein dye test?
- to detect corneal scratches, abrasions, or ulcers
- to detect nasolacrimal duct blockage
- has been used given PO to determine which cat in a multiple cat household is urinating outside the litter box
How are we able to get the fluorescein dye test?
- sterile strips of paper impregnated with fluorescein sodium
- 2% aqueous solution
What is the disadvantage of the 2% aqueous solution of fluorescein dye?
Pseudomonas can grow in the solution
How does the fluorescein dye test show a corneal injury?
- shows full thickness loss of corneal epithelium
- corneal epithelium will not stain because its outer lipid cell membranes repels the stain
- corneal stroma does stain
- Descemet's membrane does not stain
What is a nasolacrimal duct blockage?
nasolacrimal duct may become blocked due to swelling and mucus accumulation
How does fluorescein dye test show a nasolacrimal duct blockage?
- fluorescein dye can be placed in the eye (one at a time) to see if the dye will pass through the duct
- check inside the nostril and the back of the throat and tongue to the presence of the dye (means that the duct is patent)
- may need to do both eyes and compare the results
How long do we wait to check the patency for a nasolacrimal duct for a dog? Cat?
- dog: 2 - 5 minutes
- cat: up to 10 minutes
What does a positive test for the nasolacrimal duct blockage indicate?
Fluorescein dye can be added to irrigating solution for flushing of the nasolacrimal duct. Why would we do this?
makes it easier to detect the irrigating solution at the nose or back of the throat
What are the three methods of use for fluorescein dye?
- strip in syringe
- direct drop
- direct placement
How do we do the strip in syringe method?
- put fluorescein dye strip into a 3 cc syringe with a few cc of tap water
- let the dye dissolve into the water
- place a few drops from the syringe into the patient's eye
How do we do the direct drop method?
place a drop of irrigating solution (water, sterile saline) on the strip and then allow the drop to fall on the eye
What is the advantage to the direct drop method?
less diluted this way, but requires more patient cooperation and understanding gravity and a good aim
When we are doing the direct drop method, what do we need to be careful we don't do?
- when dropping the drop onto the eye, do not allow the test strip to contact the cornea
- if it touches the eye it can cause a false positive stain retention at the site of contact with the corneal epithelial cells
How do we do the direct placement method?
- the strip may be placed directly onto the patient's eye (sclera), like a Schirmer tear test strip
- strip is bent at notch and placed over lower lid
Doing the direct placement is more common in...
Why don't we use the direct placement often in animals?
too much risk of direct contact with the cornea
What are the cautions for using the fluorescein dye test?
- conjunctival or corneal epithelial cells for fluorescent antibody testing should be collected before a fluorescein dye test on the eye is done - this can cause a false positive test for several days after application of the stain
- rarely, fluorescein dye can cause hypersensitivity reactions
- temporary staining of fur and skin can occur