Cornea Flashcards
What 2 corneal measurements increase with age in adult dogs?
Corneal thickness
Descemet’s thickness
What are the 3 predominant GAGs in the cornea?
Keratin sulfate
Dermatin sulfate
Chondroitin sulfate
What is the normal cell density of endothelial cells?
2500-3100 cells/mm2
What is the average thickness of the canine cornea?
~560 um (pachymetry)
~85 um (confocal microscopy)
What nerve innervates the cornea?
Trigeminal
Who has a decr density of nerves in the cornea?
Brachycephalics
What part of the cornea is the most sensitive?
Central
Other than brachycephalics, what other group has decr corneal sensitivity?
Diabetics
What has more sensitivity: dorsal/ventral cornea, or nasal/temporal cornea?
Nasal/temporal
Does morphine 1% affect epithelialization?
No
What is the average turnover of corneal epithelium?
2 wks
What is the term for when epithelial cells move over a defect?
Epithelial sliding
What is the lag time between injury and start of epithelial sliding?
1 hr
If the entire corneal epithelium is removed, most species can cover it by sliding CONJUNCTIVAL epithelium in how much time?
48-72 hrs
Epithelial spreading results in a thin epithelial barrier. How do epithelial cells increase the thickness?
Mitosis
PMNs enter the cornea from what TWO sites?
Tear film and conjunctival vessels
What cells in the corneal stroma transform after injury? What do they transform into?
Keratocytes –> fibroblasts
Name the 6 phases of corneal wound healing?
- Immediate
- Leukocytic
- Epithelial
- Fibroblastic
- Endothelial
- Changes in organization and cellularity
Describe what happens in the immediate step of corneal wound healing?
- Outer stromal fibers and Descemet’s retract, causing gaping on either side of the wound
- Fibrinogen (from inflamed AH) come into contact with wound
- Fibrinogen is transformed into fibrin, and forms a plug
In a perforation, name the 4 ways PMNs can get to the wound (for the leukocytic phase)?
Tear film
Conj blood vessels
AH
Perilimbal vessels (if chronic)
How long does it take PMNs to reach the wound?
0.5-5 hrs
How long does it take mononuclear cells to reach the wound?
12-24 hrs
What 2 things monocytes do in the wound?
Scavenge
Transform into fibroblasts
Why is the epithelial phase so important to wound healing?
It is needed for transformation of keratocytes and mononuclear cells to fibroblasts
What happens if epithelium does not cover the wound?
Wound healing is significantly delayed
How long does the fibroblastic phase take to start?
12 hrs
Which keratocytes are transformed first?
Those closest to the wound edge
When does the endothelial phase begin (time)?
24 hrs
What occurs in the endothelial phase of wound healing?
Endothelial sliding or amitotic multiplication
What change to Descemet’s would indicate an old corneal perforation (histologically)?
Descemet’s duplication
How long does it take for corneal incisions to heal for significant strength to allow suture removal?
19 days
What 3 organisms make a ton of exogenous proteases?
Fusarium, Aspergillus, Pseudomonas
What 2 MMPs are especially important in corneal remodeling and degradation?
MMP-2 and -9
Where is pigment usually deposited in the cornea?
Basal epithelial cells
In the rabbit, 24 hrs after removal of the endothelium vs epithelium increased corneal thickness by?
Endothelium: 500%
Epithelium: 200%
Where is most of corneal edema distributed in the cornea?
Posterior lamellae
Uveitis results in cornea edema due to what 2 changes?
- Increased endothelial permeability
2. Decr function of Na-K ATPase pump
What virus causes blue eye?
Infectious canine hepatitis (canine adenovirus)
What type of reaction occurs with infection of infectious canine hepatitis?
Type III hypersensitivity
Why is blue eye seen with infection of CAV?
Immune complex formation after release of virus from infected endothelial cells
What breed is predisposed to blue eye?
Afghan hound
What vaccine is more likely to cause blue eye?
CAV-1 modified live
What vaccine is less likely to cause blue eye?
CAV-2
Of those dogs that develop blue eye (from vaccination or natural infection), what percentage will have permanent corneal edema?
30%
Intraocular irrigation can increase risk of corneal endothelium damage if volume is over how many mL?
100 mL
Intraocular irrigation can increase risk of corneal endothelium damage if time is over how many minutes?
20 min
Where do superficial vessels originate from?
Conjunctiva
Where do deep vessels USUALLY originate from?
Anterior ciliary bidy
Where do deeps vessels RARELY originate from, and when does this happen?
Iris; if anterior synechiae
Is corneal depth consistent with vessel growth?
YES–they grow along a lamellar plane
Microcornea size?
<12 mm
3 diseases associated with microcornea
- Merle ocular dysgenesis
- Multiple ocular anomalies
- Ehlers-Dalos syndrome
Megalocornea size?
> 16-18 mm
What disease commonly occurs with megalocornea?
Congenital glaucoma and buphthalmos
What part of the cornea is a dermoid most commonly seen?
Temporal limbus
What part of the cornea is a dermoid RARELY seen?
Central cornea
Are dermoids usually superficial or deep?
Usually superficial
What is the max number of superficial keratectomies that can be performed at one site?
2-3
Why is there a limit to the number of superficial keratectomies that can be performed?
Stroma doesn’t usually regenerate completely
Why is the carbon argon laser not appropriate for cornea surgery?
Generates heat = coagulates tissue = too much damage
What type of laser can be used for corneal surgery?
Argon-fluoride excimer laser
What are the 3 descriptive terms used for a corneal opacity (by density)?
Nebula, Macula, Leukoma
How does a nebula appear?
Minor, diffuse, hazy, indistinct borders
How does a macula appear?
Moderately dense, with CIRCUMSCRIBED borders
How does a leukoma appear?
Dense and white
If iris is at the back of a leukoma, what is the proper descriptive term?
Adherent leukoma
What part of the cornea (specifically) is puppy dystrophy seen?
Subepithelial
Is puppy dystrophy congenital?
No
When is puppy dystrophy seen (age)? When does it disappear?
Usually <10 wks, but def gone by 12-16 wks
What corneal opacity can be associated with a PPM?
Adherent leukoma
What causes a PPM?
Embryonic vasculature structures fail to completely regress
Specifically, where is the corneal opacity seen with Peter’s anomaly?
Paracentral
Why is a corneal opacity seen with Peter’s anomaly (what structures are involved)?
Damage to endothelium and Descemet’s
How common are colobomas or staphylomas in the dog?
RARE
Why do staphylomas appear black?
Uveal tissue is showing
What other abnormality is commonly seen with staphylomas?
Strabismus
What are 6 causes of staphylomas?
- Congenital
- Inflammation
- Glaucoma
- Trauma
- Neoplasia
- Surgery
What breed specifically has an increased prevalence to staphylomas?
Miniature doxies
What is histologically characteristic of a SCCED?
Superficial stromal acellular hyalinized zone
Avg age for a SCCED?
8-9 yrs
Where on the cornea do SCCEDs usually occur?
Axial or paraxially
How can you differentiate bullous keratopathy and secondary ulcer vs a SCCED with edema?
Edema in a SCCED is MILD and confined to affected area
How does SCCED vascularization differ if the lesion is axially vs peripheral?
Axial = can be present for MONTHS without vascularization Peripheral = more likely to vascularize
How deep can a diamond burr make a defect?
Into epithelial basement membrane but NOT beyond
When is tissue adhesive NOT recommended for ulcers?
If deep/descemetocele = ocular toxicity
What is the biggest downside to a tarsoconjunctival graft?
Eyelids are mobile = TENSION
Why should epithelium be debrided prior to graft placement? (2)
- Graft can’t adhere if epithelium is there
2. Epithelial down growth can lead to graft dehiscence
How thick is D’s membrane?
3-12 uM
At what time point (hours) after prolapse should the prolapsed iris be amputated?
If incarcerated >6-8 hrs
How do you remove prolapsed iris?
Pull slightly to get fresh uveal tissue, and cauterize newr the cornea
3 most common bacteria that infect the cornea?
- Strep
- Staph
- P. aeruginosa
What virus can cause ulcerative lesions in dogs?
CHV-1
CHV-1 ulcers in dogs– what depth are they?
Superficial = NO stromal loss
How can you diagnose CHV-1 keratitis definitely in dogs?
Virus detection (vs FHV in cats… CHV-1 is NOT ubiquitous so positive test supports diagnosis)
What burn is usually WORSE: ACID or ALKALI?
Alkali
What do acids do the the cornea? How does this affect prognosis?
They tend to cause protein coagulation in EPITHELIUM = limits penetration/depth = SUPERFICIAL and NON-PROGRESSIVE
What do alkali do to the cornea? (3)
- They a LIPOPHILIC = cause SAPONIFICATION of fatty acids of cell membranes = cell death
- Hydrolyze intracellular GAGs and denature collagen
- Above damage stimulates inflammation and poteoytic enzymes
Treatment for chemical burns?
- IRRIGATE for 10-15 min or until ocular pH is normal
- Treat with anati-collagenases
- Treat as you would an ulcer = ABx, mydriatics, systemic NSAID or steroid
- If needed, conj graft
Irrigation for chemical burn– where should you be sure to irrigate?
Under the TEL
What are 4 sources of proteases/collagenases?
- Infectious organisms
- PMNs
- Fibroblasts
- Epithelial cells
What breed(s) are prone to rapidly progressive corneal pigmentation?
Brachycephalic– esp Pug, Shih Tzu, Pek, Lhasa
Where does pigmentary keratitis usually start?
Nasally
Where do melanocytes originate from to produce pigmentary keratitis?
Perilimbal
Possible treatments for pigmentary keratitis?(4)
- Keratectomy– but scarring and recurrence!
- Strontium-90 plesiotherapy
- Cryotherapy
- Topical: CsA, tacro, steroids
