Cornea (1) Flashcards
How many layers are there to the cornea?
Depends; if not counting tear film, 4 Epithelium Stroma Descemet's membrane Endothelium
Epithelium characteristics
5-7 layers
Hydrophobic/lipophilic barrier to drugs and fluorescein
Regular turnover
Stroma characteristics
75% water, 25% collagen
Regular (parallel) arrangement of collagen fibers
Hydrophilic/lipophobic
keratocytes
Descemet’s Membrane characteristics
BM of endothelium
Thickens with age
Endothelium characteristics
Dehydrate stroma by pumping mechanism
Monolayer of cells
Non-regenerative
Functions of the cornea
Anterior structure for globe
Protection of interior structures from injury/infection
Clear medium for vision
Refraction
Keepingthe cornea transparent
Avascular No pigment Non-keratinized epithelium Precise arrangement of collagen fibers Relatively dehydrated
Corneal innervation
CN V (trigeminal) More nerve endings in epithelium/superficial cornea Decreased sensitivity can affect healing - Brachycephalics - Sick foals - Diabetics
Axonal Reflex
Any stimulation of corneal nerves = reflex stimulation of the CN V nerve branches to anterior uveal tract
All keratitis creates some degree of reflex uveitis
Corneal ulcer = stimulation of CN V = reflex uveitis with painful ciliary body muscle spasm
- Can relieve with cycloplegic drug such as atropine
More severe the keratitis = more severe the uveitis
Epithelial loss alone heals by epithelial proliferation
Fact
Things to remember about corneal wound healing
Corneal blood vessels move at ~1mm/d
WBC move in the cornea at 8.6mm/d
Epithelial cells move at ~.6mm/d
Epithelial cells may take up to 6 wk to adhere to BM
Corneal disease states
Loss of corneal thickness
Loss of transparency
Define corneal ulcer
Scratch, wound, or defect of the cornea in which there is a loss of variable amounts of corneal tissue
Most common presenting ophthalmic condition
Corneal ulcer categories
Depth - Superficial - Stromal - Desmetocele - Full-thickness - iris prolapse Etiology - Traumatic - Infectious . Bacterial . Fungal - Immune-mediated Response to therapy - Simple - Complicated . Indolent . Melting
Simple ulcer criteria
Superficial
No infection
Heals in an appropriate amount of time (a week)
No complicating factors
How can one tell if an ulcer is infected?
Presence of cellular infiltrate Melting Degree of uveitis Delayed healing Positive cytology or culture
What are complicating factors leading to delayed healing of the cornea?
Entropion KCS (dry eye) Eyelid tumors Lagophthalmos Ectopic cilia Trigeminal neuropathy Systemic disease (Cushing's, DM) Distichiasis
Define desmetoceles
Epithelium and stroma lost; DM only remains
Very thin
DM will bulge into trough dt pressure from AqH
Iris prolapse
DM has ruptured, leaving hole in cornea open to outside
Iris passively follows AqH and often plugs hole
Melting cornea
Proteases - degredation of collagen
MMPs, serine proteases (NE)
Gelatinous appearance
Can happen within 24h
Corneal ulcer CS
Blepharospasm Epiphora Serous to micropurulent discharge Miosis d/t reflex uveitis Corneal edema Corneal vascularization
Management of corneal ulcers
Rule infection in or out - C&S (48h) - Cytology Rule out KCS - Schirmer tear test Stain with fluorescein - Depth of lesion - Epithelialization - Stain every eye with signs of pain
Surface properties
Fluorescein sodium - hydrophilic
Epithelium - hydrophobic
Stroma - hydrophilic
Descemet’s membrane - hydrophobic
Therapy for corneal ulcers
Depends on: etiology, infection, depth, melting, extent of uveitis Goals: - Slow collagen breakdown - Sterilize the wound bed - Control the secondary anterior uveitis - Provide structural support
T/F: During treatment, you are not actually making the ulcer heal
True
Medical therapy for corneal ulcers
Reduce tear protease activity Topical antimicrobials: C&S guides selection - SA: mostly Abx - Horses: Abx and antifungals Treat reflex uveitis Provide analgesia Prevent self-trauma Treat etiology
Simple ulcer medical therapy
Broad-spectrum topical abx TID-QID until healed - Prevent infection - Broad spectrum, non-epitheliotoxic - Neomycin, Polymyxin B, Bacitracin, Ofloxacin, Gentamicin Treat reflex uveitis - Topical atropine 1% - Oral NSAID (5d) Analgesia Anti-protease Prevent self-trauma
NO TOPICAL STEROIDS OR NSAIDS
Complicated ulcer medical therapy
Anti-protease (q1-6h) - Serum, plasma, EDTA, NAC, Tetracyclines Antimicrobials - Based on culture/cytology - q2-4h - Abx - Antifungals Treat reflex uveitis - Topical atropine 1% solution BID-QID - Oral NSAIDs Provide analgesia Consider surgical stabilization
Topical anti-collagenase/protease q1-6h
Autologous serum
- Red top tube, let clot, spin down, draw off serum
- Fill 1cc syringes to dispense to client
- Replace q7d, must stay in fridge
EDTA sol’n
- Fill half a purple top tube with sterile water
- Can dispense in dropper bottle for ease of administration
N-acetylcysteine (5%)
- 5mL 20% Mucomyst in 15mL artificial tears
Tetracycline
- Terramycin ointment
- Doxycycline 5mg/kg PO q12h
Topical Abx
Gr+: Cefazolin 55mg/mL / Ciprofloxacin
Gr -: Tobramycin/ Ciprofloxacin
Always give broad spectrum coverage
Remember: all Abx are toxic to epithelial cells
Least to most toxic topical Abx
Chloramphenicol Tobramycin Neopolygram Gentamicin Cefazolin Ciprofloxacin
Treat reflex uveitis
Regardless of initial cause, all ulcers associated with some iridocyclitis
- Topical atropine 1%
- Systemic NSAIDs
Surgical therapy of corneal ulcers
Surgical grafting procedure (synthetic, corneal, conjunctival)
- Stromal loss >50% of corneal thickness
- Corneal perforation
Surgical procedures
Conjunctival grafts
Biological graft
Corneal transplant
Corneoconjunctival transposition
Evaluation of iris prolapse
Potential to see - Menace - Dazzle - PLR - Fundic exam Sealed perforation - Formed anterior chamber - Leaking aqueous humor
Treatment of iris prolapse
Add oral broad spec Abx - Clavamox 13.25mg/kg PO q12h Strongly consider surgical repair No ointments (granulomatous uveitis) No topical atropine (let iris plug hole) Counsel owners on risk of vision loss from retinal detachment or endophthalmitis
