Cornea (1) Flashcards by Emma Mortensen

How many layers are there to the cornea?

Depends; if not counting tear film, 4
Epithelium
Stroma
Descemet's membrane
Endothelium

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Epithelium characteristics

5-7 layers
Hydrophobic/lipophilic barrier to drugs and fluorescein
Regular turnover

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Stroma characteristics

75% water, 25% collagen
Regular (parallel) arrangement of collagen fibers
Hydrophilic/lipophobic
keratocytes

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Descemet’s Membrane characteristics

BM of endothelium

Thickens with age

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Endothelium characteristics

Dehydrate stroma by pumping mechanism
Monolayer of cells
Non-regenerative

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Functions of the cornea

Anterior structure for globe
Protection of interior structures from injury/infection
Clear medium for vision
Refraction

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Keepingthe cornea transparent

Avascular
No pigment
Non-keratinized epithelium
Precise arrangement of collagen fibers
Relatively dehydrated

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Corneal innervation

CN V (trigeminal)
More nerve endings in epithelium/superficial cornea
Decreased sensitivity can affect healing
- Brachycephalics
- Sick foals
- Diabetics

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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

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Epithelial loss alone heals by epithelial proliferation

Fact

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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

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Corneal disease states

Loss of corneal thickness

Loss of transparency

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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

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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

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Simple ulcer criteria

Superficial
No infection
Heals in an appropriate amount of time (a week)
No complicating factors

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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&amp;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 ```