6 - cornea

be able to list 5 layers of cornea and their functions

  1. pre-corneal tear film

    1. lubrication, bactericidal, nutrition, removal debris, general maintenance of optical clarity

  2. epithelium

    1. physical barrier

    2. preventing seepage of pre-corneal tear film into cornea

    3. barrier to microorganisms

  3. stroma

    1. 90% of corneal thickness

  4. Descemet’s membrane

    1. basement membrane of corneal endothelium

  5. endothelium

    1. metabolically active

    2. plays the major role in maintaining corneal transparency

    3. transports water out of the cornea

name factors that contribute to corneal transparency

  1. no pigment

  2. no blood vessels

  3. no keratinization

  4. precise arrangement of collagen lamellae

  5. relatively dehydrated

explain difference in healing of superficial ulcers and deeper ulcers involving stroma

superficial ulcers

epithelial cells migrate to defect after 4 hours (1-3 mm/day)

deeper ulcers

heals with fibrosis/fibroplasia

remodeling → gradual restoration to normal thickness, collagen fiber arrangement and transparency

explain why fluorescein adheres to corneal ulceration but not an intact cornea or Descemet’s membrane

hydrophilic fluorescein binds to hydrophilic stroma when hydrophobic corneal epithelium

list criteria for simple and complicated corneal ulcer and explain why differentiating two types of ulcers is important

simple/superficial:

only epithelium has been lost

no evidence of infection

heal in 7-10 days

no complicating factors

complicated

  1. deep/stromal loss

  2. infection/melting

  3. not healed within 7 days

  4. complicating factors present

define Descemetocele

all stroma has been lost

describe treatment of simple ulcer and complicated ulcer with evidence of stromal loss > 50% malacia and infection

simple

  1. topical antibiotics → systemic abx do not reach cornea

  2. atropine → paralyze ciliary body, alleviate pain. contraindicated in glaucoma

  3. analgesia → NSAID, opioid

  4. E-collar

complicated

surgical stabilization → conjunctival graft

manage as infected/melting ulcer

abx → fluoroquinolone (mixed), aminoglycoside (GN), triple abx (GP)

if melting → autologous serum for anti-collagenase/anti-protease (also EDTA, tetracyclines, N-acetylcysteine)

Conjunctival flap if >50% loss

explain the endogenous and exogenous factors that contribute to corneal melting

collagenase and protease

exogenous

infectious organisms

endogenous

white blood cells → neutrophils

list complicating factors that prevent healing of a corneal ulcer

KCS

entropion

eyelid tumors

lagophthalmos

ectopic cilia

glaucoma

cornea denervation

systemic immunocompromise

explain pathophysiologic reason that indolent ulcers do not heal in an appropriate amount of time and treatments for an indolent ulcer

basement membrane defect prevents adhesion between epithelium and underlying stroma

tx: diabond bur or grid geratotomy

know how to identify and treat FHV-1 keratitis

conjunctivitis, keratitis ± resp disease

PCR for definitive diagnosis

tx: cidofovir 0.5% solution (ocular dz), oral lysine, famciclovir (systemic dz)

name white, blue, red, brown, and yellow corneal opacities

yellow/white/tan = white blood cells/infection

blue = edema

red = blood

list causes of focal and diffuse corneal edema

focal

epithelial disease → focal corneal ulcer

diffuse

heptatitis

endothelial dystrophy

uveitis

glaucoma

explain difference in clinical appearance of superficial and deep corneal vessels

superficial

pigmentation

non-painful

long, branching

originate from conjunctiva, cross over limbus

deep

short, straight, non-branching

originate from anterior ciliary arteries

explain treatment of feline corneal sequestrum and eosinphilic keratitis

corneal sequestrum

keratitis with central, slightly elevated plaque of black, dried tissue

tx: superficial keratectomy with or without adjunctive tissue replacement

eosinophilic keratitis

raised white or pink plaques

tx; clyclosporine/tacrolimus, megestrol acetate

topical steroids → may need antiviral therapy

know medical and surgical treatments for corneal endothelial dystrophy

medical

hyperosmotic agents → 5% NaCl

NSAIDs

steroids

surgical

thermal keratoplasty

conjunctival flap

corneal endothelial cell transplantation