Microbial Keratitis Flashcards
What are the 5 layers of the cornea?
Epithelium Bowman's Layers Stroma ( ~90%) Descemets Membrane Endothelium
What are infiltrates?
yellow-grey-white opacities located within the anterior stroma.
Infiltrates are made from acute inflammation composed of inflammatory cells, cellular and extracellular debris (necrosis).
What is the classification for an Ulcer?
An ulcer is characterized by inflammation, necrosis, loss of tissue, progression and chronicity. (Yanoff)
What are the pathophys steps for the Bacterial Keratitis
- Compromised epithelial layer allows bacteria access to the underlying tissue
- Exo/Endo-toxins are released to aid this infestation.
- The bacteria will begin its replication process within the stroma
- This will further activate and release exotoxins that will damage the epithelial layer and initiate an inflammatory reaction.
- WBC (specifically neutrophils) will arrive at the infected area and partnered up with the exotoxins, more damaged to the epithelial layer will be caused.
What are two ways that wearing Contact Lenses promote bacterial growth?
- Naturally occurring Lipid Rafts on the corneal epithelium cells help bind bacteria. Wearing CLs increases the number of Lipids Rafts thus increases the chances of bacteria attaching.
- Side chains of corneal mucin barriers block bacteria binding, and wearing CLs reduces the length of the side chains and thus less bacteria are blocked.
What type of bacteria can penetrate a healthy corneal epithelium?
Pseudomonas species can penetrate a healthy corneal epithelium in approximately 3 hours.
What helps stops the Pseudomonas species penetrate a healthy cornea?
Complete Lid closure
Tear FIlm
Tear Flow
Why is Microbial Keratitis more aggressive in Contact Lens wearers?
Wearing CLs predisposes the cornea to a “Pro inflammatory state”, which activates Langerhans Cells in the peripheral and central cornea.
What does Dk stand for?
The "Dk" value is a measure of their oxygen permeability of a CL. Materials with a high Dk transmit more oxygen to the eye than those with a low Dk value. Low Dk is < 12 Medium Dk is 15-30 High Dk is 31-60 Super Dk is 61-100 Hyper Dk is > 100
How do Contact Lens solutions and cases effect the Cornea?
Contact lens cases and solutions harbor bacteria. The CL MPS decreases the epithelium layer turnover and thus older epithelium cells are held on on the cornea.
MPS promotes bacterial binding as it distrupts the ZO-1 proteins and thus decreases epithelial resistance.
What are the Risk Factors for MK?
Contact Lens Wear
Ocular trauma/surgeries
Ocular surface diseases
How do you differentiate between a Sterile and Infected ?
Use the PEDDAL mnemonic
What does the PEDDAL mnemonic stand for?
P - Pain (More pain the more likely it is Infected)
E - Epithelial Defect (If full thickness defect is seen its more likely infective) (Dendritic shape - Viral) (Larger than the infiltrate - more likely bacterial)
D- Discharge (Purulent - infected) (watery - viral)
D- Depth (If multiple layers are involved its more likely infected.
A - Anterior Chamber Reaction ( is present - infected)
L - Location (central - bacterial) (viral and fungal - numerous lesions
3 Other ways to differentiate between Infected and Sterile Keratitis
- Iris visibility? If iris is not visible more likely to be Bacterial
- Colour ( Grey-white, usually indicative of bacterial Infection, yellow is more likely just Bacterial.
- Borders of infiltrate - Indistinct usually means infective.
What is the clinical presentation of a Staph/Strep related Bacterial Keratitis?
Round/oval Grey/white lesions Distinct Margins Mucopurulent discharge Severe AC reaction Initally may appear as a relatively clear surrounded cornea.
