Cornea and external eye disease

Question 1

Where does the cornea meet the sclera?

Answer 1

The limbus

Cornea is transparent, sclera is not

Question 2

What structure has the highest refractive power in the eye?

Answer 2

Cornea at 40 D

Question 3

What is the average thickness of the cornea?

Answer 3

535 um

Thickest towards periphery (up to 660 um)

Question 4

How is the cornea nourished and innervated?

Answer 4

Nourished by aqueous humour posteriorly and by tear film anteriorly
Innervated by long ciliary nerves (branches from CN V1)

Question 5

What are the layers of the cornea?

Answer 5

5 layers

1. epithelial layer - stratified nonkeratinized squamous epithelium, high regenerative capacity
2. bowman layer - avascular layer, collagen fibres
3. stroll layer - thickest, scars, no regenerative potential
4. descemet membrane - elastic layer
5. endothelium - pumps excess fluid from stroma to maintain dehydration and transparency

Question 6

What organisms commonly cause bacterial keratitis?

Answer 6

Pseudomonas aeruginosa (contact lens wearers)
Staph aureus
Streptococci

Question 7

What are clinical features of bacterial keratitis?

Answer 7

Unilateral sudden-onset pain, redness, photophobia, discharge
Hypopyon in severe cases
Complications include corneal perforation with other organisms eg N gonorrhoeae, H influenzae