Terrien's marginal degeneration

There are bilateral (but asymmetrical) peripheral thinning of the superior cornea. The epithelium is intact and contains superificial vascularization. The thin area has a sloping peripheral border and a sharp central edge which contains lipid deposits. There may be pseudopterygium in advanced cases.
 

Peripheral corneal ulceration due to systemic diseases

There is thinning with ulceration involving a sector of the peripheral cornea. Inflammation may or may not be present (there may be associated scleritis).

Look for:

• associated systemic signs such as symmetrical arthropathy in rheumatoid arthritis or saddle nose in Wegener's granulomatosis 
• associated eye signs such as keratoconjunctiva sicca or scleromalacia perforan both of which may occur in rheumatoid arthritis

Mooren's ulcer

There is (but may be bilateral in younger patient) peripheral ulcerative keratitis located in the interpalpebral region. The ulceration is contiguous with the limbus without intervening clear zone. The epithelium is vascularized and there is an overhanging advancing edge. The whole corneal circumference may be involved. The sclera is not involved.