The cornea is a clear dome of skin covering the pupil, where light shines into the eye so we can see (see Diagram — Anatomy of the Eye). Corneal Ulcers and other corneal Infections (“Keratitis”) can leave permanent scars which permanently interfere with vision. Then, the only cure would be a corneal transplant.
That’s why clinicians who suspect a possible corneal ulcer or infection should never delay, and refer immediately to an Ophthalmologist. Not tomorrow. Not to an optometrist, who can make the diagnosis but can’t treat. And not to an ER without calling first to make sure an ophthalmologist is on-call (which is not the case in many smaller hospitals).
It’s easy to suspect a Corneal Ulcer or Infection. The eye is red and hurts bad; pain is a key. We can see some sort of spot on the cornea, maybe with just a simple penlight, or maybe with an ophthalmoscope (the common instrument on the wall of exam rooms). Vision may or may not be normal, depending on where the infection is. Staining the cornea with fluorescein makes the diagnosis; ophthalmologists use their large slit lamp to see it best.
Treatment depends on the likely germ. It usually includes not just eye drops, but also oral or perhaps IV antibiotics, or even by injection into the eye. Keratitis from Herpes simplex (the cold sore Herpes, not the STD) is diagnosed by its special staining pattern; treatment requires its own special anti-viral medication (see Herpes).
Contact lenses can predispose wearers to corneal ulcers, which can be sight-threatening. The annual risk of corneal ulcers from extended-wear contact lenses is 1 in 1,000, which equals 1 in 20 over 50 years. Any wearer with eye pain should remove the lens immediately. If the pain hasn’t gone away in 1-2 hours, they should see an Optometrist or Ophthalmologist. It may just be an abrasion, but could be much worse.