Corneal ulcer
A scratch, foreign body, feline herpesvirus, eyelid abnormality, or trauma can damage the corneal surface. Fluorescein stain helps detect many ulcers. Some ulcers can deepen quickly.
Uveitis and glaucoma
Inflammation inside the eye can cause pain, redness, a small pupil, or cloudiness. Glaucoma involves elevated intraocular pressure and can also be painful. These conditions require different treatment, so guessing from appearance is unsafe.
Why leftover eye drops are risky
Some steroid-containing drops can worsen corneal ulcers. Medication that was appropriate for a previous eye problem may be inappropriate for a new one.
What can I do before the visit?
Prevent rubbing if possible and gently wipe external discharge with saline or clean damp gauze. Do not put human “redness relief” drops, anesthetic drops, or unprescribed medication into the eye.