Bilateral red eyes

Painful

1. Caustic Keratoconjunctivitis
2. Foreign Body
   1. welder's flash/grinding
3. Trauma
4. Contact Lens
5. Conjunctivitis (rarely painful, more irritated)
   1. watery viral/discharge bacterial/consider chlamydia
   2. viral keratoconjucitivis assoc with URI and adeno
6. Blepharitis
7. Allergic Reaction (to eye drops w/ periorbital excoriation)
8. Dysthyroid Dz (proptotic, may have visual loss)
   1. gross chemosis (swelling of conjunctiva), diploplia
9. Carotico-cavernous fistula
   1. fistula b/n carotid and cavernous sinus p/w bilat conjunctival edema and pulsatile exophthalmos (spont in elderly, 2/2 trauma in young)

Painless

1. Conjunctivitis
2. Blepharitis
3. Allergic Reaction
4. Thyroid Dz (optic nerve compression/APD)

Source

Adapted from Pani