What should you first check with a patient complaining of diplopia?

Diplopia is the complaint of seeing double. When faced with a patient complaining of double vision, the first thing you must determine, before anything else, is whether the diplopia is monocular or binocular. That’s because:

monocular diplopia = refractive errors, such as astigmatism, cataracts, or retinal problems.

binocular diplopia = a lot more work! This could be nerve palsies, congenital tropias, convergence/divergence deficiencies, restrictions, masses.