Which test is used to determine the presence of a tropia by observing the eye's fixation when the other eye is covered?

• A. Slit-lamp exam
• B. Ishihara test
• C. Visual field test
• D. Cover-uncover test

Answer: (D)

A tropia is a constant misalignment of the eyes, a real, or manifest, deviation. To detect it, you use the cover test. Have the patient fixate on a target and cover the eye that is currently fixing. Watch the other eye; if it moves to take up fixation when the cover is placed, this reveals a misalignment that exists even with both eyes open. In other words, occluding one eye breaks fusion and the eye’s behavior shows whether there is a striking, constant deviation—that’s a tropia. The other tests don’t assess eye alignment in this way: a slit-lamp exam checks the front eye structures, the Ishihara test checks color vision, and a visual field test maps peripheral vision.