The first step is to determine whether your double vision is monocular or binocular. To do this, your doctor will ask you to cover one eye and then the other. If you have monocular diplopia, your doctor will evaluate you for conditions, such as cataracts, that could be causing the problem. You will then need to see an eye specialist (an ophthalmologist). If the problem is binocular and there has been no facial trauma, then your doctor will want to know if you have diabetes, Graves' disease or neurological disorders.
In diagnosing binocular diplopia, your doctor has to determine which eye muscles are affected. To do this, you will be asked to look at the doctor's finger as he moves it up, down, left and right. This lets the doctor see how far your eye can move in each direction. Your doctor also will cover one eye and then the other, while you focus on a target. If the doctor sees your eyes shift as the eye cover is moved, it means your eyes are not aligned properly. Prisms may be placed over your eye to shift the image, and the test is repeated. The prisms allow the doctor to measure the amount or degree of your double vision when you look in different directions. This helps to diagnose the problem and monitor the problem over time. Your doctor will use the results of this exam, together with your medical history and additional symptoms, to determine if you need more tests.
For example, if your doctor suspects that you have hyperthyroidism, then you will need blood tests to measure thyroid hormone levels. If your doctor suspects that something is affecting the nerves to your eye muscles, you may need a magnetic resonance imaging (MRI) or computed tomography (CT) scan of your head to check for signs of trauma, bleeding, tumor or blood vessel malformations in the brain.
In most cases, double vision is easy to pinpoint in adults, because they can describe what they are seeing. Symptoms are tougher to pinpoint in children, who may not be able to explain what is wrong. Parents may notice that the child is squinting, covering one eye with a hand, tilting or turning the head abnormally, or looking sideways.