Symptoms may include double vision when looking to one side (unilateral injury). In some cases, there is pain or headaches.

Testing of eye movement shows that one eye has difficulty looking to the side and the other eye moves normally. An eye examination shows the eyes do not align -- either at rest, or when looking in the direction of the weak eye.

A complete medical and neurological examination must be done to determine if other parts of the nervous sytem are affected. Depending on the suspected cause, a patient may undergo blood tests, a head imaging study like an MRI or CT scan, and possibly a lumbar puncture (spinal tap).