Patients typically easily identify double vision. In some cases the two images are completely separate, and in other cases they are overlapping. Double vision might occur in specific circumstances (for example, when looking in the distance, at near, or in a particular direction). In some cases the double vision may fluctuate throughout the day, being most prominent during periods of fatigue. Patients with some forms of double vision may find themselves closing one eye to improve their vision.
The key to understanding whether double vision is due to a problem in the eye or in the brain is to see what happens when one eye is closed. If there is double vision when looking with the right or left eye alone, then the cause is ophthalmological—such as a cataract, a problem of the retina, or another eye disease.
On the other hand, when double vision is present with both eyes open, but goes away upon looking with only one eye, the cause may be neurological. This type of double vision occurs because of abnormal eye movements that cause the eyes to become misaligned.
When the eyes are not aligned properly, one eye sees an image in one location, while the other eye sees the same image in another location. Understandably, the brain becomes confused and sees two images instead of one. When one eye is closed, the double vision immediately goes away, because the brain receives information from just one eye.
After checking the vision in each eye, the doctor will carefully examine the movements of each eye. The doctor will also use different methods to measure the alignment of the two eyes in different positions. The results of these tests will allow an understanding of whether the problem is caused by the eye muscles, the nerves that connects to the muscles, or parts of the brain that control eye movements.
The exact tests ordered will depend on the results of the history and examination. It is always important to know a patient’s other medical illnesses, such as diabetes, high blood pressure, prior strokes or heart disease, cancer, and other neurological or medical conditions. To understand why a patient has double vision, a doctor might need to perform some blood tests to screen for medical illnesses. One blood test commonly performed looks for an antibody that causes myasthenia gravis, a condition where different muscles have fluctuating weakness. In many cases, it is important to obtain imaging, such as a CT or MRI scan to check for any structural causes for the double vision.
After the causes of double vision are understood, your doctor might be able to predict approximately how long the double vision may last. Some cases recover on their own, whereas in other cases, the double vision can persist.
Based on this information, the doctor will try to offer treatments to lessen the double vision. One approach is to block vision from one eye. This can be done either by using an eye patch or by using scotch tape to cover one lens of the eyeglasses. Depending on the exact pattern of the abnormal eye movements, temporary prisms can sometimes be tried to reduce the double vision. If these are very effective, permanent prisms can be ground into the lenses of the eyeglasses. Finally, if double vision has become a permanent problem (without further improvement for about 12 months), surgery can be performed to adjust the position of the eye muscles.
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