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What causes double vision?
There are dozens of causes of double vision ranging from benign to life-threatening. Therefore, it is important for the doctor to carefully review the history and the examination to determine the cause and initiate appropriate treatment when necessary. Sometimes, emergency treatment is needed.
Most causes of monocular diplopia stem from poor focusing of light by the eye. Refractive errors (myopia, hyperopia, astigmatism) are common. Dry eye (from a variety of causes such as meibomitis, Sjögren's syndrome, and decreased tear production following refractive surgery) can produce diplopia that varies with blinking. Cataracts (clouding of the natural lens) and posterior capsule opacification (after cataract surgery) are common in people over 60 years of age. Other conditions that interfere with proper focusing of light include corneal irregularity from swelling or scars and retinal conditions, such as epiretinal membranes. Rarely is the underlying cause a medical emergency in cases of monocular diplopia.
Binocular diplopia on the other hand is produced by a misalignment of the eyes, which can be caused by life-threatening conditions. For example, aneurysms, strokes, trauma (head injury), and brain swelling (such as from brain cancer) can interfere with the nerves that control the extraocular muscles. The extraocular muscles move the eyes in different direction of gaze, much like the strings on a marionette, and when one or more muscle is weakened or paralyzed, it is referred to as cranial nerve palsy. In multiple sclerosis, lesions in regions of the brain that control eye alignment may result in diplopia that varies over time. Guillain-Barré syndrome can produce diplopia from restricted eye movement due to nerve damage. Migraine headaches can cause a sudden but temporary eye misalignment. Diseases such as myasthenia gravis can interfere with the communication between the nerves and the eye muscles to cause diplopia. And the eye muscles themselves can be damaged or compressed by conditions such as Graves' disease (often associated with thyroid disease), orbital inflammations, vascular disease (as seen with diabetes and high blood pressure), and others. Following traumatic fracture of the orbital bones, muscles and orbital tissue may be trapped in the fracture, leading to restriction of eye movement in certain directions of gaze. Sometimes the cause is relatively harmless, such as when the eye muscles or neurologic signals to the muscles weaken with fatigue or illness. Inability to align the eyes when focusing on a near object (convergence insufficiency) is a common benign cause of intermittent binocular diplopia that can often be treated with prism glasses.