When an individual sees a double image where there should only be one, it is referred to as double vision or diplopia. The two images can be side by side (horizontal), one on top of the other (vertical), or a combination of both. It can be monocular ( involves one eye) or binocular (involves both eyes). It is usually the result of impaired function of the extraocular muscles where both eyes are still functional but they cannot turn to target the desired object. Injury to muscles may be mechanical, disorders of the neuromuscular junction, disorders of the cranial nerves (III, IV, and VI) that stimulate the muscles, and occasionally disorders involving the supranuclear oculomotor pathways or ingestion of toxins. Diplopia can be one of the first signs of a systemic disease, particularly to a muscular or neurological process and it may disrupt a person’s balance, movement, and/or reading abilities.
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