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Diplopia is the medical term for double vision. Vision is the result of complex interactions between the eyes and the muscles and nerves that supply them. The eyes move up and down, side to side, and diagonally up and down. Each eye is moved by 3 pairs of muscles. Each muscle has 1 of 3 cranial nerves that signals the muscle to move the eye. All of these structures must be healthy and work together for vision to be single.Normally, eye movements are coordinated. This allows images to be focused on corresponding places on the right and left retinas, leading to single vision. The retina is the light-sensitive lining in the back of the eye. Diplopia occurs when a problem in the eyes or their muscles and nerves causes the image to be focused on different places on each retina. Systemic illnesses, or illnesses outside of these structures, can affect the structures of eye movement and lead to diplopia. In fact, diplopia can be the first sign of a systemic illness.There are numerous causes of diplopia. Some are due to local problems affecting the eye, eye muscles, and cranial nerves or surrounding tissues. Problems within the eye, such as a corneal or retinal disorder, can cause diplopia by changing where the image is focused on each retina. Anything that restricts eye movement can lead to diplopia. This includes an infection of tissues surrounding the eye, such as sinusitis or cellulitis, and trauma with fracture, swelling or bleeding into the orbit, or eye socket. A tumor within the orbit can also restrict eye movement.Diseases of the nervous system can cause diplopia. These include the autoimmune diseases of myasthenia gravis and multiple sclerosis. Myasthenia gravis affects the ability of the nerve to signal the muscle to move. It frequently affects the muscles controlling both eye movement and eye opening. Therefore, people with myasthenia gravis often have diplopia and ptosis, or drooping of the eyelids. Multiple sclerosis affects the protective covering of the nerve and may affect nerves supplying eye muscles, leading to muscle weakness and double vision. Progressive Supranuclear Palsy, or PSP, is a degenerative disease of the nervous system that affects eye movements, typically leading to impaired up and down movements. An aneurysm near the brain can cause double vision by pressing on a nerve that controls eye movements.Systemic diseases which can cause diplopia are hyperthyroidism, an excess production of thyroid hormone, and diabetes mellitus. Hyperthyroidism due to Grave's disease causes inflammation of the muscles and tissue surrounding the eyes. This may restrict movement of an eye and lead to diplopia. Diabetes mellitus can damage the nerves that control the muscles of eye movement. Rare genetic diseases have been identified that can affect the muscles of eye movement. In some cases, the cause of diplopia is unknown, or idiopathic. At times, when an individual focuses on a close object, distant objects may become blurry or double. This is normal.Strabismus, or a crossed eye, is caused by an isolated or single eye muscle weakness. It is usually identified in children. As the child's visual system is not mature, the child's brain will suppress one of the images. Therefore, the child will not complain of double vision. Routine infant and preschool eye examinations are very important as prolonged strabismus could lead to permanent visual loss or amblyopia.For adults, diplopia is difficult to ignore. It can cause significant problems for individuals who are experiencing it. Depending on the severity, diplopia can change depth perception.