When we look at an object and see a single clear image, we give no thought to the different areas of the visual system that need to work together to allow us to see this image. Light enters the cornea, the clear membrane over the front of the eye, and the lens helps to focus the light onto the retina at the back of the eye, from where information is carried via the optic nerve to the brain. The muscles of the eye move the eye in different directions. A problem with any part of this delicate system can lead to double vision or diplopia, the perception of two images of a single object. The images can be side by side, one above the other, or a combination of the two; they may overlap or appear as two separate images. Double vision can affect just one eye (monocular) or both eyes (binocular). As well as the obvious difficulties of navigating the world while viewing a double image, double vision can also cause disruptions in balance, movement and reading ability.
Double vision occurs when the eyes are unable to look in the same direction. There are a number of possible causes for this, some relatively minor and others more serious, some of which originate in the eyes, while others may be symptoms of underlying conditions within the body.
Monocular double vision is less common than binocular double vision and can be caused by conditions within the eye itself, which distort the transmission of light through the eye to the retina. These include problems with the shape of the cornea, an abnormality of the lens or retina, or an injury to the eye causing blood and fluid to build up around the eye and press on the nerves and muscles around it.
The control of eye movement depends on three sets of eye muscles working together with each other and with the brain. If damage occurs to the muscles or nerves that control eye movement, a double image may result. Anything that mechanically interferes with eye movement can prevent the eyes from aligning properly, and cause double vision. Most commonly, the eyes are misaligned because of a disorder affecting the nerves supplying the muscles that move the eyes. Sometimes the cause is unknown, however known causes include disorders that typically interfere with the ability of the nerves to control muscles throughout the body, including the muscles of the eyes. Examples of these are certain thyroid problems, stroke, diabetes, aneurysm, multiple sclerosis, brain tumour, myasthenia gravis, and head injury.
The question is sometimes asked if a squint is the same as double vision. Although a squint may cause double vision, it is not the same condition. A squint occurs when the eyes are not perfectly aligned and are looking in two different directions. Depending on the cause of the squint, it may not always lead to double vision. Factors that may cause a squint include paralysed or weak eye muscles, unbalanced eye muscles, or ineffective nerve control of the eye muscles. If a person has had a squint from childhood, he or she is unlikely to experience double vision because the brain learned at an early age to ignore the double image, using one eye at a time to view an image. If the squint is acquired in adulthood, the brain is unable to learn to ignore the double image, resulting in double vision.
Temporary double vision may be the result of certain drugs, concussion, alcohol intoxication, or extreme fatigue or eye strain. If normal vision does not return within a reasonable period of time, professional intervention may be needed.
DIAGNOSING DOUBLE VISION
Because of the large number of possible causes, diagnosing double vision can be challenging. Questions that may be asked include whether the double vision is in one or both eyes, if it is constant or comes and goes, and if it occurs only when the person is looking in a particular direction. Further questions are if there is pain, numbness, weakness of facial muscles, dizziness, and speech or swallowing difficulties, or a recent eye or head injury, any of which may be a cause for concern that the double vision may represent problems with the nervous system.
The eye examination aims to investigate bulging of one or both eyes, drooping eyelids, the pupil’s response to light, and an assessment of the internal structures of the eyes under magnification.
Some cases of double vision clear up without treatment. If symptoms and examination findings suggest no serious cause, the optometrist may recommend that your eyes be checked regularly for a few weeks to see whether the vision clears up. If necessary, he may refer you for further testing.
Because children are unaware that they may be seeing differently from others and cannot express what they are perceiving, diagnosing double vision in children can be more difficult. Signs to watch out for include squinting or narrowing the eyes, covering one eye with the hand, turning the head to view an object, or looking at objects from the side rather than facing forward.
TREATMENT OF DOUBLE VISION
Double vision can be quite debilitating and uncomfortable to live with, but it will not harm your eyes or your brain. Some of the conditions that cause double vision, however, can be harmful to the eyes and even to general health, so should be treated as early as possible. The treatment for double vision varies, depending on the underlying cause. If it is related to a medical condition, it should be managed by a medical professional.
As a short-term solution, immediate relief from the discomfort of double vision can be achieved by covering one eye. Your optometrist can advise on the best way to treat double vision and may recommend specific eye exercises to strengthen weak eye muscles or even glasses with prism lenses, a wedge-shaped piece of glass or plastic which diverts the light rays to the angle at which the eye is looking. This allows the person to see singly without moving the position of the eyes.
Occasionally Botox, a drug that paralyses muscles, may be injected into one of the extraocular muscles. By paralysing one of the muscles the balance of muscle power is re-established, and the other extraocular muscles can pull the eye into the correct position. This treatment lasts for approximately 12 weeks and then needs to be repeated.
In certain cases, surgical correction may be required to correct a squint. The operation involves strengthening or weakening certain extraocular muscles, so that the balance of muscle power is corrected and the eye is pulled into the correct position. If a muscle is not working effectively, it may be shortened by cutting out a piece of the muscle.
Some squints will resolve without any treatment and so the double vision resolves with it. This is particularly true for double vision due to stroke; in the majority of cases the squint will disappear after three to six months.
Double vision should always be evaluated by a professional, even if it is temporary. People who have warning signs and symptoms other than double vision should be evaluated by a doctor immediately. All people who have double vision, even if it has resolved, should see an optometrist within a few days, and continue to have regular check-ups.