Diplopia

Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object. These images may be displaced horizontally, vertically, or diagonally (i.e. both vertically and horizontally) in relation to each other. ^[1].

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Binocular diplopia

Binocular diplopia is double vision arising as a result of the misalignment of the two eyes relative to each other, such as occurs in Esotropia or Exotropia. In such a case whilst the fovea of one eye is directed at the object of regard, the fovea of the other is directed elsewhere, and the image of the object of regard falls on an extra-foveal area of retina.

The brain calculates the 'visual direction' of an object based upon the position of its image relative to the fovea. Images falling on the fovea are seen as being directly ahead, whilst those falling on retina outside the fovea may be seen as above, below, right or left of straight ahead depending upon the area of retina stimulated. Thus, when the eyes are misaligned, the brain will perceive two images of one target object, as the target object simultaneously stimulates different, non-corresponding, retinal areas in either eye, thus producing double vision.

This correlation of particular areas of the retina in one eye with the same areas in the other is known as Retinal correspondence. This relationship also gives rise to an associated phenomenon of binocular diplopia, although one that is rarely noted by those experiencing diplopia: Because the fovea of one eye corresponds to the fovea of the other, images falling on the two foveas are 'projected' to the same point in space. Thus, when the eyes are misaligned, the brain will 'project' two different images in the same visual direction. This phenomenon is known as 'Confusion'.

Double vision is dangerous to survival, therefore, the brain naturally guards against its occurrence. In an attempt to avoid double vision, the brain can sometimes ignore the image from one eye; a process known as suppression. The ability to suppress is to be found particularly in childhood when the brain is still developing. Thus, those with childhood strabismus almost never complain of diplopia whilst adults who develop strabismus almost always do. Whilst this ability to suppress might seem a wholly positive adaptation to strabismus, in the developing child this can prevent the proper development of vision in the affected eye resulting in amblyopia. Some adults are also able to suppress their diplopia, but their suppression is rarely as deep or as effective and takes longer to establish. They are not at risk of permanently damaging their vision as a result though. It can appear sometimes, therefore, that diplopia disappears without medical intervention. However, in some cases the cause of the double vision may still be present.

Monocular diplopia

More rarely, diplopia can also occur when viewing with only one eye; this is called monocular diplopia, or, where the patient perceives more than two images, monocular polyopia. In this case, the differential diagnosis of multiple image perception includes a structural defect within the eye, a lesion in the anterior visual cortex (rarely cause diplopia, more commonly polyopia or palinopsia) or non-organic conditions.

Temporary diplopia

Temporary diplopia can also be caused by intoxication from alcohol or head injuries, such as concussion. If temporary double vision does not resolve quickly, one should see an eye doctor immediately. It can also be a side effect of the anti-epileptic drugs Phenytoin and Zonisamide, and the anti-convulsant drug Lamotrigine, as well as the hypnotic drug Zolpidem and the dissociative drug Ketamine.

Treatment for binocular diplopia

The appropriate treatment for binocular diplopia will depend upon the cause of the condition producing the symptoms. Efforts must first be made to identify and treat the underlying cause of the problem. Treatment options includes prism lenses and/or vision therapy and/or surgery, and also botulinum toxin can be used. On occasions, in certain conditions such as Oculomotor nerve palsy for example, it may be necessary to occlude one eye either temporarily or permanently. Daily wear of prism lenses is a passive compensatory treatment. Vision therapy is an active treatment which retrains the visual and vestibular systems (brain, eye muscles, and body). Vision therapy may eliminate the need for daily wear of prism lenses but is only suitable for a minority of those with diplopic symptoms.

Voluntary diplopia

Some people are able to consciously uncouple their eyes, inducing double vision on purpose. These people do not consider their double vision dangerous or harmful, and may even consider it enjoyable. It makes viewing stereograms much easier.

References

• ^  Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.

See also

• Amblyopia
• Binocular vision
• Orthoptics
• Strabismus
• Vision therapy