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Diplopia (Double Vision)

Background

Diplopia is the subjective complaint of seeing 2 images instead of one and is often referred to as double-vision in lay parlance. The term diplopia is derived from 2 Greek words: diplous, meaning double, and ops, meaning eye. Diplopia is often the first manifestation of many systemic disorders, especially muscular or neurologic processes.
An accurate, clear description of the symptoms (eg, constant or intermittent; variable or unchanging; at near or at far; with one eye [monocular] or with both eyes [binocular]; horizontal, vertical, or oblique) is critical to appropriate diagnosis and management.

Binocular diplopia occurs only when both eyes are open and can be corrected by covering either eye. Monocular diplopia persists in one eye despite covering the other eye and can usually be corrected by using a pinhole. Monocular diplopia can be unilateral or bilateral. Physiologic diplopia is a normal phenomenon depending on the alignment of the ocular axes with the objects of regard (eg, focusing on a finger held close results in distant objects being blurry but double).

Further classification schemes for binocular diplopia include constant versus intermittent and vertical versus horizontal (or oblique) diplopia. Vertical diplopia indicates vertical alignment of the images, which usually suggests pathology in the vertical muscles, including superior oblique, inferior oblique, superior rectus, and inferior rectus. Horizontal diplopia suggests pathology of the medial or lateral rectus.

Animal models

Unless the visual fields of the eyes overlap, binocular diplopia cannot occur. Among vertebrates, the potential for diplopia (and for stereoscopic depth perception) depends on where the eyes are located in the head. Eyes located on either side of the head provide a wide visual field but with a less overlapped visual field. These animals have less field for binocular vision and less risk for diplopia when one eye becomes misaligned. However, when both eyes are located in the front of the head, a greater visual field overlap exists and, thus, a better binocular depth perception, as frequently seen in predators. Misalignment of such eyes may result in diplopia.

The eyes of birds demonstrate many unique anatomical features, one of which is the presence of multiple foveae and, in some cases, a streak fovea linking 2 foveae. Thus, they may be able to have 2 separate areas of regard without disabling diplopia. How the visual perception occurs in these cases remains debatable.

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