Excerpt from Diplopia

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Background

The term diplopia is derived from 2 Greek words: diplous, meaning double, and ops, meaning eye. Diplopia (double vision) is a common subjective complaint, or diplopia may be elicited during the course of an eye examination. 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 can be corrected by covering either eye; monocular diplopia persists in one eye despite covering the other eye. 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).

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. Monocular diplopia is often due to optical aberrations resulting in multiple images.

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.

Pathophysiology

Binocular diplopia (or true diplopia) is a breakdown in the fusional capacity of the binocular system. The normal neuromuscular coordination cannot maintain correspondence of the visual objects on the retinas of the 2 eyes. Rarely, fusion cannot occur because of dissimilar image size, which can occur after changes in the optical function of the eye following refractive surgery (eg, LASIK) or after a cataract is replaced by an intraocular lens.

The distortion of one image may be interpreted as diplopia by the patient; however, the same object does not appear to be in 2 places but rather appears differently with each eye.

Monocular diplopia may occur from abnormal ocular media (eg, corneal distortion or scarring, multiple openings in the iris, cataract or subluxation of the natural lens or pseudophakic lens implant, vitreous abnormalities, retinal conditions). Monocular diplopia must be distinguished from metamorphopsia, in which objects appear misshapen.

Frequency

United States

No figures are available as to prevalence of diplopia in the United States.

International

International incidence rates of diplopia are unknown.

Mortality/Morbidity

Divergent pathological processes, each with its own morbidity and mortality, can cause diplopia. However, irrespective of cause, diplopia has significant morbidity in terms of difficulty with depth perception and confusion with orientation of objects, especially when performing visually demanding tasks, such as driving a vehicle or operating tools. Therefore, in assessing visual disability after injuries, loss of binocularity accounts for a major percentage of loss of function.

Race

No information is available regarding differences in various racial groups.

Sex

No information is available suggesting differences in prevalence with respect to sex.

Age

Diplopia is encountered almost exclusively in adults or in those with mature visual systems because of the following:

• Young children may not be able to express this symptom. More importantly, the immature visual system deals with diplopia by suppressing the poorer image, possibly resulting in irreversible amblyopia. Children with obvious and marked ocular malalignment from strabismus are comfortable and content because the visual image from the deviating eye is suppressed and not noticed.
• In contrast, adults who have mature visual processing pathways cannot easily ignore the second image. Wearing a patch over one eye to prevent visual confusion usually is the only option for these patients.

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