diplopia

   The term diplopia comes from the Greek words diploös (double) and opsis (seeing). It translates as double vision. The ensuing coexistence ofsim-ilar images within the field of vision is called * multiplication. Diplopia can be divided into two broad classes: binocular diplopia, in which both eyes are involved, and *monocular diplopia in which only one eye is involved. When the term diplopia is used, it usually refers to binocular diplopia. For an account of monocular diplopia, see the entry Diplopia monocularis. Binocular diplopia is characterized by the visual perception of two identical images of a single object or stimulus, while looking with both eyes. The perceived image may display a horizontal, vertical, or oblique displacement, depending on the ocular muscles involved. Pathophysiologically, binocular diplopia is associated with a variety of conditions affecting the oculomotor nerve, the abducens nerve, the trochlear nerve, the eye muscles themselves, or the orbit (as in mass lesions). Etiologically, diplopia is associated with a variety of systemic conditions, including diabetes mellitus, hypertension, myasthenia gravis, herpes zoster, and aneurysm of the arteria communi-cans posterior. The local mechanical causes of diplopia binocularis include thyroid disease, orbit myositis, fracturing of the orbit wall, intraorbital tumour or haematoma, and Brown syndrome. Central causes of diplopia binocularis include mesencephalic lesions, pons lesions, increased intracranial pressure, and acute vitamin B1 deficiency (i.e. Wernicke's syndrome). Diplopia may be transferred to the content of * visual hallucinations, which then take the form of double images (also referred to as distorted hallucinations). The Scottish physicist David Brewster (1781-1868) is commonly credited with having been the first to demonstrate the mediation of distorted hallucinations experimentally. Whilst seeking to distinguish between sensory and hallucinatory visual images, he applied pressure to the eyeball of a test person, only to find that both types of percepts were doubled in the process. The term * pseudodiplopia is sometimes used to denote cases of *palinopsia, a condition which displays certain phenomenological similarities to diplopia.

   References

   Parish, E. (1897). Hallucinations and illusions. A study of the fallacies ofperception. London: Walter Scott.

   Pelak, V.S. (2004). Evaluation of diplopia: An anatomic and systematic approach. Hospital Physician, 16-25.