DIPLOPIA MONOCULARIS

   Also known as monocular diplopia and hallucinatory diplopia. The term diplopia monocu-laris is modern Latin for 'double vision with one eye'. The concomitant condition is characterized by the visual perception of two identical images based on a single object or stimulus, even when one eye is covered. The ensuing coexistence of identical images within the field of vision is called *multiplication. The displacement of multiplied images can be in either a horizontal or vertical direction. Moreover, displacement can vary in accordance with the distance to the perceived object, and with the perceived object's orientation in the visual field (i.e.to the right or to the left). The ensuing images can differ in size, in distinctness (one being 'fuzzier' than the other), and in shape (displaying visual distortions, for example). Under physiological circumstances diplopia monocularis can be induced artificially with the aid of a double prism. Within the context of pathology, diplopia monocularis may occur as a consequence of a refractive/optical defect (including irregular astigmatism, keratoconus, an early incipient cataract, a dislocated lens, and retinomacular disease), or as a result of the simultaneous employment of normal and abnormal retinal correspondences. Cortical causes of monocular diplopia include migraine and occipital lobe lesions. During the 19th century, diplopia monocularis also went by the name of diplopia monocularis hysterica. Due to its association with hysteria, as well as to medicine's failure to explain this phenomenon physiologically, it was long considered a purely functional symptom. The German neurologist Heinrich Lissauer (1861-1891) has been credited with introducing the notion that diplopia monoc-ularis may stem from the eye's failure to accommodate properly. He agreed that the symptom was often associated with hysteria, but conjectured that individuals with hysteria might occasionally suffer from nerve disturbances affecting the lens system of the eye, thus causing the retina to receive a dual optical image instead of a single one. Diplopia monocularis may be classified as a visual distortion or *metamorphopsia. When the condition is due to refractive/optical pathology, it can be classified alternatively as an *entoptic phenomenon.

   References

   Lissauer, H. (1893). Ueber Diplopia monocularis hysterica. Berlin: Dissertation. Smith, J.L. (1986). Monocular diplopia. Journal of Clinical Neuro-ophthalmology, 6, 184-185.