The term dysmetropsia comes from the Greek words dus (bad), metron (yardstick), and opsis (seeing). It is used to denote a distorted perception of image size. The term dysmetropsia was introduced in or shortly before 1916 by the British neurologist Samuel Alexander Kinnier Wilson (1878-1937) to replace the older term "dysmegalopsia. Wilson defines dysmetrop-sia as a defect in the visual appreciation of the measure or size of objects, whether by over-estimation or by under-estimation. On phenomenological grounds, dysmetropsia is commonly divided into four categories, comprising "macropsia, "micropsia, "pelopsia, and " teleopsia. However, combinations and intermediate forms (such as "porropsia, "microtelepsia, and " macroproxiopia) have been described as well. Using the alleged locus of origin as a guiding principle, Wilson divides dysmetropsia into a peripheral and a central form, and the latter into a cortical and a transcortical (or psychical) form. As noted by Wilson, dysmetropsia may occur physiologically (i.e. in the case of objects appearing larger through a fog, and in the case of the "Moon illusion), but also in the context of disease (notably tabes dorsalis, syphilitic basal meningitis, epilepsy, hysteria, tic disorder, alcoholism, and retinal conditions such as retinitis, and sarcoma of the choroid). Dysmetropsia is generally classified as a "sensory distortion, more specifically, as a variant of "metamorphopsia. However, in some hierarchical models it constitutes a class of its own, ranking at the same level as the metamorphopsias. Wilson himself motivates this arrangement by pointing out that metamorphopsias are characterized by a distortion of the visual image, whereas dysmetropsia involves a change in estimated size, without any kind of distortion of the interrelationships between the image's constituent parts.
References
Frassinetti, F., Nichelli, P., di Pellegrino, G. (1999). Selective horizontal dysmetropsia following prestriate lesion. Brain, 122, 339-350.
Wilson, S.A.K. (1916). Dysmetropsia and its pathogenesis. Transactions ofthe Ophthalmological Society UK, 36, 412-444.
Dictionary of Hallucinations. J.D. Blom. 2010.