The term metamorphopsia comes from the Greek words metamorphoun (to change the form) and opsis (seeing). It translates roughly as 'seeing an altered form'. It is not clear who introduced the term, but it appears in a medical lexicon as early as 1858, while the phenomenon itself has been described for at least two millennia. Today the term metamorphopsia is used in a restricted sense to denote a visual distortion affecting the contours of objects. In a more general sense, it is used to denote a variety of qualitative visual distortions affecting the perceived form, size, orientation, colour, and/or speed of perceived objects. In both cases, the visual distortion can be either fleeting or long term. Metamorphopsia as defined in the broad sense is an extremely varied phenomenon that can be divided into multiple categories. These include " achromatopsia (i.e. the inability or strongly diminished ability to perceive colour), " akinetopsia (the inability to perceive motion), " chromatopsia (seeing things in a single hue, as in " cyanopsia (blue vision), "chloropsia (green vision), "erythropsia (red vision), "ianothinopsia (violet or purple vision), and " xanthopsia (yellow vision)), the "corona phenomenon (in which an extra contour is visible around objects), " dysmegalopsia (a diminished ability to appreciate the size of objects), " dysmetropsia (characterized by changes in the apparent size and distance of objects), " dysmorphopsia (in which lines and contours appear to be wavy), "dysplatopsia (in which objects are perceived flattened and elongated), " enhanced stereoscopic vision (an exaggeration of depth and detail of visually perceived objects), " entomopia (seeing multiple identical images as if perceived through an insect's eye), " gyropsia (seeing an illusory, circular movement), " hemimetamorphopsia (in which only one half of an object or face appears distorted), "illusory splitting (a visual distortion consisting of an apparent vertical splitting of objects), "inverted vision (in which objects are perceived as ifrotated 180°), "kinetopsia (illusory movement), "loss of stereoscopic vision (in which things appear two dimensional or 'flat'), "macropsia (seeing things larger than they are), " macroproxiopia (in which the perceived size and distance of objects is altered), " micropsia (seeing things smaller than they are), " microtelepsia (in which the perceived size and distance of objects is altered), " mosaic vision (characterized by the fragmentation ofper-ceived objects or stimuli into irregular, crystalline, polygonal facets, interlaced as in a mosaic), " pelopsia (in which objects appear to be closer than they are), " plagiopsia (in which objects appear to be tilted), " polyopia (seeing multiple identical copies ofa single image), "porropsia (in which stationary objects are seen as moving away from the observer), " prosopometamorphopsia (in which faces appear to be distorted), " teleopsia (in which objects appear to be further away than they are), " visual allachaesthesia (in which objects are perceived as if dislocated into the opposite visual field), and "visual perseveration (i.e. the illusory reoccurrence of visual percepts, as in " illusory visual spread, " palinopsia, and the " trailing phenomenon). The Scottish philosopher Thomas Reid (1710-1796) is commonly credited with providing the first case report of metamorphopsia in 1764, after having contracted the condition himself due to a prolonged period of sungaz-ing. Reportedly, the Norwegian expressionist painter Edvard Munch (1863-1944) also suffered from metamorphopsias, caused by an intraocular haemorrhage. Cases of metamorphopsia due to a lesion of a single eye are referred to as "monocular metamorphopsia. When metamor-phopsias evoke changes in the affective assessment of the extracorporeal environment, rendering it either beautiful, ugly, or frightening, they are called " complicated metamorphopsias. When such changes are absent the term " simple meta-morphopsia is used. Pathophysiologically, meta-morphopsias are divided into two broad categories. Those attributable to an anomaly in the eye are referred to as peripheral or retinal metamorphopsias. Those attributable to a central anomaly are termed central, cortical, or cerebral metamorphopsias. Metamorphopsias tend to be transient or episodic in nature, especially when the underlying neurological condition is transient or episodic in nature. Some examples of these conditions are migraine, epilepsy, and the use of illicit substances such as mescaline or LSD. Metamorphopsias occurring in the context of an " aura or a related seizure disorder are referred to as "ictal illusion or "ictal metamor-phopsia. Long-lasting and permanent metamor-phopsias are relatively rare. Pathophysiologically, they are associated primarily with discrete lesions affecting the visual association areas. Etiologi-cally, they are associated primarily with structural damage, due, for example, to infarction, haemorrhage, or a neoplasm. Peripheral causes of meta-morphopsias include retinal oedema (which can give rise to micropsia or dysmorphopsia) and retinal scarring (which can lead to macropsia). Meta-morphopsias are commonly classified as " sensory distortions.
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