Epicanthic fold

The epicanthic is the skin fold of the upper eyelid, covering the inner corner (medial canthus) of the eye. Various factors influence whether epicanthic folds form, including ancestry, age, and certain medical conditions. The highest frequency of occurrence of epicanthic folds is found in specific ethnicities: East Asians, Southeast Asians, Central Asians, North Asians, Polynesians, Micronesians, Native Americans (as well as Mestizos), the Khoisan, and the Malagasy. In some of these populations the trait is almost universal, specifically in East Asians and Southeast Asians, where a majority, up to 90% in some estimations, of adults have this feature. Epicanthic folds also occur, at a considerably lower frequency, in other populations: Europeans (e.g., Scandinavians, English, Irish, Hungarians, Russians, Poles, Lithuanians, Finns, Estonians and Samis), Western Asians, Nilotes and Amazigh people. The degree of development of the fold between individuals varies greatly and its presence or absence is often subjective, also its frequency varies clinally across Eurasia. Its use, therefore, as a phenotypic marker to define biological populations is debatable. Epicanthic folds occur to varying degrees among South Asians (specifically at very high frequencies among the Bhutanese, Northeast Indians, Kirati, certain Adivasi tribes of eastern India and other groups and at considerably lower frequencies among the Sinhalese, Bengalis and other groups in eastern South Asia). The epicanthic fold is often associated with greater levels of fat deposition around the eyeball, a feature most accentuated in native North Siberian, Aleut and Inuit populations. The adipose tissue is thought to provide greater insulation for the eye and sinuses from the effects of cold, especially from freezing winds, and to represent an adaptation to cold climates. It has also been postulated that the fold itself might provide a level of protection from snow blindness. Though its appearance in peoples of Southeast Asia can be linked to possible descent from cold-adapted ancestors, its occurrence in various African peoples precludes a cold-adaptive explanation for it appearing in the latter groups. The epicanthic fold found in some African people has been tentatively linked to protection for the eye from the high levels of ultra-violet light found in desert and semi-desert areas. Many fetuses lose their epicanthic folds after three to six months of gestation. Epicanthic folds may be visible in the development stages of young children of any race, especially before the nose bridge fully develops. The epicanthic fold is sometimes found as a congenital abnormality. Medical conditions that cause the nasal bridge not to mature and project are associated with epicanthic folds. About 60% of individuals with Down syndrome (also known as trisomy 21) have prominent epicanthic folds. In 1862, John Langdon Down classified what is now called Down syndrome. He used the term mongoloid for the condition. This was derived from then-prevailing ethnic theory and from his perception that children with Down syndrome shared physical facial similarities (epicanthic folds) with those of Blumenbach's Mongolian race. While the term 'mongoloid' (also 'mongol' or 'mongoloid idiot') continued to be used until the early 1970s, it is now considered pejorative and inaccurate and is no longer in common use about medical conditions. In Zellweger syndrome, epicanthic folds are prominent.Other examples are fetal alcohol syndrome, phenylketonuria, and Turner syndrome.