Epicanthic fold
An epicanthic fold, epicanthal fold, or epicanthus is a skin fold of the upper eyelid (from the nose to the inner side of the eyebrow) covering the inner corner (medial canthus) of the eye. The trait arises because the eyelid muscles are weaker or lower compared with people who do not have this epicanthic fold, resulting in a lower fold in the eyelid, when the eyes are open.
The term "epicanthic fold" refers to a visually categorized feature; however, there are different underlying causes.
Population distribution
The epicanthic fold occurs commonly in people of Central Asian, East Asian and Southeast Asian descent as a result of adaptive significance, such as the Mongols, Hazaras, Chinese, Koreans, Japanese, Vietnamese, Kazakhs and some South and Southeast Asians like Burmese, Filipino, Cambodian, Malays, Thai, Bhutanese, Northern Nepali, Tibetans, Ladakhis and others. It also occurs in Inuit, Native Americans, Afro-Asians, Khoisans (Capoids) in Africa and Madagascar, and certain groups from southern Sudan such as the Dinka and the Nuer. Epicanthic folds can also be found inherited among some Oceanic peoples including Tongans, Samoans, Micronesians, and Hawaiians.
The epicanthic fold occurs more frequently among persons of East Asian descent than among southeast Asians or south Asians.
Conditions underlying expression
Evolutionary
The epicanthic fold is normal for persons of certain ethnicities mentioned above.
One hypothesis as to why epicanthic folds came about involves the climates in which populations expressing them arose. Sunlight reflects more intensely off light colored surfaces, such as those prevalent in snowy regions or savannahs and deserts. The theory is that an epicanthic fold in such an environment would expose a narrower section of the eye to incoming light, reducing the amount of damaging UV radiation entering the eye. The trait may also be useful against strong winds.[citation needed] With regard to the vast dry grasslands of the savannah regions, the yellow grasses also reflect and intensify sunlight. Additionally, analogous conditions are present among desert groups such as the Dinka or Nuer of Sudan [1] who live in environments in which sunlight reflects off the light colored sands of the desert.
The certain people of African descent (ex. the Khoisan and the Dinka) as well as the majority of East Asian descended peoples express this trait.
Medical
If epicanthic fold appears on persons that traditionally don't display the characterstics, it can be a sign of a number of disorders coupled with other symptoms like retardation, weak muscles, etc.
Epicanthic fold can be a result of low genetic diversity characteristics caused by inbreeding depression, pedigree collapse, cultural isolation, endogamy, etc. or medical conditions like Down syndrome, Cri du chat and Williams syndrome, three chromosomal abnormalities that are associated with severe intellectual disability. Another condition in which this is seen is Triple-X syndrome, a chromosomal abnormality that does not necessarily impair intelligence, fetal alcohol syndrome.[2], Cri du Chat syndrome, Triple-X syndrome, Ehlers-Danlos syndrome and/or pre-term birth.
In children
All humans initially develop epicanthic folds in the womb. Some children lose them by birth, but epicanthic folds may also be seen in young children of any ethnicity before the bridge of the nose begins to elevate. They may persist where birth is pre-term, and sometimes also where the mother is alcoholic. [3]
Epicanthic folds can cause a child's eyes to appear crossed, a scenario known as pseudostrabismus.
Inheritance
Children who have one parent with a pronounced epicanthic fold as non-medical reason and one without can have varying degrees of epicanthic fold.
Surgical alteration
In some Asian ethnicities, the presence of an epicanthic fold is associated with a less prominent upper eyelid crease, commonly termed "single eyelids" as opposed to "double eyelids". The two features are distinct; a person may have both epicanthic fold and upper eyelid crease, one or neither.
The procedure of reducing or removing epicanthic folds is epicanthoplasty. It is now an extremely rare procedure. Asian blepharoplasty, however, is a popular form of cosmetic surgery in East Asia. The use of "eyelid glue" or "eyelid tape" (sometimes referred to as "eye glue" or "eye tape") is used to create the appearance of the "double eyelid" by finding a "natural fold" in the eyelid and tucking it in and holding it with an adhesive.
See also
References
- ^ Hassan, M. (1962) “Mongolism in Sudanese Children”, J Trop Pediatr. 8: 48-50
- ^ MedlinePlus Medical Encyclopedia
- ^ "Ophthalmic Involvement in the Fetal Alcohol Syndrome: Clinical and Animal Model Studies". Journal of Alcohol and Alcoholism. Retrieved 16 November.
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