Template:Short description Template:Use British English Template:Use dmy dates Template:Infobox anatomy
An epicanthic fold or epicanthus<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is a skin fold of the upper eyelid that covers the inner corner (medial canthus) of the eye.<ref name="Anthropology"/> However, variation occurs in the nature of this feature and the presence of "partial epicanthic folds" or "slight epicanthic folds" is noted in the relevant literature.<ref>Powell, M. L. (1981) Assessment and management of developmental changes and problems in children, Mosby, Incorporated, p. 38 Template:ISBN</ref><ref>U. Schilbach, U. and Rott, H-D. (1988) Ocular Hypotelorism, Submucosal Cleft Palate, and Hypospadias: A New Autosomal Dominant Syndrome, American Journal of Medical Genetics 31, pp. 863–870</ref><ref name="Berel2000">Template:Cite book</ref> Various factors influence whether epicanthic folds form, including ancestry, age, and certain medical conditions.
The primary cause of the epicanthic fold is the hypertrophy of the preseptal portion of the orbicularis oculi muscle.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite book</ref>
EtymologyEdit
Epicanthus means 'above the canthus', with epi-canthus being the Latinized form of the Ancient Greek Template:Wikt-lang Template:Wikt-lang: 'corner of the eye'.
ClassificationEdit
Variation in the shape of the epicanthic fold has led to four types being recognised:
- Epicanthus supraciliaris runs from the brow, curving downwards towards the lachrymal sac.
- Epicanthus palpebralis begins above the upper tarsus and extends to the inferior orbital rim.
- Epicanthus tarsalis originates at the upper eyelid crease and merges into the skin near the medial canthus. This is the type most often found in East Asians.
- Epicanthus inversus runs from the lower eyelid skin over the medial canthus and extends to the upper lid.<ref>Nguyen, M.Q., Hsu, P.W. and Dinh, T.A. (2009) Asian Blepharoplasty, Semin Plast Surg. 2009 Aug; 23(3), pp. 185–197 doi: 10.1055/s-0029-1224798, p. 189</ref>
Ethnogeographic distributionEdit
High-frequency populationsEdit
The highest frequency of occurrence of epicanthic folds is found in specific populations or ethnicities: East Asians, Southeast Asians, Central Asians, North Asians, Polynesians, Micronesians, Indigenous peoples of the Americas, Sámi people and some African people (especially among Khoisan and Nilotic people). Among South Asians, they occur at very high frequencies among the Nepalis, Bhutanese,<ref name="LivingRacesofMan">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Better source needed Northeast Indians,<ref name="LivingRacesofMan" /> Kirati people and certain Adivasi<ref name="LivingRacesofMan" /> tribes of Eastern and Southern India. It is also commonly found in Northern India, especially in Kashmir. The Hazara people in Afghanistan and Pakistan commonly have this trait. Some people in Eastern/Northern Pakistan have this trait.
In some of these populations, the trait is almost universal. This is especially true in East Asians and Southeast Asians, where a majority, up to 90% in some estimations, of adults have this feature.<ref>Template:Cite journal Template:Closed access</ref>
Lower-frequency populationsEdit
Epicanthic folds also occur, at a considerably lower frequency, in other populations: Europeans (e.g., Scandinavians, English, Irish,<ref>Template:Cite news</ref> Hungarians, Russians, Poles, Lithuanians, Latvians, Finns, and Estonians),<ref>Template:Cite encyclopedia</ref><ref>Template:Cite book</ref> Jews, South Asians (Bengalis,<ref name="LivingRacesofMan" /> Sinhalese,<ref>Template:Cite journal</ref> among other groups in eastern and southern South Asia),<ref name="LivingRacesofMan" /> Nilotes, Cushites, and Amazigh people.<ref>Template:Cite journal</ref>
Perception and attributionEdit
The degree of development of the fold between individuals varies greatly, and attribution of its presence or absence is often subjective, being to a degree relative to the occurrence of the trait within the community of the specific observer. Also, its frequency varies but can be found in peoples all over the world. Its use, therefore, as a phenotypic marker to define biological populations is debatable.<ref name="Berel2000" />
Possible evolutionary functionEdit
The epicanthic fold is often associated with greater levels of fat deposition around the eyeball. 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 may 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, this does not explain its occurrence in various African peoples. The epicanthic fold found in many African people has been tentatively linked to protection for the eye from the high levels of ultraviolet light found in desert and semi-desert areas.<ref>Template:Cite book</ref>
The exact evolutionary function and origin of epicanthic folds remains unknown. Scientific explanations include either random variation and selection (presumably sexual selection), or possible adaption to desert environment and/or high levels of ultraviolet light found in high-altitude environments, such as the Himalayas.
Dr. Frank Poirier, a physical anthropologist at Ohio State University, said that the epicanthic fold among Asian people is often explained as part of an adaptation to severe cold or tropical environments, however he suggests that neither of these explanations are sufficient to explain its presence in East and Southeast Asia, and notes that the fold can also be observed in Irish and African people. He attributes the epicanthic fold to pleiotropic genes that control more than one characteristic or function. He also did not offer an explanation for the origin of epicanthic folds.<ref>Template:Cite news "Dr. Frank Poirier, a physical anthropologist at Ohio State University, says the classical explanation of epicanthic fold depicts it as an adaptation to the tropical and arctic regions where many Asians live. The fold is described as a sun visor protecting the eyes from overexposure to ultraviolet radiation or as a blanket insulating them from the cold. According to Poirier, the problem with this theory is that a substantial portion of the Asian population evolved in areas outside of the tropical and arctic regions. In addition, he says epicanthic fold is not limited to Asians. John F. Kennedy had a variance of the fold and it is found among Europeans, especially the Irish, he said. It's just less prevalent. The fold is also found among infants worldwide. Poirier attributes the fold to pleiotropic genes--single genes that control more than one characteristic or function--but he has no explanation for its origin."</ref>
Other factorsEdit
AgeEdit
Many fetuses lose their epicanthic folds after three to six months of gestation.<ref>Template:Cite journal</ref> Epicanthic folds may be visible in the development stages of young children of any ethnicity, especially before the nose bridge fully develops.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Medical conditionsEdit
Epicanthic fold prevalence can sometimes be found as a sign of congenital abnormality, such as in Zellweger syndrome<ref>Template:Cite journal</ref> and Noonan syndrome. Medical conditions that cause the nasal bridge not to develop and project are also associated with epicanthic fold. About 60% of individuals with Down syndrome (also known as trisomy 21) have prominent epicanthic folds.<ref name=Steph2010>Template:Cite book</ref><ref>Pham, V. (2010). COMMON OTOLARYNGOLOGICAL CONGENITAL ABNORMALITIES. UTMB, Dept. of Otolaryngology. [1] Template:Webarchive</ref>
Other examples are fetal alcohol syndrome, phenylketonuria, and Turner syndrome.<ref>{{#invoke:citation/CS1|citation |CitationClass=web
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Misclassification of monolids and epicanthic foldsEdit
Monolids and epicanthic folds are often used interchangeably, but they refer to different eyelid types. Monolids are characterized by the absence of an upper eyelid crease, with the tarsal plates being obscured. In contrast, epicanthic folds are defined by the presence of excess upper eyelid skin that forms a curve over the inner corners of the eyes, while the tarsal plates remain exposed completely or laterally.
See alsoEdit
- Blepharitis
- Epicanthoplasty, the surgical modification of epicanthic folds
- Human physical appearance