Epicanthic fold

An epicanthic fold or epicanthus ()

is a vertical fold of skin on either side of the nose, sometimes covering the inner canthus (medial canthus) of the eye. 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 (anterior to the orbital septum) portion of the orbicularis oculi muscle.

Etymology

Epicanthus means 'above the canthus', with epi-canthus being the Latinised form of the Ancient Greek : 'corner of the eye'.

Classification

thumb|200x200px|A [[Koreans|Korean girl with the skin fold of the upper eyelid covering the inner angle of the eye]]

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.

Ethnogeographic distribution

High-frequency populations

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, Northeast Indians, In Southern China and Southeast Asia, about 70% have this feature.

Lower-frequency populations

Epicanthic folds also occur, at a considerably lower frequency, in other populations: Europeans (e.g., Scandinavians, English, Irish, Hungarians, Russians, Poles, Lithuanians, Latvians, and Estonians), Jews, South Asians (Bengalis, among other groups in eastern and southern South Asia),

Perception and attribution

thumb|Swedish (with some [[Sámi people|Sámi roots) ski racer Jens Byggmark with an epicanthic fold over his left eye|alt=]]

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.

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.

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.

Other factors

thumb|The [[Khoekhoe, the San and some other African groups have a high frequency of the epicanthic fold.]]

Age

Many foetuses 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 ethnicity, especially before the nose bridge fully develops.

Medical conditions

Epicanthic fold prevalence can sometimes be found as a sign of congenital abnormality, such as in Zellweger syndrome 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.

Other examples are foetal alcohol syndrome, phenylketonuria, and Turner syndrome.thumb|Epicanthic folds. Note that the tarsal plates are exposed laterally.

Misclassification of monolids and epicanthic folds

Monolids and epicanthic folds are often used interchangeably, but they refer to different eyelid types. Monolids are characterised 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.thumb|In monolids, the tarsal plate is barely or not at all visible.)