An epicanthic fold or epicanthus[Powell, M. L. (1981) Assessment and management of developmental changes and problems in children, Mosby, Incorporated, p. 38 ][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]
The primary cause of the epicanthic fold is the hypertrophy of the preseptal portion of the orbicularis oculi muscle.
Etymology
Epicanthus means 'above the
canthus', with epi-canthus being the Latinized form of the Ancient Greek : 'corner of the eye'.
Classification
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.
[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]
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,
,
,
and certain
Adivasi tribes of
East India and
South India. It is also commonly found in
North India, especially in
Kashmir. The
Hazaras 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.[ ]
Lower-frequency populations
Epicanthic folds also occur, at a considerably lower frequency, in other populations:
Europeans (e.g.,
Scandinavians,
English people,
Irish people,
,
Russians,
Polish people,
Lithuanians,
Latvians,
Finns, and
Estonians),
, South Asians (
Bengalis,
,
among other groups in eastern and southern South Asia),
,
Cushitic peoples, and
Amazigh people.
Perception and attribution
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.
Possible evolutionary function
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.
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 people 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.[ "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."]
Other factors
Age
Many
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.
[Pham, V. (2010). COMMON OTOLARYNGOLOGICAL CONGENITAL ABNORMALITIES. UTMB, Dept. of Otolaryngology. [1] ]
Other examples are fetal alcohol syndrome, phenylketonuria, and Turner syndrome.
Misclassification of monolids and epicanthic folds
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 also
External links
