Epicanthic Fold in Human Eye Anatomy

Epicanthic fold is also known as epicanthal fold. It is the fold of skin across the inner corner of the eye (also called canthus). The epicanthic fold will produce the eye shape characteristic of persons from eastern and central Asia; also, it is seen in a few Native American peoples and occasionally in Europeans (for example, Poles and Scandinavians).

Classification

Differentiation in the shape of epicanthic fold has led to 4 types being recognized as given below:

• Above the upper tarsus, Epicanthus palpebral begins to the inferior orbital margin.

• Epicanthus supraciliary runs from the brow, by curving downwards towards the lachrymal sac.

• Epicanthus inversus usually runs from the lower eyelid skin over the medial canthus and also extends to the upper lid.

• Epicanthus tarsalis usually originates at the upper eyelid crease and it merges into the skin near the medial canthus. This is the most often type found in East Asians.

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Ethnogeographic Distribution

High-Frequency Populations

Southeast Asians, East Asians, North Asians, Central Asians, Micronesians, Native Americans, Polynesians, Mestizos, Khoisans, and Malagasy have the highest prevalence of epicanthic folds. Among the South Asians, they take place at very high frequencies among the Bhutanese, Kirati, and certain Adivasi tribes of eastern India and Northeast Indians.

In fewer of these populations, the trait is mostly universal, particularly in Southeast Asians and East Asians, where a majority, up to 90 percent in a few estimations, of adults contain this feature.

Lower-Frequency Populations

Epicanthic folds also occur in other populations, though at a much lower frequency: Europeans (for example, Scandinavians, English, Hungarians, Irish, Poles, Russians, Lithuanians, Estonians, Latvians, Finns, and Samis), South Asians (Bengalis, Sinhalese among other groups in eastern South Asia), Cushites, Nilotes, and Amazigh people.

Perception and Attribution

The development degree of the fold between individuals differs greatly, and attribution either of its absence or presence is often subjective, being to the degree relative to the occurrence of the trait within the community of the particular observer. In addition, its frequency differs clinically across Eurasia. Therefore, its use as a phenotypic marker to describe the biological populations is debatable.

Possible Evolutionary Function

The epicanthic fold is frequently linked to increased fat deposition around the eyeball, which is particularly noticeable in native North Siberian, Inuit, and Aleut populations. The adipose tissue is thought to provide greater insulation for both the eye and sinuses from the effects of cold, especially from the freezing winds, and to represent an adaptation to cold climates. 

Also, it has been postulated that the fold itself can provide a protection level from snow blindness. Though its appearance in the peoples of Southeast Asia may be linked to the possible descent from cold-adapted ancestors, its occurrence in different African peoples precludes the cold-adaptive explanation for it appearing in the latter groups. The epicanthic fold found in fewer African people has been tentatively linked to protection for the eye from the levels of high ultraviolet light, which is found in semi-desert and desert areas.

Causes

Epicanthal folds can be normal for people of Asiatic descent and a few non-Asian infants. Also, epicanthal folds can be seen in young children of any race prior to the nose bridge beginning to rise.

Also, however, they can be due to certain medical conditions, including the below-given ones:

• Fetal alcohol syndrome

• Down syndrome

• Phenylketonuria (PKU)

• Turner syndrome

• Noonan syndrome

• Williams syndrome

• Blepharophimosis syndrome

• Rubinstein-Taybi syndrome

Other Factors

Age

Several fetuses lose their epicanthic folds after 3 - 6 months of gestation. Epicanthic folds can be visible in the development stages of young children of any ethnicity, especially before the nose bridge fully develops.

Etiology of Epicanthic Fold

Cadaver specimens with the epicanthal folds have represented a connection between the lower and upper preseptal orbicularis muscle fibers. This was not evident in the specimens without the epicanthus.

Medical Conditions

In some cases, such as in Zellweger syndrome and Turner syndrome, the presence of epicanthic folds can be a sign of congenital abnormalities. Medical conditions, which cause the nasal bridge not to develop and project are associated with the epicanthic fold. Up to 60 percent of individuals with Down syndrome (also called trisomy 21) have prominent epicanthic folds. In the year 1862, John Langdon Down classified what is now known as Down syndrome.

He used the word mongoloid for the condition. This particular theory was derived from then-prevailing ethnic theory and from his own perception that children with Down syndrome shared physical facial similarities (epicanthic folds) with those of Blumenbach's Mongolian race. While the word "mongoloid" (also "mongoloid idiot" or "Mongol") continued to be used until during the early 1970s, it is now considered pejorative and inaccurate and no longer in common use since the 1970s about such types of medical conditions. Also, in Zellweger syndrome, epicanthic folds are prominent. The other examples are phenylketonuria, fetal alcohol syndrome, and Turner syndrome.