Eye colour is determined by the distribution and concentration of melanin in the iris and is based on genetic factors. While typical eye colours range from dark brown to sapphire blue, some people have abnormal colouring patterns.
Heterochromia in eyes is a condition in which the eyes have two different colours. There are two recognised types of heterochromia in eyes: complete and sectoral.
In complete heterochromia, the eyes are different colours. Autosomal disorders like Hirschsprung's disease and Waardenburg syndrome can cause complete heterochromia.
Sectoral heterochromia, also known as partial heterochromia, is far rarer than complete heterochromia. In sectoral heterochromia, part of the iris is a different colour than the rest, with colours appearing in sections, typically one large section and one small section. It is more common in cats and dogs than in humans.
Central heterochromia is a type of sectoral heterochromia in which a ring of colour surrounds the pupil that is a different colour from the rest of the iris. This condition is often referred to as "cat eyes" and is caused by low amounts of melanin.
Heterochromia is typically genetic, but it can be brought about by inflammation, injury and illness. Iron deposits in tissue, or siderosis, can lead to heterochromia, as can conditions that cause an overstimulation of melanin synthesis, which can darken the eye. Tumours like neoplasms can also cause abnormal eye colouration.
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