Heterochromia is a variation in an animal's coloration, most often a difference in the colors of the left and the right eyes' irises, but also a variation in the color of hair or skin. Heterochromia is determined by the production, delivery, and concentration of melanin (a pigment). It may be inherited or caused by genetic mosaicism, chimerism, disease, or injury. It occurs in humans and certain breeds of domesticated animals.
Types Heterochromia of the eye is called heterochromia iridum (heterochromia between the two eyes) or heterochromia iridis (heterochromia within one eye). It can be complete, sectoral, or central. In complete heterochromia, one iris is a different color from the other. In sectoral heterochromia, part of one iris is a different color from its remainder. In central heterochromia, there is a ring around the pupil or possibly spikes of different colors radiating from the pupil.
Causes Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia, at least in domestic animals. This is due to a mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity. Though common in some breeds of cats, dogs, cattle and horses due to inbreeding, heterochromia is uncommon in humans, affecting fewer than 200,000 people in the United States, and is not associated with lack of genetic diversity. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, an increase of melanin production in the eyes indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia.
Etymology The term is derived from Ancient Greek: ἕτερος, héteros "different" and χρῶμα, khrôma "color".
Background
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin. Although the processes determining eye color are not fully understood, it is known that inherited eye color is determined by multiple genes. Environmental or acquired factors can alter these inherited traits. The color of the mammalian, including human, iris is very variable. However, there are only two pigments present, eumelanin and pheomelanin. The overall concentration of these pigments, the ratio between them, variation in the distribution of pigment in the layers of the stroma of the iris and the effects of light scattering all play a part in determining eye color.
Classification
Heterochromia is classified primarily by onset: as either genetic or acquired. Although a distinction is frequently made between heterochromia that affects an eye completely or only partially (sectoral heterochromia), it is often classified as either genetic (due to mosaicism or congenital) or acquired, with mention as to whether the affected iris or portion of the iris is darker or lighter. Most cases of heterochromia are hereditary, or caused by genetic factors such as chimerism, and are entirely benign and unconnected to any pathology, but some are associated with certain diseases and syndromes. Sometimes one eye may change color following disease or injury.
Genetic
Abnormal iris darker Lisch nodules – iris hamartomas seen in neurofibromatosis. Ocular melanosis – a condition characterized by increased pigmentation of the uveal tract, episclera, and anterior chamber angle. Oculodermal melanocytosis (nevus of Ota) Pigment dispersion syndrome – a condition characterized by loss of pigmentation from the posterior iris surface which is disseminated intraocularly and deposited on various intraocular structures, including the anterior surface of the iris. Sturge–Weber syndrome – a syndrome characterized by a port-wine stain nevus in the distribution of the trigeminal nerve, ipsilateral leptomeningeal angiomas with intracranial calcification and neurologic signs, and angioma of the choroid, often with secondary glaucoma.
Abnormal iris lighter
Simple heterochromia – a rare condition characterized by the absence of other ocular or systemic problems. The lighter eye is typically regarded as the affected eye as it usually shows iris hypoplasia. It may affect an iris completely or only partially. Congenital Horner's syndrome – sometimes inherited, although usually acquired. Waardenburg syndrome – a syndrome in which heterochromia is expressed as a bilateral iris hypochromia in some cases. A Japanese review of 11 children with albinism found that the condition was present. All had sectoral/partial heterochromia. Piebaldism – similar to Waardenburg's syndrome, a rare disorder of melanocyte development characterized by a white forelock and multiple symmetrical hypopigmented or depigmented macules. Hirschsprung's disease – a bowel disorder associated with heterochromia in the form of a sector hypochromia. The affected sectors have been shown to have reduced numbers of melanocytes and decreased stromal pigmentation. Incontinentia pigmenti Parry–Romberg syndrome
Acquired
Acquired heterochromia is usually due to injury, inflammation, the use of certain eyedrops that damage the iris, or tumors, both benign and malignant.
Abnormal iris darker Deposition of material Siderosis – iron deposition within ocular tissues due to a penetrating injury and a retained iron-containing, intraocular foreign body. Hemosiderosis – long standing hyphema (blood in the anterior chamber) following blunt trauma to the eye may lead to iron deposition from blood products. Certain eyedrops – prostaglandin analogues (latanoprost, isopropyl unoprostone, travoprost, and bimatoprost) are used topically to lower intraocular pressure in glaucoma patients. A concentric heterochromia has developed in some patients applying these drugs. A stimulation of melanin synthesis within iris melanocytes has been postulated. Neoplasm – Nevi and melanomatous tumors. Iridocorneal endothelium syndrome Iris ectropion syndrome
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