Heterochromia is a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is a result of the relative excess or lack 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 dogs and cats.
Heterochromia of the eye (heterochromia iridum or heterochromia iridis) is of three kinds. In complete heterochromia, one iris is a different color from the other. In segmental heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder and finally in central heterochromia there are spikes of different colors radiating from the pupil.
Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia. 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.
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, usually, an excess of melanin indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia. The term is from ancient Greek: ἕτερος, héteros meaning different and χρώμα, chróma meaning 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 (segmental 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, caused by certain diseases and syndromes. Sometimes one eye may change color following disease or injury.
Segmental or sectoral heterochromia
In segmental heterochromia, sometimes referred to as sectoral heterochromia, areas of the same iris contains two completely different colors.
Segmental heterochromia is rare in humans; it is estimated that only about 1% of the population have it.
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 heterochromia
Acquired heterochromia is usually due to injury, inflammation, the use of certain eyedrops that damages the iris, or tumors.
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. The stroma around the iris sphincter muscle becomes darker than the peripheral stroma. A stimulation of melanin synthesis within iris melanocytes has been postulated.[medical citation needed]
Neoplasm – Nevi and melanomatous tumors.
Iridocorneal endothelium syndrome
Iris ectropion syndrome
Abnormal iris lighter
Fuchs heterochromic iridocyclitis – a condition characterized by a low grade, asymptomatic uveitis in which the iris in the affected eye becomes hypochromic and has a washed-out, somewhat moth eaten appearance. The heterochromia can be very subtle, especially in patients with lighter colored irides. It is often most easily seen in daylight. The prevalence of heterochromia associated with Fuchs has been estimated in various studies with results suggesting that there is more difficulty recognizing iris color changes in dark-eyed individuals.
Acquired Horner’s syndrome – usually acquired, as in neuroblastoma, although sometimes inherited.
Neoplasm – Melanomas can also be very lightly pigmented, and a lighter colored iris may be a rare manifestation of metastatic disease to the eye.
Parry–Romberg syndrome – due to tissue loss.
Heterochromia has also been observed in those with Duane syndrome.
Chronic iritis
Juvenile xanthogranuloma
Leukemia and lymphoma
Central heterochromia
Central heterochromia is an eye condition where there are two colors in the same iris; the central (pupillary) zone of the iris is a different color than the mid-peripheral (ciliary) zone, with the true iris color being the outer color.(requires reference)
Eye color is determined primarily by the concentration and distribution of melanin within the iris tissues. 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 human iris can be seen in a number of various colors. There are three true colors in human eyes that determine the outward appearance: brown, yellow, and grey. The amount of each color an individual has determines the appearance of the eye color.
Eyes displaying central heterochromia are often referred to as “cat eyes” because of their multi-colored iris. Central heterochromia appears to be prevalent in irises containing low amounts of melanin.
A famous case of a person with central heterochromia was Baroness Rózsika Edle von Wertheimstein, whose daughter wrote: “She was a very beautiful woman… She had dark, dark brown eyes, but each eye had a purple ring to it, about a quarter of an inch of purple around these dark brown eyes.”
In other animals
Although infrequently seen in humans, complete heterochromia is more frequently observed in other species, where it almost always involves one blue eye. The blue eye occurs within a white spot, where melanin is absent from the skin and hair. These species include the cat, particularly breeds such as Turkish Van, Turkish Angora, Khao Manee and (rarely) Japanese Bobtail. These so-called odd-eyed cats are white, or mostly white, with one normal eye (copper, orange, yellow, green), and one blue eye. Among dogs, complete heterochromia is seen often in the Siberian Husky and few other breeds, usually Australian Shepherd and Catahoula Leopard Dog and rarely in Shih Tzu. Horses with complete heterochromia have one brown and one white, gray, or blue eye—complete heterochromia is more common in horses with pinto coloring. Complete heterochromia occurs also in cattle and even water buffalo. It can also be seen in ferrets with Waardenburg syndrome, although it can be very hard to tell at times as the eye color is often a midnight blue.
Sectoral heterochromia, usually sectoral hypochromia, is often seen in dogs, specifically in breeds with merle coats. These breeds include the Australian Shepherd, Border Collie, Collie, Shetland Sheepdog, Welsh Corgi, Pyrenean Shepherd, Mudi, Beauceron, Catahoula Cur, Dunker, Great Dane, Dachshund and Chihuahua. It also occurs in certain breeds that do not carry the merle trait, such as the Siberian Husky and rarely, Shih Tzu. There are example of cat breeds that have the condition such as Van cat.
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