Heterochromia

Complete heterochromia in human eyes: one brown and one hazel

In anatomy, heterochromia refers to 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, due to genetic mosaicism, or due to disease or injury.

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.

Heterochromia of the eye (heterochromia iridis or heterochromia iridum) is of two kinds. In complete heterochromia, one iris is a different color from the other. In partial heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder.

Sectoral heterochromia: a blue iris with a brown section.

Partial or sectoral heterochromia is much less common than complete heterochromia and is typically found in autosomally inherited disorders such as Hirschsprung's disease and Waardenburg syndrome.

Famous comedian Dan Aykroyd has heterochromia, as do singer/songwriter Carly Simon, actresses Kate Bosworth, Elizabeth Berkley, Mila Kunis, Jane Seymour, Eleni Willmott, actor Christopher Walken, actor/comedian Simon Pegg, American mixed martial artist Jens Pulver, Rock singer Tim McIlrath, and Major League Baseball pitcher Max Scherzer.

Heterochromia in other animals

Complete heterochromia in a cat: one eye blue, one yellow

Complete heterochromia in a dog: one eye blue, one eye brown

Although 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 (see Leucism). These species include the cat, particularly breeds such as Turkish Van, Turkish Angora, 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. Horses with complete heterochromia have one brown and one white, gray, or blue eye. Complete heterochromia occurs also in cattle and even water buffalo.

Sectoral hypochromia in a blue merle Border Collie

Sectoral heterochromia, usually sectoral hypochromia, is often seen in dogs, specifically in breeds with merle coats. These breeds include Australian Shepherd and Border Collie.

Classification based on etiology

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.

Congenital heterochromia: inherited in autosomal dominant fashion (from men or women).

Congenital heterochromia

Heterochromia that is congenital is usually inherited as an autosomal dominant trait.

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, homolateral meningeal angioma 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's syndrome – a syndrome in which heterochromia presents as a bilateral iris hypochromia in some cases. A Japanese review of 11 albino children with the disorder found that 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

Heterochromia that is acquired is usually due to injury, inflammation, the use of certain eyedrops, 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
• Use of 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.
• Neoplasm – Nevi and melanomatous tumors.
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Iridocorneal endothelium syndrome
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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 Fuch's 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.

Heterochromia has also been observed in those with Duane syndrome.

• Chronic iritis
• Juvenile xanthogranuloma
• Leukemia and lymphoma

Central heterochromia

A grey-blue iris with a greenish-yellow ring showing Central Heterochromia

A grey iris featuring Central Heterochromia

Whereas Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an eye condition in which one iris is a different colour from the other (complete heterochromia), Central Heterochromia is an eye condition in which there are two different colours in the same iris. Central Heterochromia is where the central (pupillary) zone of the iris is a different colour than the mid-peripheral (ciliary) zone.

Eye colour is determined primarily by the concentration and distribution of melanin pigment within the iris tissues, Anything affecting those factors may result in a difference of colour being observed.

The human iris can be seen in a number of various colours. There are three true colors in the eyes that determine the outward appearance; brown, yellow, and grey. How much of each colour an individual has determines the appearance of his or her eye colour.

Eyes displaying Central Heterochromia are often referred to as "cat eyes" because of the appearance of a multi-coloured iris. Central Heterochromia appears to be prevalent in irides containing low amounts of melanin. Central Heterochromia does not label an eye as hazel. This is because the outer ring of an eye affected by Central Heterochromia is that iris' true colour.