Heterochromia

# Overview
In anatomy, heterochromia refers to a difference in coloration, usually of the irises but also of hair or skin.
Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an eye condition in which one iris is a different color from the other (complete heterochromia), or where part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be genetically inherited or due to mosaicism,  or acquired by disease or injury.
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin pigment within the iris tissues. 
Consequently, anything affecting those factors may result in a difference of color being observed.
The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). An excess of pigmentation is usually associated with hyperplasia of the iris tissues whereas a lack of pigmentation is associated with hypoplasia. Although seen in humans, heterochromia in which one iris differs in color from the other iris is more frequently observed in non-human species such as cats (for example, Japanese Bobtails and Turkish Vans; white, shorthaired cats can be "odd-eyed" with one copper or orange eye and one blue eye, and the Turkish Angora), dogs (for example, Siberian Huskies as well as Australian Shepherds, Border Collies, and other breeds with merle coats), horses (so-called "wall-eyed" horses have one brown and one white or blue eye), and even water buffalo. 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 Akroyd has this condition.

# Classification based on etiology
Although a distinction is frequently made between heterochromia that affects an eye completely or only partially, 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
Heterochromia that is congenital is usually inherited as an autosomal dominant trait.
- 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.
- 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.
- 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
- 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.
- Iridocorneal endothelium syndrome
- Iris ectropion syndrome
- 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 irises. 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

# Causes

## Life Threatening Causes
- Leukemia
- Lymphoma
- Neoplasm

## Common Causes
- Acquired horner's syndrome
- Chronic iritis
- Congenital horner's syndrome
- Duane syndrome
- Eye hemorrhage
- Eye injury
- Foreign body in eye
- Glaucoma
- Hypoplasia
- Incontinentia pigmenti
- Iridocorneal endothelium syndrome
- Iris ectropion syndrome
- Pigment dispersion syndrome
- Rubeosis iridis
- Uveitis

## Causes by Organ System

## Causes in Alphabetical Order
- Acquired horner's syndrome
- Bimatoprost
- Chromosome 8 trisomy syndrome
- Chronic iritis
- Congenital horner's syndrome
- Duane syndrome
- Eye hemorrhage
- Eye injury
- Foreign body in eye
- Fuchs' heterochromic cyclitis
- Fuch's heterochromic iridis
- Fuchs' heterochromic iridocyclitis
- Glaucoma
- Hemosiderosis
- Hirschsprung's disease
- Hypoplasia
- Incontinentia pigmenti
- Inherited trait
- Iridocorneal endothelium syndrome
- Iris ectropion syndrome
- Isopropyl unoprostone
- Juvenile xanthogranuloma
- Latanoprost
- Leukemia
- Lisch nodules
- Lymphoma
- Neoplasm
- Neurofibromatosis
- Ocular melanosis
- Parry-romberg syndrome
- Piebaldism
- Pigment dispersion syndrome
- Rubeosis iridis
- Siderosis
- Sturge-weber syndrome
- Travoprost
- Uveitis
- Waardenburg's syndrome