Heterochromia - Eyes of Different Color by G.Bertelli

Generality

Heterochromia is a characteristic in which two homologous parts of an individual 's body, such as eyes and hair, take on a different color . In medicine, this term generally refers to irises .

In most cases, iridochromia does not have a pathological significance. The reason for this manifestation is to be attributed to the different quantity of melanin in the two eyes: if the pigment is not very concentrated, it will undergo a blue color, while in the opposite case the iris will turn to brown tones.

Heterochromia can be present at birth (therefore it is congenital and recognizes a genetic origin) or acquired, that is, it occurs following particular events and pathologies.

Eyes of different colors can manifest themselves, for example, following trauma, adverse reactions to some drugs and eye diseases, such as in the case of Fuchs' heterochromic iridocyclitis, Horner's syndrome and pigmentary glaucoma.

If the heterochromia is due to an eye pathology, the treatment may be directed to the underlying cause.

What's this

Heterochromia is a characteristic in which the iris of one eye has a different color from the iris of the other eye. This somatic trait is very rare and can be congenital (present at birth) or acquired (when it derives from an illness or injury).

The heterochromia is due to a different concentration of melanin in the two eyes: the iris in which the pigment is more concentrated appears tending to brown, while the other is lighter.

Iris: what it is and what it is used for (in short)

The iris is a thin membrane of an annular shape, visible from the front through the transparency of the cornea . This eye structure contains pigmented cells, blood vessels and two layers of smooth muscle, whose contraction allows the variation of the pupil diameter. In addition to determining the color of our eyes, in fact, the iris acts as a muscular diaphragm, regulating the amount of light reaching the retina.

What does eye color depend on?

The color of the eyes is mainly determined by the pigmented cells present in the iris, which give it a variable color shade : green, blue, brown, hazelnut, gray, etc. In practice, how much more melanin is present in the iris, so the eyes are darker.
This somatic trait depends on the interaction of several genes : some of these intervene to provide the color spectrum, others can determine the pattern and positioning of the pigment in the iris. Environmental or acquired factors can alter these inherited traits. The inheritance model that follows is therefore very complex.
Normally, the two irises of an individual are of the same color. In heterochromia, the eye can be hyperpigmented (darker or hyperchromic) or hypopigmented (lighter or hypochromic) than the other.

Complete heterochromia

The heterochromia is defined as complete (or total) when the variation of color between an iris and the other is clear (that is, each eye has a distinct color, which is observed in the totality of the irises).

Example: in complete heterochromia one eye can be brown and the other green.

Partial heterochromia

Heterochromia is called partial (or sectorial) when only a part or halves of the iris have a different color from the rest of the same eye.

Example: in partial heterochromia, one eye may be cerulean and the other may have a green section and the remaining blue or gray.

Artificial heterochromia

Artificial heterochromia is obtained with the use of colored contact lenses, applied for an aesthetic effect.

Causes and Risk Factors

The heterochromia is the result of the different quantity, in excess or defect, of the melanin, in the two eyes: if the pigment is not very concentrated it will undergo a blue coloration, while in the opposite case the iris will turn towards the tones of brown.

The reason for heterochromia is therefore attributable to the different concentration and distribution of melanin in the tissues of the iris.

Warning! Heterochromia should not be confused with somatic mosaicism, ie the presence of two or more tonal variations (ie shades of a color) in the same iris.

In most cases, the origin of the heterochromia of the iris is genetic and manifests itself as a somatic characteristic present from birth . Sometimes, congenital heterochromia is associated with a hereditary syndrome (eg neurofibromatosis type 1, Horner syndrome, etc.). Other cases are acquired and caused by an illness or injury.

Congenital heterochromia

Most cases of heterochromia at birth are genetic and are not associated with any other ocular or systemic abnormality. This condition is known simply as congenital iridocromia . Melanin deficiency is usually seen in the clearer eye.

In addition to genetic reasons, heterochromia present at childbirth or shortly after birth, however, can also be caused by a hereditary syndrome or an intrauterine and / or perinatal lesion .

Heterochromia: can it depend on other pathologies?

Heterochromia is not always congenital, but can also appear throughout life as a result of pathological conditions involving the eye or other parts of the body.

Possible causes of acquired heterochromia include:

Injuries, injuries or eye trauma, where it occurs:
- Penetration of a foreign body;
- Bleeding (hemorrhage);
- Severe inflammation of the iris, even secondary to diseases such as: tuberculosis, herpes simplex and sarcoidosis;
Reactions to drugs (including prostaglandins, anti-glaucomatous eye drops that can cause a non-symmetrical iris discoloration in the two eyes);
Ocular diseases, as in the case of:
- Fuchs' heterochromic iridocyclitis (inflammation that usually affects only one eye, causing a loss of iridium pigment);
- Pigmentary glaucoma (a particular form of glaucoma caused by the abrasion of the pigment of the iris that is released in the eye and can be distributed differently in the two eyes);
- Nevus of Ota (or congenital ocular melanoma, is a very rare tumor, which gives a typical pattern of the iris, which makes the eye appear very dark);
- Some forms of uveitis .

Congenital syndromes that can be characterized by heterochromia include:

Waardenburg syndrome : genetic condition that can cause hearing loss with abnormal pigmentation of eyes, hair and skin;
Horner syndrome : it is a congenital pathology, but it is not genetic, in the sense that it is due to intrauterine brain damage or perinatal complications. These injuries involve the innervation of the sympathetic nervous system of an eye. On the affected side, the pupil is small, the eyelid is drooping and the iris is lighter;
Sturge-Weber syndrome : is a rare congenital disease, characterized by a "Port wine" stain along the course of the trigeminal nerve on the face and vascular malformations to the eye, which result in ocular and neurological anomalies of varying degree;
Type 1 neurofibromatosis : is a neuro-cutaneous genetic disorder characterized by disorders of melanin, Lisch's iris nodules, freckles in the axillae or groin and multiple neurofibromas;
Posner-Schlossman syndrome : or glaucomatocyclitic crisis, is a rare uveitis that accompanies ocular hypertonus and hypochromia of an iris;
Pigment dispersion syndrome : it is a condition characterized by the presence of melanin granules that are released into the aqueous humor and accumulate in different structures of the anterior segment, including the surface of the iris, making it darken. When the granules accumulate in the trabecular meshwork, they can cause an increase in intraocular pressure.

Heterochromia: how widespread is it?

Heterochromia is rarely found in humans, while it is quite common in animals, especially in dogs, cats and horses. In this case, the different color may refer to an area of the mantle or to the eyes.

Heterochromia is a somatic trait that affects about 1% of the population. However, this condition is often difficult to perceive and is not associated with any other abnormality.

Symptoms and Complications

Heterochromia: how does it manifest itself?

The different coloring between the two eyes is usually the only symptom or sign of heterochromia. Many times the difference between irises is so slight that it is only noticed under certain lighting conditions. In most cases, congenital iridochromia is mild and does not have consequences or damage to health.

If the cause of heterochromia is a disease, such as trauma, inflammation or an inherited disease, other signs or symptoms of the eye or body may be present.

Diagnosis

If heterochromia develops after birth, it could be an indication of an eye disease or other pathological condition. Therefore, if a newborn shows this sign, he must be visited by both a pediatrician and an ophthalmologist to check for other possible problems. In some cases, blood tests or chromosomal tests may be required to confirm any clinical suspicions.

Treatment and Remedies

In itself, heterochromia does not represent a health risk, so no treatment would be necessary.

However, if there is a sudden change in the color of one or both eyes, it is important to consult an eye doctor. Only a thorough examination can reveal, in fact, if heterochromia is caused by an underlying medical problem .

If the heterochromia is due to an eye pathology, the treatment must be directed to the triggering cause.