Heterochromia: A Beautiful Eye Mutation

Heterochromia is the mutation in the color of the eye, hair, or skin. When it comes to the variation in the color of eye or iris it is referred to as Heterochromia iridis. Basically, it is the benign type of eye mutation and it remains throughout life without causing any pathogenicity in the eye.  People who have heterochromia iridis looks very beautiful and attractive due to different eye color. In most cases, there are two colors observed in heterochromia iridis i.e. one eye color is blue and the other eye has a brown color. This mutation is rare in human i.e. almost two lac people in the United States have heterochromia iridis which is not related to any disease while it is most common in animal breeds like horses, cats and dogs. If this mutation is causing changes in other parts of the body like change in hair or skin color then it is associated with diseases or develops after the birth of the baby.

How Heterochromia develops?

Heterochromia develops due to the abnormal production of melanin in the iris around the pupil, it may be green, blue, brown, and grey. If there is less production of melanin then eye color will be lighter i.e. hypoplasia whereas if there is the high production of melanin in the iris then it will be known as hyperplasia. Melanin is the pigment responsible to give color to eyes, skin, and hair and it is produced by a process known as melanogenesis in which amino acids are oxidized following polymerization. Mutation in the genes of melanin distribution in the 8-HTP pathway results in heterochromia iridis. It may be due to genetic factors, congenital (innate), or acquired (diseases of the eye). [1]

History

Heterochromia was first described by Aristotle as heteroglaucos. Some famous personalities from the past are also affected by heterochromia such as Anastasius Ⅰ named dikoros because both eyes had different colors. It is also stated by Plutarch that both eyes of Alexander the Great had different colors. [2]

Types of Heterochromia iridis

There are three types of heterochromia which depends upon the changing color of the iris in the eye:

1.     Complete Heterochromia means the iris of one eye have different color form the iris of another eye e.g. one eye is brown and the other one is blue.

2.     Central Heterochromia is a condition in which there are two colors produced in the same iris. A ring of different colors arises from the pupillary zone while the mid-peripheral zone of the iris having a different color than the ring.

3.     Segmental/Sector Heterochromia is also known as heterochromia iridium. In this condition, some parts of the same iris in the eye have different colors.

  

Causes of Heterochromia

In most cases, heterochromia is not caused by any health risk or it may be due to genetic or inheritance factors but in rare cases it is caused by congenital diseases or acquired heterochromia.

1.   Congenital diseases

Those diseases which are present from the birth of infants are known as congenital diseases. It may occur as a result of genetic malfunction or inheritance.

      Benign heterochromia is due to inherited reasons because eye color is inherited mostly but sometimes it may occur as a result of interaction between different genes that result in a change in eye color. It may not be related to any other health risk.

      Piebaldism is a harmless condition which causes a lack of production of melanin pigment leading towards lighter skin, hair, or eyes color.

      Hirschsprung disease a condition in which the large intestine is affected and infants have difficulties passing the stool. It causes sector heterochromia in babies.

      Tuberous sclerosis or Bourneville Disease in which a benign tumor is formed in the eyes, brain, heart, kidneys, skin, and lungs.

      Waardenburg Syndrome is associated with a group of different genetic diseases that leads to hearing loss. There is a change in the color of hair, eyes, and skin due to Waardenburg syndrome.

      Sturge-Weber Syndrome in this condition blood vessels is formed abnormally due to mutations in the GNAQ gene of somatic cells which causes malfunctioning of the eyes, brain, and skin. There is a pinkish or purplish birthmark on the skin of babies.

      Parry-Romberg Syndrome it is a rare syndrome in which malfunction occurs in the skin cells, tissues, muscles, and bones cells.

      Horner’s Syndrome in this condition, nerves are damaged which connects eyes and brain leading towards droopy eyelids and small pupil on one side of the face. [3][4]

2.   Acquired Heterochromia

When people get heterochromia after the birth due to some eye injuries or other severe diseases then this is referred to as acquired heterochromia.

·       Glaucoma is a disease in which the optic nerve is damaged due to fluid production or pressure increases in the eyes. It may also result in loss of vision.

·       Melanoma is also known as ocular cancer, in which there is an abnormal production of melanin in the iris of eyes. Dark spots start forming on the iris which can result in sudden vision loss.

·       Neuroblastoma this is a type of cancer in which nerves of the sympathetic nervous system starts to damage which results in the drooping eyelids or small pupil in the children under 10 age. It later causes heterochromia.

·       Eye injury may happen due to sports injuries or projects of houses or any accidental trauma which may cause heterochromia.

·       Some medicine or eye drops some medications are used to treat glaucoma i.e. bimatoprost, latanoprost, etc. which can cause a change in the eye color.

·       Eye surgeries are done to treat eyes against any tumor or other infection which may develop heterochromia.

·       Diabetes this is a rare cause of heterochromia which may be due to age and disease factors.[5]

Diagnosis and Treatment

Ophthalmologists usually diagnose and treat heterochromia. If heterochromia is not acquired or does not show any underlying health issue then there is no need to worry about it. It just changes the iris color of both eyes which gives exotic look to a person. People with non-cancerous heterochromia only wear contact lenses to avoid showing different colors of their eyes. If heterochromia is caused by underlying diseases then it should be treated by medications or treatments. Eye surgeries are done to prevent melanoma of the eye and other treatments to prevent malfunction of nerves. Currently, Selective Laser Trabeculoplasty or SLT is used to treat glaucoma and sectoral heterochromia.[6]

By: Maryam Baig

References

1. HAESSLER, F. H. (1929). HETEROCHROMIA IRIDES. Archives of Ophthalmology, 2(3), 333-347.
2. Gladstone, R. M. (1969). Development and significance of heterochromia of the iris. Archives of neurology, 21(2), 184-192.
3. Partington, M. W. (1964). Waardenburg's syndrome and heterochromia iridum in a deaf school population. Canadian Medical Association Journal, 90(17), 1008.
4. Barrea, C., Vigouroux, T., Karam, J., Milet, A., & MISSON, J. P. (2016). Horner syndrome in children: a clinical condition with serious underlying disease. Neuropediatrics, 47(4), 268-272.
5. Diesenhouse, M. C., Palay, D. A., Newman, N. J., To, K., & Albert, D. M. (1992). Acquired heterochromia with Homer syndrome in two adults. Ophthalmology, 99(12), 1815-1817.
6. Basoglu, A., & Çelik, U. (2018). The effect of SLT laser application on iris to treat sectorial heterochromia: a promising technique. Eye & Contact Lens, 44, S352-S354.