What different forms of albinism exist?
The most well-known form of albinism is oculocutaneous albinism, with its most severe form being called Oculocutaneous Albinism type I. Patients who suffer from this condition are completely unable to produce eumelanin and have therefore very white or pinkish skin, white hair and red eyes independently from their ethnicity. The condition is due to a defect in the gene that encodes for the enzyme tyrosinase. Tyrosinase catalyzes the last step in the synthesis of eumelanin.
Oculocutaneous Albinism Type II, is due to a defect in a gene that regulates the production of tyrosinase, called OCA2. Depending on its residual activity, patient with Oculocutaneous Albinism Type II, can appear like Type I patients completely without pigment, or they can have lightly colored skin and hair. Moles can also be pigmented, and the eye color ranges from red to blue and hazel. Both forms of albinism also have characteristic vision problems.
Patients have significantly reduced acuity, are often cross-eyed (a symptom that is called strabismus), have a refractive error and astigmatism, an involuntary movement of the eyes called nystagmus, and an increased sensitivity to light and glare called photophobia.
In contrast to theses two forms of albinism that affect the skin, hair and the eyes, ocular albinism affects mainly the eyes. Patients with ocular albinism can have skin and hair that is lighter colored than the average of their ethnicity, but it is usually within the normal range.
Ocular albinism is an X-linked hereditary disease which means that most females who carry the defective gene do not know that they carry this genetic defect since they have a functioning copy of the gene on the other X-chromosomes and therefore have little or no symptoms. 50-80% of female carriers, however, have mottled pigmentation of the retina that can be detected in an eye examination. Female carriers can also have depigmented areas on their skin, but they never have the other eye symptoms like macular hypoplasia and transillumination of the iris.
Males have only one X-chromosome. Therefore they will develop the disease, if they have inherited a X-chromosome with the defective version of the gene.
What are the symptoms of ocular albinism?
The eye color appears normal in patients with ocular albinism at first glance. However, on further examination by a specialist the iris turns out to be able to let light through. This is called iris transillumination. In healthy people, independently of their eye color, the iris epithelium contains a dark layer of eumelanin, rendering the iris impenetrable to light.
In ocular albinism patients, the fundus of the eye which is the inner side of the back part of the eyeball is blonde, or too light in color. The most severe defect is in the fovea centralis, the small part of the retina that is responsible for the acuity of central part of the picture we see. The fovea is not completely developed in patients with ocular albinisms. This is also the case for patient with other forms of albinism like oculocutaneous albinism of either Type I or II.
Scientists believe that normal pigment is needed in the eye for the correct development of the fovea during the development of the child in the womb. The malformation of the fovea that is seen in albinism is called macular hypoplasia. Another malformation found in different forms of albinism including ocular albinism is that the nerves from the eye that in healthy person connect to the same half of the brain and the other half in equal parts, cross over to the other half of the brain to a much larger extend than normal.
The skin is also affected in people with ocular albinism. The cells in the skin produce enlarged granules with pigment in them called macromelanosoms. However, this might only be visible by microscope. The skin of people with ocular albinism is more sensitive to the sun and other sources of UV-radiation.
How is ocular albinism treated?
The treatment for ocular albinism is symptomatically. This means the treatment aims at the symptoms rather than the cause, since the cause is a genetic defect that leads to malformations in the early development of the eye before birth. Treatment consists of regular eye exams, corrective lenses for the refractive error and astigmatism and treatment of the strabismus, as it can lead to further vision loss, if left untreated. Corrective lenses can be in the form of glasses or contact lenses.
In more severe cases, glasses with small telescopes mounted on the lenses or hand held telescopes can improve the results. If the vision fields of the eyes cross (strabismus), the brain receives a double picture. To be able to interpret this picture, the brain disregards the information from one of the eyes. This can lead to permanent vision loss in this eye. Early treatment of strabismus with eye drops, patches, or corrective eye surgery are therefore very important to preserve the vision in both eyes. Therapy to facilitate and stimulate vision development in infants and toddlers is also important. Protecting the skin from the harmful effect of the sun and other UV-radiation sources by staying out of the sun as much as possible, covering the skin with light clothing and applying sun tan lotion with a high sun protection factor can help avoiding permanent damage to the skin.