Excerpt from Ocular Manifestations of Albinism

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Background

Albinism refers to a group of hereditary disorders that involve an abnormality of melanin synthesis or distribution. The term albinism comes from the Latin word albus, which means white, and, in 1908, Garrod first scientifically described it. Clinically, albinism presents as a pigmentation abnormality of the skin, hair, and/or eyes. It can be divided into 2 broad categories, oculocutaneous albinism and ocular albinism. Oculocutaneous albinism involves both the skin and eyes, whereas ocular albinism mainly affects the eyes with minimal to no skin involvement.

Both forms of albinism have similar ocular symptoms and signs, which include photophobia, refractive errors, monocular vision, strabismus, pendular nystagmus, iris transillumination defects, foveal hypoplasia, and abnormal decussation of the optic nerve fibers. These ocular manifestations are almost always present in both forms of albinism; however, the degree of their presentation can vary depending on the type of albinism and the racial background of the patient.

The inheritance pattern of albinism also is quite variable. Oculocutaneous albinism is mostly an autosomal recessive disorder, whereas ocular albinism is transmitted as a sex-linked or autosomal recessive disease. Oculocutaneous albinism is divided into approximately 10 different types. Two of the more common forms are type I (tyrosinase negative) and type II (tyrosinase positive) oculocutaneous albinism. Patients with type I disease have no skin or ocular pigmentation, whereas those with type II disease can develop some pigmentation as they grow older.

Another important form is type IB, which previously was called yellow mutant oculocutaneous albinism. Patients with type IB are similar to type I but can exhibit some pigmentation of their skin, hair, and ocular structures. Two additional rare types of oculocutaneous albinism with important systemic findings and an increased risk of mortality are Hermansky-Pudlak syndrome and Chediak-Higashi syndrome.

Ocular albinism has 4 different forms; the most important being Nettleship-Falls that is X-linked. The visual acuity in patients with albinism usually is poor and ranges from 20/60 to 20/400.

Pathophysiology

Melanin is the pigment responsible for skin, hair, and eye coloration. Albinism is caused by a disorder of melanin metabolism, and the defect can lie with either melanin synthesis or distribution. Melanin is synthesized in melanocytes from the amino acid tyrosine. This process takes place in special organelles called melanosomes. The pathophysiology of oculocutaneous albinism involves a reduction in the amount of melanin present in each of the melanosomes. The pathophysiology of ocular albinism is a reduction in the number of melanosomes, although each melanosome may be fully pigmented.

The most important enzyme in the synthesis of melanin is tyrosinase, which converts tyrosine to dopa. The gene for the enzyme tyrosinase has been localized to chromosome 11. A number of mutations have been found at this locus, which can result in an absent or defective tyrosinase enzyme. This results in type I oculocutaneous albinism, which is characterized by complete absence of skin and eye pigmentation, despite a normal number of melanosomes. In contrast, the type II (tyrosine positive) albinism defect is within the P polypeptide, which is a melanosomal tyrosine transporter. The gene for this P polypeptide has been mapped to chromosome 15.

The exact mechanism by which the absence of pigmentation leads to foveal hypoplasia and abnormal decussation of optic nerve fibers is not completely understood. It has been postulated that retinal pigment epithelial pigmentation around the macula is necessary for the normal development of the fovea. It also has been suggested that the absence of dopa, which is an antimitotic agent, can influence normal retinal development. This foveal hypoplasia is responsible for some degree of decreased visual acuity in almost all types of albinism. The abnormal decussation of optic nerve fibers is responsible for strabismus and monocular vision. Photophobia results from iris transillumination defects.

Frequency

United States

The frequency of type I (tyrosinase negative) oculocutaneous albinism is approximately 1 in 17,000 to 1 in 20,000. The frequency of type IB albinism appears to be higher among Amish people. The frequency of type II (tyrosinase positive) oculocutaneous albinism is higher in the African American population where it can be 1 in 10,000 compared to 1 in 36,000 in Caucasian Americans. The prevalence of X-linked ocular albinism is estimated to be 1 in 50,000.

International

The frequency of type I oculocutaneous albinism worldwide is about the same as in the US. However, type II oculocutaneous albinism is higher in African countries where it can range from 1 in 2000 to 1 in 5000. Similarly, the frequency of Hermansky-Pudlak syndrome is much higher in Puerto Rico where it is approximately 1 in 2700.

Mortality/Morbidity

The main cause for morbidity in patients with albinism is decreased visual acuity.

• In tropical regions, there can be higher mortality among these patients secondary to an increased incidence of skin cancer due to sun exposure.
• Mortality also is increased in patients with Hermansky-Pudlak syndrome and Chediak-Higashi syndrome. Patients with Hermansky-Pudlak syndrome have a bleeding diathesis secondary to platelet dysfunction and also experience restrictive lung disease, inflammatory bowel disease, cardiomyopathy, and renal disease. Patients with Chediak-Higashi syndrome are susceptible to infection and also can develop lymphofollicular malignancy.

Race

Albinism affects all racial groups. However, type II oculocutaneous albinism occurs more frequently in African American and African populations. Similarly, there is a much higher incidence of Hermansky-Pudlak syndrome among people of Puerto Rican origin.

Sex

Oculocutaneous albinism affects both sexes equally. Ocular albinism is a disease primarily of males because of its sex-linked transmission.

Age

Most people with albinism are diagnosed during infancy or early childhood.

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