Oculocutaneous albinism is an inherited defect in melanin formation that causes diffuse hypopigmentation of the skin, hair, and eyes. Ocular albinism affects the eyes and usually not the skin. Ocular involvement causes strabismus, nystagmus, and decreased vision. Diagnosis of oculocutaneous albinism is usually obvious from the skin examination, but ocular evaluation is necessary. No treatment for the skin involvement is available other than protection from sunlight.
(See also Overview of Pigmentation Disorders.)
Pathophysiology of Albinism
Oculocutaneous albinism (OCA) is a group of rare inherited disorders in which a normal number of melanocytes are present but melanin production is absent or greatly decreased. OCA occurs in people of all races throughout the world. Cutaneous and ocular pathologies (ocular involvement) are both present. Findings in ocular involvement include abnormal optic tract development manifested by foveal hypoplasia with decreased photoreceptors and misrouting of optic chiasmal fibers.
Most cases of OCA are autosomal recessive; autosomal dominant inheritance is rare. There are 7 genetic forms of OCA, of which the first 4 are well characterized:
- Type I is caused by absent (OCA1A; 40% of all OCA) or reduced (OCA1B) tyrosinase activity; tyrosinase catalyzes several steps in melanin synthesis.
- Type II (50% of all OCA) is caused by mutations in the P (pink-eyed) gene. The function of the P protein is not yet known but may involve regulation of organelle pH and accumulation of vacuolar glutathione. Tyrosinase activity is present.
- Type III occurs only in people with otherwise dark skin (Fitzpatrick skin types III to V—see table Fitzpatrick Skin Type Classification). It is caused by mutations in the tyrosinase-related protein 1 gene whose product is important in the synthesis of eumelanin, the most common of the 3 types of melanin that give skin, hair, and eyes their color.
- Type IV is an extremely rare form in which the defect is in a gene (SLC45A2) that codes a membrane transporter protein involved in tyrosinase processing and trafficking of proteins to melanosomes. Type IV is the most common form of OCA in Japan.
- Type V has been linked to chromosome 4q24, a region that may code for lysosomal proteins.
- Type VI has been linked to mutations in gene SLC45A5, which encodes a membrane transporter protein.
- Type VII is due to mutations in gene C10orf11, which encodes a leucine-rich protein that may play a role in melanocyte differentiation.
Nettleship-Falls (OA1) and Forsius-Eriksson (OA2) are extremely rare compared to OCA. They are inherited in an X-linked dominant fashion. Usually findings are confined to the eyes, but skin may be hypopigmented. Patients with OA1 may have late-onset sensorineural deafness.
In another group of inherited diseases, a clinical phenotype of OCA occurs in conjunction with bleeding disorders. In Hermansky-Pudlak syndrome, OCA-like findings occur with platelet abnormalities and a ceroid-lipofuscin lysosomal storage disease (which can lead to pulmonary fibrosis and granulomatous colitis). This syndrome is rare except in people with family origin in Puerto Rico, where its incidence is 1 in 1800. In Chédiak-Higashi syndrome, OCA-like cutaneous and ocular findings occur, hair is silvery gray, and a decrease in platelet-dense granules results in a bleeding diathesis. Patients with Chédiak-Higashi syndrome have severe immunodeficiency due to abnormal lymphocyte lytic granules and progressive neurologic degeneration.
Symptoms and Signs of Albinism
The different genetic forms of oculocutaneous albinism have a variety of phenotypes.
OCA type I (OCA1A) is classic tyrosinase-negative albinism; skin and hair are milky white, and eyes are blue-gray (decrease in visual acuity is the most severe in this form of oculocutaneous albinism). Pigmentary dilution in OCA1B ranges from obvious to subtle.
OCA type II has phenotypes with pigmentary dilution that ranges from minimal to moderate. Pigmented nevi and lentigines may develop if skin is exposed to the sun; some lentigines become large and dark. Eye color varies greatly.
In OCA type III, skin is brown, hair is rufous (reddish), and eye color can be blue or brown.
In OCA type IV, the phenotype is similar to that for type II.
Patients with ocular involvement may have decreased retinal pigmentation, leading to sensitivity to light and light avoidance. In addition, nystagmus, strabismus, reduced visual acuity, and loss of binocular stereopsis likely result from defective routing of the optic fibers.
Diagnosis of Albinism
- Clinical evaluation
Diagnosis of all types of OCA and OA is based on examination of the skin and eyes. Early ocular examination may detect iris translucency, reduced retinal pigmentation, foveal hypoplasia, reduced visual acuity, strabismus, and nystagmus.
Treatment of Albinism
- Strict sun protection
- Sometimes surgical intervention for strabismus
There is no cure for albinism.
Patients are at high risk of sunburn and skin cancers (especially squamous cell carcinoma) and should avoid direct sunlight, use sunglasses with ultraviolet (UV) filtration, wear sun-protective clothing, and use sunscreen with a broad-spectrum sun protection factor (SPF) as high as possible (eg, 50 or higher) that protects against UVA and UVB wavelengths (see sun exposure prevention).
Some surgical interventions may lessen strabismus.
Key Points about Albinism
- Oculocutaneous albinism is a group of rare, usually autosomal recessive disorders, resulting in hypopigmentation of the skin, hair, and eyes.
- Ocular involvement causes photosensitivity and often nystagmus, strabismus, reduced visual acuity, and loss of binocular stereopsis.
- Examine the eyes and skin to make the diagnosis.
- Instruct patients on how to strictly protect the skin and eyes from sun exposure.