Oculocutaneous Albinism Type 1b

A form of oculocutaneous albinism type 1 (OCA1) characterized by skin and hair hypopigmentation, nystagmus, reduced iris and retinal pigment and misrouting of the optic nerves.

Epidemiology

The worldwide prevalence of OCA1 is estimated at 1/40,000. OCA1B is considered to account for about half of all overall OCA1 cases among non-Hispanic, Caucasian patients

Clinical description

Newborns have white or very light yellow hair but with age the hair can darken to blond or light brown. Eyelash hair can be darker than scalp and eyebrow hair. Skin remains creamy white but a minimal amount of tanning is possible along with freckles and pigmented nevi. Nystagmus is sometimes visible at birth but in others not until 3 to 4 months of age. It continues throughout life but becomes less rapid with age and is usually more noticeable in times of stress, anger or tiredness. Iris color is blue at birth and can change to brownish tan or greenish hazel or remain unchanged. Visual acuity ranges from 20/100 to 20/200. With time, skin can become rough, coarse and thickened if sun protection procedures are not followed. Patients have an increased risk of developing basal and squamous cell carcinomas but melanomas are rare.

Etiology

OCA1B is caused by a mutation in the TYR gene located on chromosome 11q14.2 encoding tyrosinase. The mutation causes the production of a partially active or hypomorphic tyrosinase enzyme that leads to minimal melanin formation in melanocytes.

Diagnostic methods

The characteristic clinical findings along with confirmatory genetic testing are used to diagnose OCA1B. Ophthalmologic examination reveals visualization of the choroidal blood vessels, reduced retinal pigment and foveal hypoplasia. Alternating strabismus, reduced stereoscopic vision, and an altered visual evoked potential (VEP) are associated with the characteristic misrouting of the optic nerves at the chiasm. Molecular genetic testing is necessary to obtain a definitive diagnosis, as some OCA1B patients have a certain degree of phenotypical variation which may lead to confusion in distinguishing it from other OCAs. This overlap of clinical symptoms emphasizes the importance of genetic analysis in the diagnosis of albinism.

Differential diagnosis

Differential diagnoses include the other forms of OCA and X-linked recessive ocular albinism (XLOA) as well as syndromes with albinism as a feature, such as Hermansky-Pudlak syndromes 1-11, Chediak-Higashi syndrome, Griscelli syndromes 1-3, and Waardenburg syndrome type II.

Antenatal diagnosis

Prenatal testing is possible for at risk pregnancies by molecular genetic testing.

Genetic counseling

This disorder is inherited autosomal recessively and genetic counseling is recommended for at-risk couples (both individuals are carriers of a disease-causing mutation) informing them of the 25% risk of having an affected child at each pregnancy.

Management and treatment

Annual ophthalmologic examination is necessary and corrective lenses or glasses are given to improve visual acuity. Dark glasses may be needed to relieve photophobia. Strabismus surgery can be performed for functional or cosmetic reasons. Protection from sunlight is imperative and patients should wear clothing and sunscreen on exposed skin to prevent burning and reduce the risk of skin cancer. Annual skin examinations should also be performed to identify any pre-cancerous or cancerous lesions.

Prognosis

OCA1B is not life threatening and remains stable after childhood. The medical and social consequences can however have major impacts on a patient's daily life.