Albinism pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Shadan Mehraban, M.D.[2]

Pathophysiology

Physiology

• Melanocytes are derived from neural crest ectoderm and are found in hair follicles, skin, eyes, and inner ear
• Melanocytes account for 5% to 10% of cells in epidermal basal layers
• Melanocytes contain melanosomes which produce melanin
• Melanin protects skin from ultraviolet; with sun exposure melanin pigment increases in the skin
• Apart from the photo-protective effect of melanin, it has some roles in the development of ocular structures as well as oculoneural pathways
• Melanin converts tyrosine to 2 forms named eumelanin and pheomelanin
• Eumelanin is responsible for black or brown skin color and protects skin from ultraviolet B
• Pheomelanin is responsible for red or blond hair and light-colored, and ruddy skin
• On melanocytes, activation of melanocortin one receptors (MC1R) lead to synthesis of eumelanin over pheomelanin

Pathogenesis

• Mutation in Tyrosinase enzyme is responsible for causing albinism
• Tyrosinase converts tyrosine to DOPA and then dopaquinone; subsequenstly, dopaquionone converts to either eumelanin or pheomelanin
• Tyrosinase mutation is seen in oculocutaneous albinism 1 (OCA1) and autosomal-recessive ocular albinism (AROA)
• Lack of melanin increase chances of sun-damage related diseases including actinic keratosis and UV-related malignancies
• Ocular albinism pathway:
  • In uterus, melanin is responsible for developement of the fovea, optic nerves, optic tracts, and visual cortex
  • Decussation of some optic nerve fibers at optic chiasem are essential for binocular vision
  • In people without albinism, about 45% of optic nerve fibers from the temporal part of retina do not cross the optic chiasem to controlateral lateral geniculate nucleus
  • In albinism, most of nerve fibers decussate at optic chiasem and cause monocluar vision
  • Monocular vision is manifested as strabismus
  • In albinism, macula pigemnt is absent and fovea hypoplasia leads to decreased visual acuity
  • Visual acuity ranges from 20/60 to 20/400