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Albinism

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   CAPTION: Albinism
   Classification & external resources

   Albinistic boy
   ICD- 10 E 70.3
   ICD- 9 270.2
   OMIM 203100 103470, 203200, 203280, 203290, 203300, 203310, 256710,
   278400, 214450, 214500, 220900, 300500, 300600, 300650, 300700, 600501,
   604228, 606574, 606952, 607624, 609227
   DiseasesDB 318
   MedlinePlus 001479
   eMedicine derm/12

   Albinism (from Latin albus; extended etymology), more technically
   hypomelanism or hypomelanosis, is a form of hypopigmentary congenital
   disorder, characterized by a lack of melanin pigment in the eyes, skin
   and hair (or more rarely the eyes alone). Albinism results from
   inheritance of recessive genes. The condition is known to affect
   mammals, fish, birds, reptiles, and amphibians. While the most common
   term for an organism affected by albinism is "albino" (noun and
   adjective), the word is sometimes used in a derogatory way towards
   people; more neutral terms are "albinistic" (adjective) and "person
   with albinism" (noun). Additional clinical adjectives sometimes used to
   refer to animals are "albinoid" and "albinic".

Causes

   Albinism is a genetic disorder; it is not an infectious disease and
   cannot be transmitted through contact, blood transfusions, etc. The
   principal gene which results in albinism prevents the body from making
   the usual amounts of the pigment melanin. Most forms of albinism are
   the result of the biological inheritance of genetically recessive
   alleles ( genes) passed from both parents of an individual, though some
   rare forms are inherited from only one parent. There are other genetic
   mutations which are proven to be associated with albinism. All
   alterations, however, lead to changes in melanin production in the
   body.

   Albinism used to be categorized as tyrosinase-positive or -negative. In
   cases of tyrosinase-positive albinism, the enzyme tyrosinase is
   present. The melanocytes (pigment cells) are unable to produce melanin
   for any one of a variety of reasons that do not directly involve the
   tyrosinase enzyme. In tyrosinase negative cases, either the tyrosinase
   enzyme is not produced or a nonfunctional version is produced. This
   classification has been rendered obsolete by recent research.

   The chance of offspring with albinism resulting from the pairing of an
   organism with albinism and one without albinism is low, as discussed in
   more detail below. However, because organisms can be carriers of genes
   for albinism without exhibiting any traits, albinistic offspring can be
   produced by two non-albinistic parents. Albinism usually occurs with
   equal frequency in both genders. The only exception from this is ocular
   albinism, because it is passed on to offspring through X-linked
   inheritance. Thus, males more frequently have ocular albinism.

   Because organisms with albinism have skin that lacks (sufficiently or
   entirely) the dark pigment melanin, which helps protect the skin from
   ultraviolet radiation coming from the sun, they can sunburn easily from
   overexposure. (See human skin colour for more information). Lack of
   melanin in the eye also results in problems with vision, related and
   unrelated to photosensitivity, which are discussed further below.

   Most organisms with albinism appear white or very pale; the multiple
   types of melanin pigment are responsible for brown, black, gray, and
   some yellow colorations. In some animals, especially birds and
   reptiles, ruddy and yellow hues or other colors may be present on the
   entire body or in patches (as is common among pigeons), due to the
   presence of other pigments unaffected by albinism such as porphyrins,
   pteridines and psittacins, and carotenoids derived from the diet. Some
   animals are white or pale due to chromatophore (pigment cell) defects,
   do not lack melanin production, and have normal eyes; they are referred
   to as leucistic. The direct opposite of albinism, an unusually high
   level of melanin pigmentation (and sometimes absence of other types of
   pigment in species that have more than one), is known as melanism, and
   results in an appearance darker than non-melanistic specimens from the
   same genepool. Albinism-like conditions may affect other pigments or
   pigment-production mechanisms in some animals (e.g. "whiteface", a lack
   of psittacins that can affect some parrot species.). Another is common
   in reptiles and amphibians: axanthism, in which xanthophore metabolism,
   instead of synthesis of melanin, is affected, resuling in reduction or
   absence of red and yellow pteridine pigments. Of all these conditions,
   only albinism and melanism affect humans.
   An albino Wistar rat, a strain commonly used for both biomedical and
   basic research.
   An albino Wistar rat, a strain commonly used for both biomedical and
   basic research.

   The eyes of an animal with albinism occasionally appear red due to the
   underlying retinal blood vessels showing through where there is not
   enough pigment to cover them. In humans this is rarely the case, as a
   human eye is quite large and thus produces enough pigment to lend
   opacity to the eye. However, there are cases in which the eyes of an
   albinistic person appear red or purple, depending on the amount of
   pigment present. The albinistic are generally (but see related
   disorders below) as healthy as the rest of their species, with growth
   and development occurring as normal, and albinism by itself does not
   cause mortality (though the lack of pigment is an elevated risk for
   skin cancer and other problems.) Many animals with albinism lack their
   protective camouflage and are unable to conceal themselves from their
   predators or prey; the survival rate of animals with albinism in the
   wild is usually quite low. However the novelty of albino animals has
   occasionally led to their protection by groups such as the Albino
   Squirrel Preservation Society.

   Intentionally-bred albinistic strains of some animal species are
   commonly used as model organisms in biomedical study and
   experimentation. Examples include the BALB/c mouse and Wistar and
   Sprague Dawley rat strains, while albino rabbits were historically used
   for Draize toxicity testing. Albino axolotls, zebrafish, medaka and
   frogs are other common laboratory animals.

Types of human albinism

   About 1 in 17,000 human beings has some type of albinism, although up
   to 1 in 70 is a carrier of albinism genes.

   There are two main categories of albinism in humans:
     * In oculocutaneous albinism (despite its Latin-derived name meaning
       "eye-and-skin" albinism), pigment is lacking in the eyes, skin and
       hair. (The equivalent mutation in non-humans also results in lack
       of melanin in the fur, scales or feathers.)
     * In ocular albinism, only the eyes lack pigment. People with
       oculocutaneous albinism can have anywhere from no pigment at all to
       almost-normal levels. People who have ocular albinism have
       generally normal skin and hair colour, and many even have a normal
       eye appearance.

   While there is only one major type of ocular albinism, there are
   several varieties of oculocutaneous albinism (and disorders which
   produce the same or similar results), some of which have subtypes. Some
   are easily distinguished by appearance, but in most cases, genetic
   testing is the only way to be certain.

Oculocutaneous albinism family

   Girl from Honduras with probable OCA1b.
   Girl from Honduras with probable OCA1b.
     * Oculocutaneous albinism type 1 (OCA1) ( Mendelian Inheritance in
       Man (OMIM) 203100) is the type with (usually) the least amount of
       pigment. People with this type generally have very pale skin,
       "white" (actually translucent) hair and light blue eyes, however
       there are cases in which the irises can appear pink or violet,
       depending on the amount of blue (non-melanin) pigment present in
       the irises and the level and direction of light available to the
       observer. OCA1 is caused by an alteration of the tyrosinase gene,
       and can occur in two variations. The first is OCA1a, and means that
       the organism cannot develop pigment at all. Vision usually ranges
       from 20/200 to 20/400. The second is OCA1b, which has several
       subtypes itself. Some individuals with OCA1b can tan and develop
       pigment. One subtype of OCA1b is called OCA1b TS (temperature
       sensitive), where the tyrosinase can only function below a certain
       temperature, which causes the body hair in cooler body regions to
       develop pigment (i.e. get darker). (An equivalent mutation produces
       the coat pattern in Siamese cats.) Another variant of OCA1b, called
       Albinism, yellow mutant type (OMIM: 606952) is more common among
       the Amish than in other populations, and results in blonde hair and
       the eventual development of skin pigmentation during infancy,
       though at birth is difficult to distinguish from other types. About
       1 in 40,000 people have some form of OCA1.

     * Oculocutaneous albinism type 2 (OCA2) (OMIM: 203200), the most
       common type of albinism, is caused by mutation of the P gene.
       People with OCA2 generally have more pigment and better vision than
       those with OCA1, but cannot tan like some with OCA1b. A little
       pigment can develop in freckles or moles. People with OCA2 usually
       have fair skin but not as pale as OCA1, and pale blonde to golden
       or reddish-blonde hair, and most commonly blue eyes. Affected
       people of African descent usually have a different phenotype
       (appearance): yellow hair, pale skin, and blue, gray or hazel eyes.
       About 1 in 15,000 people have OCA2.

     * Oculocutaneous albinism type 3 (OCA3, or rufous albinism) (OMIM:
       203290) has only been partially researched and documented. It is
       caused by mutation of the tyrosinase-related protein-1 (Tyrp1)
       gene. Cases have been reported in Africa and New Guinea. Affected
       individuals typically have red hair, reddish-brown skin and blue or
       gray eyes. Variants may be the recently-identified minimal pigment
       type albinism (OMIM: 203280) and rufous oculocutaneous albinism
       (ROCA or xanthism) (OMIM: 278400). The incidence rate of OCA3 is
       unknown.

     * Oculocutaneous albinism type 4 (OCA4) (OMIM: 606574) is very rare
       outside of Japan, where OCA4 accounts for 24% of albinism cases.
       OCA4 can only be distinguished from OCA2 through genetic testing,
       and is caused by mutation of the membrane-associated transporter
       protein (MATP) gene.

     * Hermansky-Pudlak syndrome (HPS) (OMIM: 203300) is not a type of
       OCA, technically, but has similar features. HPS has a great range
       of degrees of pigmentation, from OCA1a-like to almost-normal
       coloring. Vision usually ranges from 20/60 to 20/200. Apart from
       the hypopigmentation and impaired vision, people with HPS lack
       dense bodies in their blood platelets which are responsible for
       releasing clotting factors. For this reason, HPS patients bruise
       easily and have a hard time stopping bleeding once it begins (
       bleeding diathesis, similarly to hemophilia). HPS has seven known
       forms (HPS-1 through HPS-7), each caused by a different autosomal
       recessive gene mutation. HPS-1 and HPS-4 may also include pulmonary
       fibrosis, or scarring of lung tissue that prevents the necessary
       expansion and contraction during breathing. It is believed that
       this is due to a buildup of fatty ceroid in the lungs. Colitis, or
       inflammation in the large intestine, is another symptom of most
       types of HPS, which may cause diarrhea, nausea, and blood in the
       stool. HPS is rare generally, but affects 1 in 1800 Puerto Ricans,
       and is typically fatal by middle age.

     * Chediak-Higashi syndrome (CHS) (OMIM: 214500), like HPS, is not
       technically a form of OCA, but produces similar results. CHS,
       caused by mutation of the LYST gene, is very rare, and is
       associated with other medical problems, such as immune system
       dysfunction that leads to a high infant mortality rate, HPS-like
       hemophilia, and neurological problems, among many others, in 85% of
       sufferers.

     * Griscelli syndrome (GS) is similar to CHS in symptoms (and also
       very rare). It is divided into three types, GS1 (OMIM: 214450), GS2
       (OMIM: 607624) and GS3 (OMIM: 609227). Each type is due to a
       different autosomal recessive gene mutation. Type 1 produces mainly
       neurological problems in addition to albinism, while type 2
       produces mainly immunological issues as well as the
       hypopigmentation, and type 3 only evidences hypomelanosis without
       either of the other sorts of problems. People affected by GS differ
       in appearance from those with OCA, having silvery-grey hair. A
       fourth and even rarer variant, partial albinism and
       immunodeficiency syndrome (PAID) (OMIM: 604228), has been
       identified and requires further study. An additional type called
       Elejalde syndrome (OMIM: 256710) may exist, but some researchers
       believe it is actually simply GS1. GS2, because of its immune
       system effects, results in a very high mortality rate among
       children and young adults that have it.

Ocular albinism family

     * Ocular albinism, type 1 (OA1) (OMIM: 300500), also known as
       Nettleship-Falls syndrome, is the most common variety of ocular
       albinism, which affects the eyes but generally not the skin or
       hair. OA1 is usually associated with nystagmus, and difficult to
       otherwise detect in females; males show more readily observable
       symptoms. There are several other identified types of OA, though
       researchers are not all agreed on the distinctions and
       classification. Most are caused by a mutation in a gene on the X
       chromosome, and are X-linked recessive traits.

     * Ocular albinism, type 2 (OA2) (OMIM: 300600), also known as
       Forsius-Eriksson syndrome or "Åland Island eye disease", mostly
       only affects males though females are often carriers and can
       sometimes be symptomatic; it is frequently linked with protanopic
       dichromacy (a form of colour blindness) and with night blindness (
       nyctalopia).

     * Ocular albinism, type 3 (OA3) (OMIM: 203310), also known as ocular
       albinism, autosomal recessive (OAR) is a non-X-linked variant,
       which may be more common among the Amish than in other populations.

     * Ocular albinism with sensorineural deafness (OASD) (OMIM: 300650),
       and as its name implies is associated with loss of hearing.

     * Waardenburg syndrome, type 2, with ocular albinism (WS2-OA) (OMIM:
       103470) is a rare non-X-linked recessive gene variant.

   The skin colour of people affected by OA can be slightly lighter than
   those of the rest of their families. The eye colour can vary greatly,
   and in some cases only examination of the retina or genetic testing can
   reveal OA for certain. Some form of OA afflicts 1 in 50,000 people,
   though certain isolated populations are at greater risk.

Other types

   Other rare variants of albinism are theorized (by ongoing research as
   of 2007) to exist, such as
     * Albinism-deafness syndrome (ADFN) (OMIM: 300700, which may actually
       be closer related to vitiligo); it is predominantly observed among
       Hopi Native Americans (with an incidence estimated at 1 in 200
       individuals)
     * Recessive total albinism with congenital deafness (OMIM: 220900)
     * Albinism black-lock cell-migration disorder syndrome (ABCD) (OMIM:
       600501)

Symptoms and conditions associated with albinism

   Genetic testing can confirm albinism and what variety it is, but offers
   no medical benefits except in the cases of non-OCA disorders (see
   below) that cause albinism along with other medical problems which may
   be treatable. The symptoms of albinism can be treated by various
   methods detailed below.

   Eye conditions common in albinism may or may not include:
     * Nystagmus, irregular rapid movement of the eyes back and forth, or
       in circular motion.
     * Strabismus, eye misalignment ("crossed eyes" or "lazy eye").
     * Refractive errors such as myopia or hyperopia and especially
       astigmatism are more likely
     * Photophobia, hypersensitivity to bright light and glare.
     * Foveal hypoplasia, underdevelopment of the fovea, the centre of the
       retina
     * Optic nerve hypoplasia, underdevelopment of the optic nerve
     * Abnormal decussation (crossing) of the optic nerve fibers in the
       optic chiasm
     * Amblyopia, decrease in acuity of one or both eyes due to poor
       transmission to the brain, often due to other conditions such as
       strabismus

   Albinistic girl from Papua New Guinea
   Albinistic girl from Papua New Guinea

   Organisms with albinism usually have impaired vision due to one or more
   of the listed conditions. While a person with albinism may suffer from
   common refractive errors like nearsightedness or farsightedness, the
   visual problems particularly associated with albinism arise from a
   poorly-developed retinal pigment epithelium (RPE) due to the lack of
   melanin. This degenerate RPE causes foveal hypoplasia (a failure in the
   development of normal foveae), which results in eccentric fixation and
   lower visual acuity, and often a minor level of strabismus. Nystagmus
   is usually seen, as is photophobia or light sensitivity (see below).

   The iris is a sphincter with pigmented tissue (which makes up the
   colour of the eyes) that contracts to limit the amount of light that
   can enter through the pupil and relaxes again to allow for better
   vision in darkness. This mechanism can be observed in humans and
   mammals (like in cat's eyes) and is needed because too much light is
   uncomfortable or even painful and decreases vision. In people with
   albinism, the iris does not have enough pigment to block the light,
   thus the decrease of pupil diameter is only partially successful in
   reducing the amount of light that enters the eye.. Additionally, the
   improper development of the RPE, which in normal eyes absorbs most of
   the reflected sunlight, further increases glare due to light scattering
   within the eye. The resulting sensitivity (photophobia) generally leads
   to a dislike of and discomfort in bright light, but does not prevent
   people with albinism enjoying the outdoors, especially when using
   sunglasses and/or brimmed hats.

   The lack of pigment also makes the skin unusually sensitive to sunlight
   and thus susceptible to sunburn, so people with albinism should either
   avoid prolonged exposure to bright sunlight or protect their skin.

Treatment of the symptoms

   Albinism is a condition that cannot be "cured" or "treated" per se, but
   small things can be done to improve the quality of life for those
   affected. Most importantly to improve vision, protect the eyes from
   bright lights, and avoid skin damage from sunlight. The extent and
   success rate of these measures depend on the type of albinism and
   severity of the symptoms; in particular, people with ocular albinism
   are likely to have normally-pigmented skin, and thus do not need to
   take special precautions against skin damage.

Surgical treatment

   For the most part, treatment of the eye conditions consists of visual
   rehabilitation. Surgery is possible on the ocular muscles to decrease
   nystagmus, strabismus and common refractive errors like astigmatism.
   Strabismus surgery may improve the appearance of the eyes.
   Nystagmus-damping surgery can also be performed, to reduce the
   "shaking" of the eyes back and forth. The effectiveness of all these
   procedures varies greatly and depends on individual circumstances. More
   importantly, since surgery will not restore a normal RPE or foveae,
   surgery will not provide fine binocular vision. In the case of
   esotropia (the "crossed eyes" form of strabismus), surgery may help
   vision by expanding the visual field (the area that the eyes can see
   while looking at one point).

Sun protection

   Albinistic man from Niger, with melanomas.
   Albinistic man from Niger, with melanomas.

   It is vital that people with albinism use sunscreen when exposed to
   sunlight to prevent premature skin aging or skin cancer. This poses a
   problem for those who cannot afford sunscreen, especially in regions
   with high exposure to sunlight, as in Africa. Special sun protective
   clothing and swimsuits are available and are a good alternative to
   excessive use of sunscreen.

   Use of sunglasses and hats with wide brims can make the glare outside
   bearable. Other things that can help people with albinism are avoiding
   sudden changes of the lighting situation (switching the light on in
   complete darkness), using dimmable switches and adding tint to car
   windows or blinds to normal windows. Lights should be yellowish rather
   than blue and not point towards the usual position of a person with
   albinism (like their seat at a table). When possible, people with
   albinism generally prefer to have the light on their backs rather than
   face it.

Misconceptions

   While some of the very rare albinism disorders that are coupled with
   deafness and immunodeficiency appear to be linked with inbreeding, the
   vast majority of sufferers of common albinism are not the product of
   such unions; the more usual albinism genes are widespread enough that
   they can easily produce albinistic offspring from parents that are not
   related.

   A common misconception is that albinistic individuals of a species are
   sterile; they are in fact fully capable of reproducing. It is also
   thought by many that people with albinism live short life spans. This
   is not true in general, but may be a distorted view of a more
   reasonable fact that people with albinism have a higher risk of skin
   cancer if they do not use proper skin protection when in the sun. (Some
   very rare variants of albinism are lethal in and of themselves by
   adulthood or sooner, but they are so little-known by the general public
   that they are unlikely to have contributed to this belief.)
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