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FAQ about Ocular Albinism (OA) and Oculocutaneous Albinism (OCA)
1.) What is ocular albinism (OA)?
Human albinism can be divided broadly into two types, oculocutaneous albinism (OCA) and ocular albinism (OA), where “oculo” means eye and cutaneous means skin. These terms were devised in the late 1940s, when medical science was less sophisticated than it is now. The terminology sounds simple, but in reality is probably incorrect, since all forms of albinism have relative deficiencies of pigment in the hair, skin, and eyes. Historically, ocular albinism is an inherited disorder in which the eyes are deficient in the amount of melanin, which gives the eye its color or pigment, while the skin and hair appear normal or near normal in coloration.
2.) What causes OA?
Ocular albinism (OA) results from the inability of the normal pigment cells in the eyes (especially the iris and the retinal pigment epithelium) to produce normal amounts of pigment. Exactly how the reduced amount of pigment leads to reduced visual acuity (aka eyesight), nystagmus (aka involuntary rapid movement of the eye), and sensitivity to sunlight is not yet clear, nor is it necessarily true that the pigment itself is primarily responsible, but rather some other developmental pathway in the assembly line of the eye and of visual development.
3.) How many genetic variations of OA have been identified?
In most cases, ocular albinism is X-linked. This means that the gene for it is located on the X-chromosome, which makes men more susceptible to ocular albinism than women since men have only 1 X-chromosome versus women who have 2 X-chromosomes. Historically, X-linked ocular albinism is also called Nettleship-Falls ocular albinism, after the two physicians who defined its X-linked inheritance. Some years ago a series of families were described in which children of normally pigmented parents had the ocular features of albinism but did not appear to have significant skin involvement (also known as cutaneous hypopigmentation).
4.) How common is OA?
The population frequency of ocular albinism is unknown. Healthcare statistics in North America do not tabulate the frequency of these disorders. Educated estimates vary between 1 person in 20,000 to 1 person in 50,000. It is likely that the incidence of Nettleship-Falls albinism is more prevalent than 1 person in 50,000 due to frequent misdiagnosis.
5.) What are the features of OA?
The features of ocular albinism include: reduced visual acuity (typically from approximately 20/30 to approximately 20/400); nystagmus (an uncontrollable, pendular, rapid movement of the eyes back and forth); strabismus (a muscle imbalance of the eyes in which the eyes are “crossed” rather than straight and parallel); and sensitivity to bright light. The reduced visual acuity may result in difficulty at school, such as trouble reading what is on a blackboard, except when the reading material is held very close, and difficulty with sports, particularly with small projectile objects. The reduced visual acuity may ultimately limit an affected individual's ability to obtain a driver's license, because most states require at least 20/70 vision (best corrected with glasses or contact lenses) to obtain at least a daylight-restricted driver's license.
6.) How soon after birth can these features be detected in an infant?
That depends, especially on the care with which the parents look for subtle changes in the newborn infants. Oftentimes, the nystagmus is not present at birth but develops between three weeks and eight weeks after birth. The sensitivity to sunlight may not be appreciated for many months, and the reduced visual acuity may not be detected until an infant is cooperative enough to understand the task of recognizing figures, numbers, and other objects. However, in a family with Nettleship-Falls ocular albinism in which there are other known affected males and the mother is known to be a carrier, examination of a male infant at or shortly after birth should clarify whether any individual male is or is not affected.
7.) Will the nystagmus lessen with age?
Generally, the nystagmus does dampen or reduce with time, especially after age 5-8 years. Nystagmus never completely disappears, even in adults, although most adults with nystagmus are able to control it to a significant extent.
8.) How does OA affect the development of the eye from conception onward?
Two major problems with the eye and ocular development include abnormalities in the development of the central vision area of the retina (called the macula or the fovea) and in the conduct of impulses from the retina through the optic nerve to the brain. The most obvious feature inside the eyes of individuals with ocular albinism is underdevelopment of the fovea, the small area in the center of the retina that provides fine central vision (reading, recognizing faces). In OA, the fovea does not develop completely, presumably because the amount of melanin and pigment needed for growth processes that normally occur after birth is not present. As a result, and perhaps for other reasons, the eye cannot process sharply focused light images. Because this sensitive nerve tissue in the retina does not develop normally, visual acuity cannot be corrected to normal even with glasses or contact lenses, because the “film in the camera” is incapable of taking a complete picture and transmitting it to the brain normally. The second alteration is the transmission of images through the optic nerve. The nerve pathways through the optic nerve to the brain do not follow the usual pattern of routing. In the normal eye, nerve fibers from each eye go to both sides of the brain, that is, the same side as the eye and the opposite side of the eye. In eyes with ocular albinism, more of the nerve fibers cross from one eye to the opposite side of the brain. These same eye problems occur in individuals with oculocutaneous albinism (where the albinism more dramatically affects the skin and the hair as well as the eye).
9.) What is the general range of acuity measured in individuals with OA?
Reduced visual acuity less than 20/20 is essentially always present. However, visual acuity best corrected in young adults and adults with ocular albinism may be as good as 20/30 or as bad as 20/400 and anywhere in between.
10.) Why can't OA be corrected by wearing corrective glasses or contact lenses?
Two features affect the final visual outcome. Underdevelopment of the center vision of the retina (the fovea) does not allow the “film of the retina” to take an accurate picture and thus transmit an accurate picture to the brain. Second, because both the transmission of images and the balance of wiring from each eye to each side of the brain are altered, the brain cannot interpret the impulses correctly and the image is less than perfect. Thus, even when the image is accurately focused with the proper glasses prescription or contact lens prescription, the film in the back of the camera (i.e. the retina) does not take a good picture and the computer (i.e. the brain) does not receive a good transmission.
11.) Is there any examination that can reliably prognosticate visual acuity for infants with OA?
A test called a Visually Evoked Potential (VEP), or sometimes called a Visually Evoked Cortical Potential (VECP), is performed like an EEG or brain wave test. This test will show, when each eye is stimulated independently, the differences of transmission from the stimulated eye to each side of the brain. However, in infants, since the visual pathways continue to develop until at least 7-9 years of age, nothing in an infant will predict exactly what the final visual acuity will be.
12.) Is OA a progressive or degenerative condition?
In general, as children with ocular albinism mature, their visual acuity seems to improve. Part of this improvement may be progressive maturation of the nerve pathways in the brain. Some of this, of course, is maturation of the infant or child to understand the task of visual acuity, reading letters and recognizing figures on a screen. However, there is no evidence that OA is degenerative, and most adolescents and young adults maintain vision throughout life. Individuals with OA never go completely blind from this disorder, although they may be legally blind.
13.) Are there other physical conditions that commonly occur in conjunction with OA?
Males with X-linked ocular albinism tend to have lighter skin and lighter hair than their unaffected brothers and sisters. Some of them will also have blotchy skin changes with relative hyper- and hypo- pigmentation. However, no other systemic constitutional disease is associated with X-linked OA. Parents of children with OA need to be watchful for development of strabismus (when the eyes do not fixate and track together). Uncorrected strabismus leads to amblyopia, also known as a lazy eye, which can further impair vision severely.
14.) Is OA commonly misdiagnosed? If so, why?
There are several pitfalls in the diagnosis of OA, just as there are for oculocutaneous albinism (OCA) in general. The first is the frequent misdirection of the infant who presents with nystagmus (the pendular or rhythmic uncontrollable movement of the eyes). Children with this finding are sometimes referred to neurologists for evaluation of brain tumors or other developmental aberrations of the brain, and are not referred to ophthalmologists. Both should be consulted until a definite diagnosis is reached. Sometimes, well-intended but unsuspecting ophthalmologists fail to look at the whole child and compare the color of the skin and hair to other siblings (if there are any) at comparable ages. Third, infants and children of European-American ancestry tend to be blonde anyway and may escape notice if other siblings are not available for comparison. Lastly, the ophthalmologist may fail to look carefully at the iris for trans-illumination, or they mistake the absence of normal retinal development for 'foveal hypoplasia' or other neurologic problems. Even if the diagnosis of albinism is made, the ophthalmologist may frequently fail to examine the mother, and most importantly, to dilate the mother's pupil to look for the classical features of the carrier state of X-linked OA described in 1951 by Dr. Harold Falls.
15.) What is the difference between ocular albinism (OA) and oculocutaneous albinism (OCA)?
The vision problems experienced by those with OA and OCA are virtually the same. The hair color and skin pigment can vary significantly between OA and OCA, even though those with either condition typically have reduced pigment compared to unaffected siblings.
16.) Do some kids with OCA have normal hair and skin pigment?
Yes, some people with OCA (primarily type OCA1B) have nearly normal hair and skin pigment just like people with OA.
17.) Is there a DNA test available for OCA? Where can I find out about it?
Testing for the types OCA1 and OCA2 is available on a research basis in the Pigment Laboratory at the University of Minnesota. The tyrosinase gene is responsible for OCA1. Sequence analysis identifies 50-80% of the causative mutations depending on the phenotype (OCA1 vs OCA1B). The P gene is responsible for OCA2. The sensitivity of the sequence analysis is currently unknown. The P gene analysis is complex because the gene has many missense non-pathologic polymorphisms, and because there is no readily available assay of the gene product. No prenatal diagnosis is available. Information can be obtained by calling 612-624-0144.
For OA, and for other genetic eye conditions, there are limited facilities that can run the tests. Depending on the particular genetic disorder, testing can take up to two months to complete. Vision of Children recommends the Medical Genetics Laboratory at Baylor College of Medicine in Houston. Dr. Richard Lewis, a professor of Ophthalmology and Molecular and Human Genetics, is on the faculty at Baylor, and has served as a Scientific Advisor to The Vision of Children for several years. He is an international expert on genetic eye diseases and a valuable resource.