Optometry & Vision Science:
Albinism: Classification, Clinical Characteristics, and Recent Findings
Summers, C Gail*
Departments of Ophthalmology and Pediatrics, University of Minnesota, Minneapolis, Minnesota.
This work was supported, in part, by an unrestricted grant from Research for the Prevention of Blindness, New York, NY.
Received September 3, 2008; accepted December 11, 2008.
Purpose. To describe the clinical characteristics and recent findings in the heterogeneous group of inherited disorders of melanin biosynthesis grouped as “albinism.”
Methods. The current classification of albinism, and the cutaneous, ocular, and central nervous system characteristics are presented. Recent clinical findings are summarized.
Results. Albinism is now classified based on genes known to be responsible for albinism. Foveal hypoplasia is invariably present and individuals with albinism often have delayed visual development, reduced vision, nystagmus, a positive angle kappa, strabismus, iris transillumination, and absent or reduced melanin pigment in the fundi. A visual-evoked potential can document the excessive retinostriate decussation seen in albinism. Grating acuity can be used to document delayed visual development in preverbal children. Glasses are often needed to improve visual acuity and binocular alignment.
Conclusions. Albinism is caused by several different genes. Heterogeneity in clinical phenotype indicates that expressivity is variable.
Albinism, derived from the Latin, albus, meaning white, is a group of inherited disorders in which melanin biosynthesis is reduced or absent. Several genes have been found to be responsible for albinism. The current classification of albinism is determined by the affected gene, making the previously used terms, “partial or complete” and “tyrosinase-positive or tyrosinase-negative” obsolete.1,2 The gene for tyrosinase on chromosome 11q14-21 and the P gene on chromosome 15q11.2 are the most commonly affected genes, mutations on these genes cause oculocutaneous albinism type 1 (OCA1; OMIM 203100) and oculocutaneous albinism type 2 (OCA2; OMIM 203200), respectively. These types of albinism are inherited in an autosomal recessive manner and are expressed in males and females. Another type of albinism caused by mutations on Xp22.3, ocular albinism (OA1; OMIM 300500), affects males because of X-linked inheritance; 85 to 90% of obligate carriers show pigmentary mosaicism in the fundi, representing the lyonization effect (X-inactivation), although there are no functional sequelae for them. Other types of albinism occur more infrequently, including those associated with systemic manifestations, such as Hermansky-Pudlak syndrome (OMIM 203300; bleeding disorder due to absence of dense bodies in platelets) and Chédiak Higashi syndrome (OMIM 214500; immunodeficiency and neurologic problems). The prevalence of albinism in the United States is estimated to be 1 in 18,000.2 Table 1 summarizes the genes responsible for albinism. For available testing for mutations, the reader is referred to www.genetests.org.
CLINICAL FEATURES OF ALBINISM
The cutaneous phenotype often permits the clinical diagnosis to be made, although gene testing is sometimes required to specify the type of albinism.1,3 However, foveal hypoplasia is common to all types of albinism, even though a rudimentary annular reflex has been described in a few patients with better visual acuity.4,5 Siblings with albinism can show variable expression in visual function and clinical phenotype,6–8 suggesting that other genes modify the classical phenotype.9 Ocular findings do not allow one to determine the type of albinism.
Individuals with OCA1 typically have white hair at birth. Some with a “leaky mutation” (OCA1B) will develop some melanin pigment in their lashes and hair over time, whereas others with OCA1A will fail to show any melanin pigment in their hair, skin, or eyes during their lifetimes.2 Those with OCA2 are typically born with blond or red hair.2 Individuals with OCA1 and individuals with OCA2 both have pale skin at birth. The phenotype and the normal presence of an immature fovea at birth can delay diagnosis, particularly, in families of European descent in whom reduced melanin pigment in the skin, hair, and eyes is common. However, examination with the slit lamp biomicroscope will show transillumination of the irides in those with albinism, and rarely is any pigment found at the level of the retinal pigment epithelium with careful examination of the fundi in an individual with albinism.
Delayed Visual Maturation
Delayed visual maturation has been reported in albinism.10–15 It is not unusual for parents of an infant with albinism to note poor fixation on faces and objects and a delay in visual development.15,16 Nystagmus typically develops by 6 to 8 weeks of age. If the phenotype of albinism is not recognized, patients may undergo neurologic examination, including magnetic resonance imaging of the brain, before the diagnosis of albinism, with no abnormalities being detected.16 In fact, neurodevelopment was carefully measured in 78 children with albinism from ages 4 to 18 years and was generally felt to be normal, despite reduced visual acuity.17 However, an increased prevalence of attention deficit and hyperactivity disorder was found in this prospective study.
As infants with albinism develop a fixation response, additional findings may be noted. Nystagmus is initially slow and has a large amplitude, but the amplitude typically decreases within the first year of life. One study found that, with monocular fixation on a light, 99.6% of patients had a corneal light reflex deviated nasal to the center of the pupil, known as a positive angle kappa.18 Prism and alternate cover test at a more rapid rate than usual (to diminish effect of increased nystagmus amplitude with monocular occlusion) often discloses strabismus. Because of the positive angle kappa, an esotropia may be masked or appear diminished in amount on causal gaze, compared with the measured deviation, whereas an exotropia may appear larger than measured. Even individuals with albinism who are orthophoric seem to have a small exotropia, causing parents to report that the child does not look directly at them. A study of 178 individuals with albinism showed a mean exoshift of 17.11 prism diopters because of the presence of a positive angle kappa when prism and alternate cover measurements were compared with Krimsky assessment of binocular alignment.18
Other ocular findings in albinism are iris transillumination and absent or poor foveal development. Iris transillumination is best performed in a dark room after the examiner has become adapted to the darkness. The tonometer platform is rotated 90° to avoid having it touch the child’s chest, and a small, bright light is directed through the pupil while the examiner fixates on the iris. The amount of iris transillumination can vary and a grading scheme has been described: grade 1, punctate areas of transillumination, indicating that a marked amount of pigment is present in the posterior iris epithelium; grade 2, moderate iris pigment; grade 3, minimal iris pigment; and grade 4, full transillumination of the iris because of the absence of melanin pigment.19 Examination of the fundi typically shows that foveal development is absent. A few patients with albinism, who have vision 20/50 or better, have some rudimentary foveal development, and some thinning of the retina in the foveal area has been demonstrated with optical coherence tomography.4,5,20 The appearance of the macula has been graded as follows: grade 1, choroidal vessels easily seen in macula; grade 2, choroidal vessels less distinctly seen because of translucent retinal pigment epithelium; and grade 3, opaque macula so that choroidal vessels are not visible.19 These grading scales for the iris and macula can be useful in clinical studies to more precisely describe the study population because these characteristically persist over time. In addition, careful inspection can show granular melanin pigment in the macula in a few patients with albinism and occasionally finely granular pigment has been identified beyond the macula. The presence of melanin pigment in the macula correlates with better visual acuity.15
Central Nervous System Findings
Pattern visual-evoked potentials performed with monocular visual stimulation demonstrate the excessive retinostriate decussation that is characteristic of albinism.21 In individuals with a questionable phenotype for albinism, the visual-evoked potentials can be useful in identifying those with the disorder before proceeding with more expensive gene testing. This abnormal decussation may account for absent stereoacuity that is often found in albinism. A study of 45 individuals with albinism and strabismus ≤10 prism diopters identified 19 with stereoacuity measured with the Titmus vectrograph (Stereo Optical Co., Chicago, IL).22 These individuals had significantly better visual acuity, more pigment in their iris, reduced or absent nystagmus, and more frequently had melanin pigment identified in their maculae and the development of a rudimentary foveal reflex when compared with those without measurable stereoacuity.
TREATMENT OF ALBINISM
Nystagmus in albinism is typically pendular in nature. The amplitude of nystagmus diminishes as the child matures and in some cases may be detected clinically only as latent nystagmus with monocular occlusion in older children and adults. Patients typically report that nystagmus becomes more noticeable with fatigue and illness. These factors often confound the results of any treatment for nystagmus in albinism. Often patients develop an anomalous head posture to damp the nystagmus, referred to as the null point, where visual acuity is usually improved. When the head posture is sufficiently large and stable in amount, consideration can be given to extraocular muscle surgery in an attempt to transfer the null point to a position closer to primary gaze (Kestenbaum-Anderson procedure).23,24 Another surgical procedure, retroequatorial recession of the horizontal rectus muscles, has been described to improve visual acuity in patients with nystagmus due to many etiologies.25,26 Most recently, tenotomy of the horizontal rectus muscles with reattachment at the original insertion has been introduced to improve visual acuity and calculated acuity function based on the proposal that the dynamics of the proprioceptive loop are altered.27 A similar mechanism might exist for the previously described surgeries in which the muscles are either recessed or resected. Based on cumulative results in the literature, slightly more than half of individuals with albinism undergoing any of these procedures will experience up to one line of improvement in measured visual acuity, in addition to alterations in the nystagmus waveform.27,28 The reason for not recording better postsurgical visual acuity in those with albinism is likely due to the associated anatomic abnormalities. However, even one line improvement in visual acuity may be sufficient to allow some individuals with albinism to obtain a driver’s license so surgery should at least be discussed with the patient, in addition to other potential methods to improve visual function.
RECENT FINDINGS IN ALBINISM
Before the clinical use of the Teller Acuity Cards14,29 (TAC; Vistech Consultants, Dayton, OH), measurement of visual acuity in a child with albinism was often limited to the ability to fix and follow a moving target. With the TAC, resolution acuity could be measured. When 14 adults with albinism had resolution acuity measured with the TAC presented horizontally and vertically, it was found that vertical presentation (horizontal orientation of grating) yielded significantly better vision than when measured with standard horizontal presentation of the cards, likely due to the predominantly horizontal character of the nystagmus.30 Infants with albinism who are beginning to learn to fixate will typically respond more readily to vertical presentation of the cards. However, as children mature, standard horizontal presentation of the TAC is most often used. Because of the presence of increased nystagmus amplitude with monocular occlusion, monocular grating acuity is frequently worse than binocular grating acuity.
Parents often ask how impaired their child’s vision is when vision is measured with the TAC and what their child’s vision will be when the children are older. A cross-sectional study of 64 children with albinism showed that visual acuity measured with the TAC was 2.3, 2.1, and 1.7 octaves lower than norms at ages 1, 2, and 3 years, respectively.31 Another study of children with albinism compared grating acuity measured with the TAC at ages 1 (n = 30), 2 (n = 29), and 3 years (n = 19) with recognition acuity measured at ages 4 to 6 years.32 In this study, the mean binocular grating acuity was 2.0, 1.9, and 1.5 octaves below norms at ages 1, 2, and 3 years, respectively. Grating acuity at ages 1, 2, and 3 years correlated moderately with recognition acuity at ages 4 to 6. This correlation improved when grating and recognition acuities were analyzed in a subgroup of nine patients who were followed up longitudinally. Binocular grating acuity in both the subgroup and the larger group at ages 1 and 2 was significantly worse than recognition acuity, but was not significantly different from binocular letter acuity at age 3.
Recognition visual acuity among persons with albinism varies from 20/20 to 20/400, but is commonly close to 20/80. Foveal hypoplasia, nystagmus, and refractive errors contribute to reduced acuity.33 A prospective evaluation of 35 individuals with albinism (median age 9.5 years) who had received glasses (spherical equivalent from −9.75 D to +8.88 D) had a mean uncorrected visual acuity of 20/107.6 at 6 m, improving to 20/80.9 with glasses.34 Mean uncorrected near acuity was 20/41, improving to 20/28.4 with correction. In addition, with glasses, mean strabismus improved from 10 to 7.2 prism diopters at distance, and from 14 to 10.8 prism diopters at near. Compliance with wearing refractive correction was generally good, despite the absence of normal corrected visual acuity. Guidelines for prescribing glasses in children with albinism are given in Table 2.
In summary, patients with albinism have delayed visual development that can be monitored by measuring grating acuity. Cycloplegic refraction can determine refractive errors that should prompt consideration of glasses. A spectrum of structure and function exists in albinism. Examination may disclose absence of nystagmus, measurable stereopsis, melanin pigment in the macula, and presence of a rudimentary annular reflex in the macula that tend to be associated with better vision than when these findings are absent. As research progresses, other methods of improving visual acuity in children and adults may emerge.
I gratefully acknowledge the constant encouragement and support of my friend and colleague, Dr. Velma Dobson, during this study of visual development in albinism.
C. Gail Summers
University of Minnesota
Department of Ophthalmology, MMC 493
420 Delaware St. SE
Minneapolis, MN 55455
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© 2009 American Academy of Optometry
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