When evaluating patients with retinal degeneration, it is important to consider metabolic diseases. Some disorders, such as albinism and conditions associated with CNS abnormalities, are covered in BCSC Section 6, Pediatric Ophthalmology and Strabismus. Other metabolic disorders, such as abetalipoproteinemia and Refsum disease, are among the differential diagnostic concerns for RP, even though the retinopathy associated with these disorders may be granular and atypical.
In albinism, the synthesis of melanin is reduced or absent. When the reduction in melanin biosynthesis affects the eyes, skin, and hair follicles, the disease is called oculocutaneous albinism. These disorders usually have an autosomal recessive inheritance pattern. If the skin and hair appear normally pigmented and only ocular pigmentation is affected, the condition is called ocular albinism. Ocular albinism typically has an X-linked inheritance pattern. Female carriers of X-linked ocular albinism may show partial iris transillumination and fundus pigment mosaicism.
Regardless of the type of albinism, ocular involvement generally conforms to 1 of 2 clinical patterns: (1) congenitally subnormal visual acuity (typically 20/100–20/400) and nystagmus or (2) normal or minimally reduced visual acuity without nystagmus. The first pattern is true albinism; the second has been termed albinoidism because of its milder visual consequences. Both patterns share the clinical features of photophobia, iris transillumination, and hypopigmented fundi. They differ according to whether or not the fovea develops normally; in true albinism, the fovea is hypoplastic, with no foveal pit or reflex and no evident luteal pigment (Fig 14-3). The gold standard for diagnosis of true albinism is the finding of characteristic abnormalities of the flash and pattern visual evoked potentials (VEPs). Compared with a normal symmetric response, in albinism, a single eye stimulation will result in an asymmetric occipital response because there is a greater number of decussating fibers.
Oculocutaneous albinism has 2 forms with potentially lethal systemic implications. The first, Chédiak-Higashi syndrome, combines albinism with neutropenia and an extreme susceptibility to infections as well as other complications such as bleeding (caused by deficient platelets). The second, Hermansky-Pudlak syndrome, is characterized by a platelet defect that causes easy bruising and bleeding. In the United States, most patients with Hermansky-Pudlak syndrome are of Puerto Rican descent.
King RA, Jackson IJ, Oetting WS. Human albinism and mouse models. In: Wright AF, Jay B, eds. Molecular Genetics of Inherited Eye Disorders. Chur, Switzerland: Harwood Academic; 1994:89–122.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.