Contributor: Gordon K. Klintworth
Trisomy 21 (mongolism, Down syndrome, trisomy D) is the most common chromosomal disorder and the most common cause of mental retardation. The prevalence of this systemic syndrome is 1 in 650-700 newborns. Down syndrome is most often due to an extra copy of chromosome 21, but other cases result from a fusion of the long arm of chromosome 21, or a partial reciprocal translocation. A prenatal diagnosis can be suspected by finding increased levels of hyperglycosylated variants of human chorionic gonadotropin in the urine of women carrying a fetus with Down syndrome. Ophthalmic manifestations are common in this trisomy and include: keratoconus (which may be complicated by acute corneal hydrops [hydrops - cornea]), epicanthal folds with a mongoloid slant, narrow almond-shaped palpebral fissures, ectropion, blepharitis, a hypoplastic iris [hypoplasia - iris], Brushfield spots, lens opacities, cataract, strabismus, esotropia, and nystagmus. Ophthalmoscopic features include scanty pigmentation of the retinal pigment epithelium and choroid, and prominent choroidal vasculature (regardless of skin or iris pigmentation), attenuation of fundus pigmentation, peripapillary and patchy peripheral areas of retinal pigment epithelium atrophy. Persons with Down syndrome appear to have more retinal vessels traversing the disk margin than do normal individuals. A traumatic retinal detachment may occur. The optic disc appears pinker than usual due to the large number of retinal vessels. Myopia, hypertelorism, and ametropia may also occur.