Trisomy 18, initially identified in 1960,83 causes serious birth defects including a low birth weight; microcephaly; a characteristic facies with fawnlike ears, micrognathia, high arched palate, a narrow bifrontal diameter, and prominent occiput; hypertonicity; poor muscle development; hand flexion with overlapping of second and fifth fingers; and limited hip abduction (Fig. 3-10). Rocker-bottom feet, webbing of toes, and dorsiflexion of a short great toe are common. Renal anomalies including horseshoe kidney and collecting duct malformations, cardiac malformations, pyloric stenosis, eventration of the diaphragm, and Meckel diver-ticulum may occur. Apnea and failure to thrive are serious complications. The majority (90%) die before 1 year of age.
The most common ophthalmic anomalies include epican-thus, hypertelorism, and hypoplastic supraorbital ridges. Less frequently, corneal opacities, congenital glaucoma, cataract, microcornea, retinal depigmentation, colobomatous microph-thalmia, and cyclopia are evident (Fig. 3-10).156,262,312
Pathological findings include corneal opacities with lamellar disorganization and fibrosis of the stroma, anomalies of the
Trisomy 18 is caused by the presence of an extra chromosome 18; rarely, an unbalanced translocation may cause the syndrome. The incidence increases with maternal age.
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