Low frequency mosaicism of normal cells in a 16-year-old girl with trisomy 18.

A 16-year-old girl with mosaicism of trisomy 18 has been followed from birth in our department. She had stigmata characteristic for trisomy 18. Chromosome analysis of lymphocytes showed trisomy 18 both at birth and at age 15, whereas analysis of fibroblasts at age 16 showed trisomy 18 with low frequency mosaicism of normal cells (4%). In most case reports, karyotype analyses have been performed in lymphocytes only. The low frequency mosaicism of normal cells found in fibroblasts from the present patient may raise the question of mosaicism in other long-living patients previously reported to be non-mosaic trisomy 18. The main disorders in the present patient were limited to severe mental deficiency, structural cerebral malformations and skeletal deformities, including bilateral equinovarus deformities. At birth, she had a ventricular septal defect which closed spontaneously. Frequent respiratory infections subsided after age 2. At age 7 she developed a seizure disorder. Since then, her medical condition has been stable. Even though patients with trisomy 18 rarely survive early childhood, the possibility that they may reach their teens must be kept in mind when treatment is planned. In our case, the decision not to treat her equinovarus deformities means that she cannot stand, a major problem in her everyday life.