Trisomy 18.

Abstract

Since Tijio and Levan established the normal human chromosome number as 46, numerous abnormalities of the human chromosome complement have been described. Four reported clinical syndromes are associated with autosomal (non-sex chromosomes) anomalies. The first of these was described in 1959 by Lejeune et al., who found that in mongolism there was an extra chromosome in group 21–22 (also known as the G group). The extra chromosome presumably resulted from nondisjunction in meiosis. There is a correlation between increased maternal age and mongolism. Because of this, it is thought that in most instances the nondisjunction occurs in the ovary, particularly in an older ovary. Thus, instead of two pairs of chromosomes in the G group, mongoloids have one pair plus a triplicate set. By convention this is called “trisomy 21–22.” In 1960, Edwards et al. published the first case report of trisomy 18 (3). In this syndrome there is also a correlation between older maternal age and nondisjunction. At present over 100 accounts of this autosomal abnormality have been published. Only brief mention has been made of trisomy 18 in the radiologic literature, and differential features between this and other autosomal abnormalities have not been discussed (1, 15). After a review of our 23 cases at Boston Children's Medical Center and Massachusetts General Hospital, it was apparent that characteristic radiologic features permit the differentiation and diagnosis of trisomy 18. Karyotype The karyotype in trisomy 18 contains 47 chromosomes instead of the normal 46 (8) with 7 in the 16–18 group (group E) rather than the usual 6 (13). From morphological considerations and analysis of synthesis patterns of deoxyribonucleic acids it has been established that in almost all instances of this clinical syndrome, there is trisomy of the 18 chromosome (22). Trisomy at the 18 position is thought to result from nondisjunction. Clinical Features In trisomy 18 generalized abnormalities affect all systems. Infants with trisomy 18 have a pitiable appearance with low birth weight (two-thirds below 2,500 g) (24), growth and mental retardation, muscular hypertonicity, and characteristic hand configuration and facies (12, 19, 20). The prenatal history is usually normal. Because this autosomal abnormality is almost always lethal in early infancy (the oldest reported case is a 10.5-year-old female), the mean age at death is about ninety days. Of 100 cases in the literature, 50 per cent survived two months, and 33 per cent survived three months (23). Typical facies of trisomy 18 consist of micrognathia, small triangular mouth, short upper lip, high palate, and a prominent occiput (2). There are often redundant cervical skin folds not unlike those seen with neck “webbing” (4,11,18) (Fig. 1). Thoracic and cardiac structures are usually affected. A “shield-chest” deformity with wide subcostal angle and a short sternum (Fig. 2) is almost always associated with congenital heart disease.