Turner syndrome is the only monosomy compatible with life, but it is also the most common aneuploidy in first-trimester losses, accounting for 20 percent. his is explained by the wide range in phenotype. Approximately 98 percent of afected conceptuses are so abnormal that they abort early in the first trimester. Of the remainder, many manifest large, septated cystic hygromas in the late irst or early second trimester (Fig. 10-22, p. 198). When cystic hygromas are accompanied by hydrops fetalis, fetuses nearly always die in utero (Chap 15, p. 309).
Less than 1 percent of pregnancies with Turner syndrome yield a liveborn neonate. And, only half of these actually have monosomy X. Approximately a fourth have mosaicism, such as 45,X/46,XX or 45,Xl46,XY, and another 15 percent have isochromosome X, that is, 46,X,i(Xq) (Milunsky, 2004; Nuss baum, 2007).
Abnormalities associated with Turner syndrome include left sided cardiac defects-such as coarctation of the aorta, hypo plastic left heart syndrome, or bicuspid aortic valve-in 30 to 50 percent; renal anomalies, particularly horseshoe kidney; and hypothyroidism. Other features are short stature, broad chest with widely spaced nipples, congenital lymphedema-pui ness over the dorsum of hands and feet, and a "webbed" pos terior neck resulting from cystic hygromas. Intelligence scores are generally in the normal range, but afected individuals are at risk for diiculties with visual-spatial organization, nonver bal problem solving, and interpretation of social cues Q ones, 2006). Growth hormone is typically administered in childhood to ameliorate short stature (Kappelgaard, 201i1). More than 90 starting just before adolescence. An exception is mosaicism involving the Y chromosome, as this confers risk for germ cell neoplasm-regardless of whether the child is phenotypically male or female. Accordingly, eventual prophylactic bilateral gonadectomy is indicated (Cools, 2011; Schorge, 2016).
47,XXX. Approximately 1 in 1000 female newborns has an additional X chromosome-47,XXX. The extra X is maternally derived in more than 90 percent of cases (Milunsky, 2004). Afected infants do not have a characteristic appearance, and in the past most children did not come to attention until school age. However, the incidence of 47,XXiis weakly associated with maternal age, and cell-free DNA screening has resulted in increased diagnoses (Table 14-5, p. 281). Frequent features include tall stature, hypertelorism, epicanthal folds, kyphoscoliosis, clinodactyly, and hypotonia (Tartaglia, 2010; Wigby, 2016). More than a third are diagnosed with a learning disability, half have attention deicit disorder, and overall cognitive scores are in the low-average range. No speciic pattern of malformations has been described, but genitourinary problems and seizure disorders are more common (Wigby, 2016). Pubertal development is unafected. Primary ovarian insuiciency has been reported (Holland, 2001). Because of variable presentation and subtle abnormal indings, it is estimated that this diagnosis is ascertained clinically in only 10 percent of afected children.
Females with two or more extra X chromosomes-48,XXior 49,XXXX-are likely to have physical abnormalities apparent at birth. These abnormal X complements are associated with intellectual disability. For both males and females, the IQ score is lower with each additional X chromosome.