Chromosome - Turners Syndrome

 

Turner's syndrome is caused by the absence of one of the sex chromosomes, monosomy of X chromosome. Cells contain only 45 chromosomes with a single X chromosome, rather than the usually 46XX in a female or 46XY in a male. Half of Turner's syndrome cases have karyotype 45XO (i.e. single X chromosome) and the remainder have one normal X and one abnormal chromosome (partial deletion, ring formation or short/long iso-chromosomes). Turner's syndrome occurs sporadically, and there is no association with increased maternal age as it arises from mitotic, not meiotic, error. Having an affected child does not appear to alter the risk in subsequent pregnancies.

ANTENATAL

Generally all cases of monosomy except Turner's syndrome are non-viable. Turner's syndrome is frequent in first trimester spontaneous abortions as most conceptuses are lethal. The ovaries form normally until the end of 2nd trimester but do not make primary follicles, they then degenerate and may not be detected on ultrasound. Many cases are associated with cystic hygroma (an accumulation of fluid within the skin around the fetal neck), this is often present in association with more general fetal fluid accumulation with inn other body cavities, such as the fetal chest (pleural effusion), pericardium (pericardial effusion) and abdomen (ascites). This general accumulation of fluid within the fetus is termed "fetal hydrops" and is an extremely poor prognostic sign, with almost all cases dying in-utero. At the time of birth, the fluid within cystic hygromas is usually been reabsorbed, leading to loose skin and so-called webbing of the neck. Coarctation of the aorta is also common, as is growth retardation and asymptomatic renal tract anomalies (horseshoe kidney, duplex ureters).

POSTNATAL

Many cases of Turner's syndrome are not diagnosed until childhood and many present at puberty with primary amenorrhoea, failure of secondary sexual development, or short stature with no pubertal growth spurt. Other features include a broad (shield-shaped) chest with widely spaced nipples, a low posterior hairline, hypoplastic or malformed nails, pigmented naevi, cervical lymphatic cysts, and an increased carrying angle at the elbow (cubitus valgus). Most girls are infertile but menstruation and secondary sexual development can be induced by oestrogen replacement. Intelligence is within the normal range. Life span is normal but osteoporosis may develop if oestrogen replacement is not used.