KEY FACTS
Terminology
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Monosomy X is preferred term
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Definition: Complete or partial deficiency of X chromosome
Imaging
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Cystic hygroma (CH) is hallmark finding
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Large back of neck fluid collection with septations
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CH is nuchal lymphangioma
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Thin septations lateral + thick central nuchal ligament
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60% of fetuses with CH have monosomy X
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Associated nonimmune hydrops is common
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Must see fluid in 2 cavities (CH counts as 1 area)
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Other associated anomalies
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Hypoplastic left heart, coarctation of aorta
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Horseshoe kidney
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Short femur and humerus
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1st-trimester findings in > 90% of cases
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Typically very large nuchal translucency, CH, hydrops, normal nasal bone, reversed ductus venosus flow
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Top Differential Diagnoses
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Noonan syndrome: Can look identical to Turner
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Trisomy 21: 2nd most likely diagnosis with CH
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Chest lymphangioma: Not associated with aneuploidy
Clinical Issues
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Very high in utero demise rates
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Cell-free fetal DNA: 89% detection rate
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Advanced maternal age not at higher risk
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Survivors variably affected
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Webbed neck, broad chest, short limbs
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Normal verbal IQ, delayed motor skills
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Infertility, metabolic and autoimmune disorders
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Scanning Tips
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Look for subtle left heart findings: Aorta smaller than pulmonary artery, mildly small left ventricle
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When massive, CH can mimic amniotic fluid (can tap CH for genetic testing)
. Multiple septations 
are seen in addition to the central nuchal ligament 
.
as well as anasarca 
and ascites 
. Hydrops is often associated with cystic hygroma and Turner syndrome. Prognosis is poor when these features are seen.
