Triploidy syndrome

Triploidy syndrome is a chromosomal condition in which an individual has a complete extra set of chromosomes, resulting in 69 chromosomes instead of the normal 46 (69,XXX; 69,XXY; or 69,XYY). It is one of the most common chromosomal abnormalities seen in human conceptions, but the vast majority of affected pregnancies end in early miscarriage. Triploidy can arise from fertilization of an egg by two sperm (diandric or paternal origin), fertilization involving a diploid sperm, or retention of a polar body in the egg (digynic or maternal origin). The parental origin of the extra chromosome set significantly influences the clinical presentation. Dispermic (diandric) triploidy is typically associated with a partial hydatidiform mole, a large but abnormally formed placenta, and a relatively small fetus with few malformations. Digynic triploidy, in contrast, is characterized by severe asymmetric intrauterine growth restriction with a small, non-molar placenta and a fetus that may survive longer in utero but displays multiple congenital anomalies. Common clinical features in liveborn infants include syndactyly (especially of the third and fourth fingers), neural tube defects, congenital heart defects, hydrocephalus, eye abnormalities (such as coloboma and microphthalmia), adrenal hypoplasia, renal malformations, and characteristic craniofacial dysmorphism including a large posterior fontanelle and low-set ears. Affected infants are typically small for gestational age with a disproportionately large head relative to the body. Triploidy is almost universally lethal. Most affected pregnancies result in first-trimester miscarriage, and liveborn infants rarely survive beyond the first days or weeks of life. Extremely rare cases of mosaic triploidy (where only some cells carry the extra chromosome set) may allow longer survival, sometimes into childhood, but are associated with intellectual disability, body asymmetry, and skin pigmentation abnormalities. There is no curative treatment for triploidy syndrome; management is supportive and palliative. Pregnancies with diandric triploidy and partial molar changes require careful monitoring for gestational trophoblastic disease. Genetic counseling is recommended, though the recurrence risk in subsequent pregnancies is generally very low.

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