triploidy
Triploidy is a rare lethal chromosomal (aneupliodic) abnormality caused by the presence of an entire extra chromosomal set.
Epidemiology
It is considered the 3 commonest fatal chromosomal anomaly . While it is thought to affect as much as 1-2% of conceptions, the vast majority are thought to undergo spontaneous abortions in the early intrauterine period.
Pathology
Genetics
The occurrence of triploidy is considered sporadic. A fetus with triploidy has 69 chromosomes. This can happen in three ways:
- failure of the division of meiosis I or II in spermatocyte: extra set of paternal chromosomes
- failure of the division of meiosis I or II in oocyte: extra set of maternal chromosomes
- dispermy: double fertilisation of the ovum with two sperms
In ~60-75% of cases, the extra set is paternally derived (diandric triploidy). The reported chromosomal compositions are
- 69XXY: ~60 %
- 69XXX: ~37%
- 69XYY: ~3 %
In some cases, there can be diploid/triploid mosaicism.
Serological markers
The serological markers can vary depending on whether the extra set of chromosomes are maternally (digynic) of paternally (diandric) derived.
- maternal beta HCG and alpha fetoprotein:
- maternally-derived triploidy: markedly decreased
- paternally-derived triploidy: markedly increased
- PAPP-A (pregnancy-associated plasma protein A):
- maternally-derived triploidy: mildly decreased
- paternally-derived triploidy: markedly decreased
Associations
Radiographic features
Antenatal ultrasound
Sonographic features of pregnancies complicated by fetal triploidy are not uniform and the diagnosis cannot be made by ultrasound alone
However common features include:
- second trimester-onset (early) fetal intrauterine growth restriction (IUGR) with a reduced growth potential pattern of anthropometric growth
- 69 XXX: tends to have early-asymmetrical IUGR
- 69 XXY: tends to have symmetrical IUGR
- body asymmetry with relative macrocephaly and an elevated head: abdominal circumference ratio (HC >> AC)
- fetal hydrocephalus
- oligohydramnios
- placental changes:
- abnormally large placenta (placentomegaly) and/or hydropic placenta: in cases of paternal origin
- small placenta: in maternally derived cases
- syndactyly: tends to affect the 3 and 4 digits of the hands
Plain radiograph
Not part of routine investigation and most fetuses are not liveborn. Reported features according to one study were :
- harlequin orbits
- small anterior fontanelle
- gracile ribs
- diaphyseal overtubulation of long bones
- upswept clavicles
- antimongoloid pelvis
Treatment and prognosis
Triploidy is incompatible with life and most fetuses miscarriage in the 1trimester. Extremely rare cases with survival few months into postnatal life have been reported . Since most cases are sporadic, the risk of recurrence is, usually, not increased.
Complications
- maternal: development of pre-eclampsia
Differential diagnosis
Considerations on early ultrasound scans include:
- fetal demise with placental hydropic change: beta HCG levels are invariably low in this case
See also
- partial mole: some consider this term to be synonymous with triploidy
Siehe auch:
und weiter:
Assoziationen und Differentialdiagnosen zu Triploidie:
