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Tetrasomy 18p

Synonym(s) Isochromosome 18p
Prevalence 1-9 / 1 000 000
Inheritance -
Age of onset Neonatal
  • Q99.8
  • C0795868
MeSH -
MedDRA -


Tetrasomy 18p is a structural chromosome anomaly characterized by moderate to severe intellectual deficit, pre- and postnatal growth retardation, pyramidal signs, a peculiar cranial dysmorphism and renal malformations. The prevalence in the general population of Europe has been estimated at approximately 1 in 180 000. The chromosome anomaly consists of an extra isochromosome 18p resulting from the duplication of the short arm of one chromosome 18 (i(18p)). All cases reported so far have been sporadic. Diagnosis is made by karyotyping of blood lymphocytes from a child with clinical features suggestive of a chromosomal anomaly. Prenatal diagnosis is feasible if there is an indication for karyotyping such as systematic screening (chorionic villus sampling or amniocentesis) for advanced maternal age, after identification of serum markers indicating an increased risk of Down syndrome, or after detection of an anomaly (not always evident) by ultrasonography. Conventional banding techniques reveal an extra metacentric chromosome, with two identical arms. Hybridization signals for both the normal chromosome 18 and the additional chromosome can be observed by Fluorescence In Situ Hybridisation (FISH) with a chromosome 18-specific probe. Genetic counseling should inform parents of the clinical features of the syndrome and of the prognosis of affected children (intellectual deficit). Parents should also be informed that the anomaly is sporadic and is not associated with an increased risk of familial recurrence.

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