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Medical Definition of Chromosome 18
1. Edwards syndrome (trisomy 18), the second most common autosomal trisomy with an occurrence rate of 1 in 8,000 live births, is associated with a distinct pattern of dysmorphogenesis and congenital abnormalities which results in early death. Marked growth deficiency is a major trait of the phenotype. Other striking clinical features of this syndrome include congenital heart abnormalities, mental and developmental delays, prominent occiput, faunlike ears, micrognathia, short sternum, narrow pelvis, overlapping fingers and rocker-bottom feet. The 18q12.2 subband is identified as the critical region in the trisomy 18 phenotype. Trisomy 18 mosaicism can present with minimal non-specific signs and no typical stigmata of the full trisomy. Although Edwards syndrome is typically associated with duplication of the entire chromosome 18, several individuals with partial trisomies of chromosome 18 (18q2 trisomy, 18p & q1 trisomy) are reported. These patients display a range of severity from a relatively mild phenotype with no internal organ malformations to the classic characteristics of Edwards syndrome. Isochromosome 18p [i(18p)] is described 6 and is associated with a distinctive syndrome, tetrasomy 18p, which includes low birth weight, characteristic facies, neonatal hypotonia with subsequent limb spasticity, short stature, microcephaly, mental retardation and seizure disorders. Isochromosome 18q is a rare cytogenetic abnormality resulting in trisomy for the q arm and monosomy for the p arm of chromosome 18. Phenotypic characteristics are not yet fully delineated. Clinical and pathological examination reveals features characteristic of both trisomy 18 and monosomy 18p or features characteristic of trisomy 18 alone. Severe cephalic malformations such as holoprosencephaly, cebocephaly and cyclopia are reported in i(18q), but are rarely described in trisomy 18 and are only occasional findings in monosomy 18p. Isodicentric chromosome 18 was described and the clinical findings are associated with both the trisomy 18 and 18p- syndromes. The 18q- syndrome or 18q2 monosomy is a well-described partial aneusomy disorder resulting from the deletion of a portion of the long arm of chromosome 18. Typically, patients with this syndrome show growth deficiency, dysmorphic facial features, limb defects, genitourinary malformations, neurologic and ocular anomalies, and developmental delay with mental retardation. The dysmorphic features are mild and include midfacial hypoplasia, carp-shaped mouth and abnormally folded ears. Neurologic findings consist of hypotonia, poor coordination, choreoathetotic movements and ophthalmologic abnormalities. Proximal interstitial deletion of 18q involving band 18q12.3 is recognised with a distinctive phenotype including moderate to severe MR, high forehead, deep-set eyes, hypotelorism, flat nasal bridge, dysplastic ears, excess dermatoglyphic whorls and aggressive behaviour. In the majority of deletions of the short arm of chromosome 18 (18p monosomy or 18p- syndrome), dysmorphism is not very distinctive. Psychomotor retardation is more or less severe. The ring 18 phenotype shares features of the 18p- syndrome and those of the 18q- syndrome with predominance of the features of the 18q monosomy. Mosaicism is frequently observed with ring chromosomes. The gene for peptidase A is localised to 18q23. (05 Mar 2000)
Lexicographical Neighbors of Chromosome 18
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