Pallister-Killian syndrome in older children and adolescents.

The Pallister-Killian syndrome is caused by a mosaic tetrasomy of the short arm of chromosome 12. Although analysis of peripheral blood lymphocytes usually reveals a normal karyotype, an isochromosome 12p mosaicism is detectable in fibroblast cultures; therefore, in this rare chromosomal aberration, clinical recognition is crucial for appropriate cytogenetic investigations. The phenotype of younger children has already been well documented. During childhood and adolescence, however, the phenotype changes markedly. The disorder in older children and young adults is characterized by a coarse and flat facies, macroglossia prognathia, everted lower lip, and severe psychomotor retardation with muscular hypertonia and contractures. Two severely mentally retarded patients are reported whose diagnoses were confirmed by fibroblast cultures at ages 16 and 21 years.