Tetrasomy 18p is a very rare structural chromosomal anomaly affecting multiple body systems and characterized clinically by craniofacial abnormalities, delayed development, cognitive impairment, changes in muscle tone, distinctive facial features, and rarely renal malformations. The disease is Monarch Disease Ontology identifier MONDO_0013668 (tetrasomy 18p). Also known as: Isochromosome 18p, tetrasomy type 18P, tetrasomy type 18p.