Syndromology: an updated conceptual overview. IV. Perspectives on malformation syndromes

In Part IV, the terms sequence and syndrome are defined and the difference between them is emphasized. Malformations and sequences are non-specific; each may occur as an isolated defect and each may occur as a component part of various syndromes. True malformation syndromes are characterized by embryonic pleiotropy in which a pattern of developmentally unrelated malformations occurs, that is, the malformations that make up the syndrome occur in embryonically non-contiguous areas. It is emphasized that syndrome diagnosis is never made from obligatory anomalies, but from the overall pattern of defects. Finally, the relationships among etiology, pathogenesis, and the phenotype are discussed.