Malformations occur during formation of structures, for example during organogenesis. They may result in complete or partial absence of a structure or in alterations of its normal configuration. Malformations are caused by environmental and/or genetic factors acting independently or in concert. Most malformations have their origin during the third to eighth weeks of gestation.
Disruptions result in morphological alterations of already formed structures and are due to destructive processes. Vascular accidents leading to bowel atresias (see Chapter 13; p. 296) and defects produced by amniotic bands are examples of destructive factors that produce disruptions.
Deformations are due to mechanical forces that mold a part of the fetus over a prolonged period. Clubfeet, for example, are due to compression in the amniotic cavity. Deformations often involve the musculoskeletal system and may be reversible postnatally.
A syndrome is a group of anomalies occurring together that have a specific common cause. This term indicates that a diagnosis has been made and that the risk of recurrence is known. In contrast, association is the nonrandom appearance of two or more anomalies that occur together more frequently than by chance alone, but whose cause has not been determined. Examples include CHARGE (Colobomas, Heart defects, Atresia of the choanae, Retarded growth, Genital anomalies, and Ear abnormalities) and VACTERL (Vertebral, Anal, Cardiac, Tracheo Esophageal, Renal, and Limb anomalies). Although they do not constitute a diagnosis, associations are important because recognition of one or more of the components promotes the search for others in the group.
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