Table 7.6 Causes of Congenital Malformations in Humans (B)Infantwitholigohydramnios(Potter)sequence.Noteflattenedfacialfeaturesanddeformedfoot(talipesequinovarus).
survival is not possible. If the embryologic connection between these defects and the initiating event is not recognized, a sequence may be mistaken for a malformation syndrome.
Malformation syndrome refers to the presence of several defects that cannot be explained on the basis of a single localizing initiating error in morphogenesis. Syndromes most often arise from a single causative condition (e.g., viral infection or a specific chromosomal abnormality) that simultaneously affects several tissues.
In addition to these global definitions, some general terms are applied to organ-specific malformations. Agenesis refers to the complete absence of an organ or its anlage, whereas aplasia and hypoplasia indicate incomplete development and underdevelopment, respectively. Atresia describes the absence of an opening, usually of a hollow visceral organ or duct such as intestines and bile ducts.
*Livebirths.DatafromStevensonRE,HallJG,GoodmanRM,editors:Human malformations and related anomalies, NewYork,1993,OxfordUniversityPress,p115.
http://ebooksmedicine.net scourge of the 19th and early 20th centuries. Fortunately, maternal rubella and the resultant rubella embryopathy have been virtually eliminated in developed countries as a result of vaccination. As mentioned later, maternal infection with Zika Virus can give rise to severe malformations of the central nervous system. A variety of drugs and chemicals have been suspected to be teratogenic, but perhaps less than 1% of congenital malformations are caused by these agents. The list includes thalidomide, alcohol, anticonvulsants, warfarin (oral anti-coagulant), and 13-cisretinoic acid, which is used in the treatment of severe acne. For example, thalidomide, formerly used as a tranquilizer in Europe and currently used for treatment of certain cancers, causes an extremely high incidence (50% to 80%) of limb malformations. Alcohol, perhaps the most widely used agent today, is an important environmental teratogen. Infants born of mothers who abuse alcohol show prenatal and postnatal growth retardation, facial anomalies (microcephaly, short palpebral fissures, maxillary hypoplasia), and psychomotor disturbances. These features in combination are designated the fetal alcohol syndrome. Although cigarette smoke–derived nicotine has not been convincingly demonstrated to be a teratogen, there is a high incidence of spontaneous abortions, premature labor, and placental abnormalities among pregnant smokers, and babies born to mothers who smoke often have a low birth weight and may be prone to the sudden infant death syndrome (SIDS). In light of these findings, it is best to avoid nicotine exposure altogether during pregnancy. Among maternal conditions listed in
Table 7.6, diabetes mellitus is a common entity, and despite advances in antenatal obstetric monitoring and glucose control, the incidence of major malformations in infants of diabetic mothers stands between 6% and 10% in most reported series. Maternal hyperglycemia–induced fetal hyperinsulinemia results in fetal macrosomia (organomegaly and increased body fat and muscle mass); cardiac anomalies, neural tube defects, and other CNS malformations are some of the major anomalies seen in diabetic embryopathy.