Congenital anomalies are structural defects that are present at birth, although some, such as cardiac defects and renal anomalies, may not become clinically apparent until years later. As will be evident from the ensuing discussion, the term congenital does not imply or exclude a genetic basis. It is estimated that about 120,000 babies are born with a birth defect each year in the United States, an incidence of 1 in 33. As indicated in
Table 7.5 , congenital anomalies are an important cause of infant mortality. Moreover, they continue to be a significant source of illness, disability, and death throughout the early years of life.
Before considering the etiology and pathogenesis of congenital anomalies, it is essential to define some of the terms used to describe errors in morphogenesis.
• Malformations are primary errors of morphogenesis. In other words, there is an intrinsically abnormal developmental process. Malformations usually are multifactorial, rather than the result of a single gene or chromosomal defect.
*Causesarelistedindecreasingorderoffrequency.Allcausesandratesarebasedon2008(final)and2009(preliminary)data.
†Ratesareexpressedper100,000populationfromallcauseswithineachagegroup.
‡Excludescongenitalheartdisease.Datasource:CentersforDiseaseControlandPrevention/NCHS,NationalVitalStatisticsSystem:mortality,2014.
www.cdc.gov/nchs/data/dvs/lcwk1_2014.pdf http://ebooksmedicine.net
Fig. 7.21 Examplesofmalformations.Malformationscanrangeinseverityfromtheincidentaltothelethal.(A)Polydactyly(oneormoreextradigits)andsyndactyly(fusionofdigits)havelittlefunctionalconsequencewhentheyoccurinisolation.(B)Similarly,cleftlip,withorwithoutassociatedcleftpalate,iscompatiblewithlifewhenitoccursasanisolatedanomaly;inthiscase,however,thechildhadanunderlyingmalformationsyndrome(trisomy13)andexpiredbecauseofseverecardiacdefects.(C)Stillbirthassociatedwithalethalmalformation,inwhichthemidfacestructuresarefusedorill-formed;inalmostallcases,thisdegreeofexternaldysmorphogenesisisassociatedwithsevereinternalanomaliessuchasmaldevelopmentofthebrainandcardiacdefects.(A and C, Courtesy of Dr. Reade Quinton, Department of Pathology, University of Texas Southwestern Medical Center, Dallas,Texas. B, Courtesy of Dr. Beverly Rogers, Department of Pathology,University of Texas Southwestern Medical Center,Dallas,Texas.)
They may manifest in any of several patterns. In some presentations, such as congenital heart diseases, single body systems may be involved, whereas in others, multiple malformations involving many organs and tissues may coexist (
Fig. 7.21 • Disruptions result from secondary destruction of an organ or body region that was previously normal in development; thus, in contrast with malformations, disruptions arise from an extrinsic disturbance in morphogenesis. Amniotic bands, stemming from rupture of amnion with resultant formation of “bands” that encircle, compress, or attach to parts of the developing fetus, constitute the classic example of a disruption (
Fig. 7.22 ). A variety of environmental agents may cause disruptions (see later). Disruptions are not heritable, of course, and thus are not associated with risk of recurrence in subsequent pregnancies.