Birth defects

Congenital disorders
Classification and external resources
MeSH D009358

A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life (neonatal disease), regardless of causation. Of these diseases, those characterized by structural deformities are termed "congenital anomalies" and involve defects in or damage to a developing fetus.

A congenital disorder may be the result of genetic abnormalities, the intrauterine (uterus) environment, errors of morphogenesis, infection, or a chromosomal abnormality. The outcome of the disorder will depend on complex interactions between the pre-natal deficit and the post-natal environment.[1] Animal studies indicate that the mother's (and possibly the father's) diet, vitamin intake, and glucose levels prior to ovulation and conception have long-term effects on fetal growth and adolescent and adult disease.[2] Congenital disorders vary widely in causation and abnormalities. Any substance that causes birth defects is known as a teratogen. Some disorders can be detected before birth through prenatal diagnosis (screening).

The term "congenital"[3] disorder does not necessarily refer to a genetic disorder despite the similarity of the words.

Some congenital conditions are idiopathic, and sometimes the word "congenital" is used synonymously with "idiopathic"; but careful usage prefers to reserve the word "congenital" for conditions to which the literal sense of the word applies (that is, those whose pathophysiology has existed since the neonatal period).

Classification

Much of the language used for describing congenital conditions predates genomic mapping, and structural conditions are often considered separately from other congenital conditions. It is now known that many metabolic conditions may have subtle structural expression, and structural conditions often have genetic links. Still, congenital conditions are often classified in a structural basis, organized when possible by primary organ system affected.

Primarily structural

Several terms are used to describe congenital abnormalities. (Some of these are also used to describe noncongenital conditions, and more than one term may apply in an individual condition.)

  • A congenital physical anomaly is an abnormality of the structure of a body part. An anomaly may or may not be perceived as a problem condition. Many, if not most, people have one or more minor physical anomalies if examined carefully. Examples of minor anomalies can include curvature of the 5th finger (clinodactyly), a third nipple, tiny indentations of the skin near the ears (preauricular pits), shortness of the 4th metacarpal or metatarsal bones, or dimples over the lower spine (sacral dimples). Some minor anomalies may be clues to more significant internal abnormalities.
  • Birth defect is a widely used term for a congenital malformation, i.e. a congenital, physical anomaly which is recognizable at birth, and which is significant enough to be considered a problem. According to the CDC, most birth defects are believed to be caused by a complex mix of factors including genetics, environment, and behaviors,[1] though many birth defects have no known cause. An example of a birth defect is cleft palate.
  • A congenital malformation is a congenital physical anomaly that is deleterious, i.e. a structural defect perceived as a problem. A typical combination of malformations affecting more than one body part is referred to as a malformation syndrome.
  • Some conditions are due to abnormal tissue development:
    • A malformation is associated with a disorder of tissue development.[4] Malformations often occur in the first trimester.
    • A dysplasia is a disorder at the organ level that is due to problems with tissue development.[4]
  • It is also possible for conditions to arise after tissue is formed:
    • A deformation is a condition arising from mechanical stress to normal tissue.[4] Deformations often occur in the second or third trimester, and can be due to oligohydramnios.
  • A disruption involves breakdown of normal tissues.[4]
  • When multiple effects occur in a specified order, it is known as a sequence. When the order is not known, it is a syndrome.

Other

  • Genetic disorders or diseases are all congenital, though they may not be expressed or recognized until later in life. Genetic diseases may be divided into single-gene defects, multiple-gene disorders, or chromosomal defects. Single-gene defects may arise from abnormalities of both copies of an autosomal gene (a recessive disorder) or of only one of the two copies (a dominant disorder). Some conditions result from deletions or abnormalities of a few genes located contiguously on a chromosome. Chromosomal disorders involve the loss or duplication of larger portions of a chromosome (or an entire chromosome) containing hundreds of genes. Large chromosomal abnormalities always produce effects on many different body parts and organ systems.
  • A congenital metabolic disease is also referred to as an inborn error of metabolism. Most of these are single gene defects, usually heritable. Many affect the structure of body parts but some simply affect the function.
  • Other well defined genetic conditions may affect the production of hormones, receptors, structural proteins, and ion channels.

Causes

Cell division errors can be due to a lack of nutrients or availability of atomic building blocks, or the presence of toxins that impede normal growth. Division errors which occur very early in the development of a multicellular organism can result in large scale structural and functional differences in the organism's final shape. For example it is now understood that a lack of folic acid in the diet of a mother can cause cellular neural tube deformities that result in Spina Bifida.

External physical shocks or constrainment due to growth in a restricted space, may result in unintended deformation or separation of cellular structures resulting in an abnormal final shape or damaged structures unable to function as expected.

For multicellular organisms that develop in a womb, the physical interference or presence of other similarly developing organisms such as twins can result in the two cellular masses being integrated into a larger whole, with the combined cells attempting to continue to develop in a matter that satisfies the intended growth patterns of both cell masses. The two cellular masses can compete with each other, and may either duplicate or merge various structures. This results in conditions such as conjoined twins, and the resulting merged organism may die at birth when it must leave the life-sustaining environment of the womb and must attempt to sustain its biological processes independently.

Epidemiology


Congenital anomalies resulted in about 510,000 deaths globally in 2010.[6]

Many studies have found that the frequency of occurrence of certain congenital malformations depends on the sex of the child (table).[7][8][9][10][11] For example, pyloric stenosis occurs more often in males while congenital hip dislocation is four to five times more likely to occur in females. Among children with one kidney, there are approximately twice as many males, whereas among children with three kidneys there are approximately 2.5 times more females. The same pattern is observed among infants with excessive number of ribs, vertebrae, teeth and other organs which in a process of evolution have undergone reduction—among them there are more females. Contrary, among the infants with their scarcity, there are more males. Anencephaly is shown to occur approximately twice as frequently in females.[12] The number of boys born with 6 fingers is two times higher than the number of girls.[13] Now various techniques are available to detect congenital anomalies in fetus before birth.

The sex ratio of patients with congenital malformations
Congenital anomaly Sex ratio, ♂♂:♀♀
Defects with female predominance
Congenital hip dislocation 1 : 5.2;[14] 1 : 5;[15] 1 : 8;[11] 1 : 3.7[16]
Cleft palate 1 : 3[15]
Anencephaly 1 : 1.9;[14] 1 : 2[12]
Craniocele 1 : 1.8[14]
Aplasia of lung 1 : 1.51[14]
Spinal herniation 1 : 1.4[14]
Diverticulum of the esophagus 1 : 1.4[14]
Stomach 1 : 1.4[14]
Neutral defects
Hypoplasia of the tibia and femur 1 : 1.2[14]
Spina bifida 1 : 1.2[16]
Atresia of small intestine 1 : 1[14]
Microcephaly 1.2 : 1[16]
Esophageal atresia 1.3 : 1;[14] 1.5 : 1[16]
Hydrocephalus 1.3 : 1[16]
Defects with male predominance
Diverticula of the colon 1.5 : 1[14]
Atresia of the rectum 1.5 : 1;[14] 2 : 1[16]
Unilateral renal agenesis 2 : 1;[14] 2.1 : 1[16]
Schistocystis 2 : 1[14]
Harelip 2 : 1;[15] 1.47 : 1[16]
Bilateral renal agenesis 2.6 : 1[14]
Congenital anomalies of the genitourinary system 2.7 : 1[11]
Pyloric stenosis, congenital 5 : 1;[15] 5.4 : 1[11]
Meckel's diverticulum More common in boys[14]
Congenital megacolon More common in boys[14]
All defects 1.22 : 1;[17] 1.29 : 1[11]
  • Data[11] obtained on opposite-sex twins. ** — Data[16] were obtained in the period 1983-1994.

P. M. Rajewski and A. L. Sherman (1976) have analyzed the frequency of congenital anomalies in relation to the system of the organism. Prevalence of men was recorded for the anomalies of phylogenetically younger organs and systems.[14]

In respect of an etiology, sexual distinctions can be divided on appearing before and after differentiation of male's gonads in during embryonic development, which begins from eighteenth week. The testosterone level in male embryos thus raises considerably.[18] The subsequent hormonal and physiological distinctions of male and female embryos can explain some sexual differences in frequency of congenital defects. It is difficult to explain the observed differences in the frequency of birth defects between the sexes by the details of the reproductive functions or the influence of environmental and social factors.

Fossil record

Main article: Paleopathology

Evidence for congenital deformities found in the fossil record is studied by paleopathologists, specialists in ancient disease and injury. Fossils bearing evidence of congenital deformity are scientifically significant because they can help scientists infer the evolutionary history of life's developmental processes. For instance, because a Tyrannosaurus rex specimen has been discovered with a block vertebra, it means that vertebrae have been developing the same basic way since at least the most recent common ancestor of dinosaurs and mammals. Other notable fossil deformities include a hatchling specimen of the bird-like dinosaur, Troodon, the tip of whose jaw was twisted.[19] Another notably deformed fossil was a specimen of the choristodere Hyphalosaurus, which had two heads- the oldest known example of polycephaly.[20]

See also

References

External links

  • CDC’s National Center on Birth Defects and Developmental Disabilities
  • Birth Defects- Types and Causes

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