Congenital disorder

A congenital disorder is a medical condition that is present at birth. (It is contrasted with the term "acquired disorder") A congenital disorder can be recognized before birth (prenatally), at birth, or many years later. Congenital disorders can be a result of genetic abnormalities, the intrauterine environment, or unknown factors. A congenital condition can arise from the genetic make-up of the fertilized egg or be acquired at any time during fetal development. The causes of many congenital disorders are not known.

Congenital disorders include minor physical anomalies (e.g., a birthmark), severe malformations of single systems (e.g., congenital heart disease or amelia of the legs), and combinations of abnormalities affecting several parts of the body. Defects of metabolism are also considered congenital disorders. A congenital disorder can have trivial or grave effects. The most severe, such as anencephaly, are incompatible with life. Birth defects are a leading cause of death in early infancy, accounting for the deaths of nearly 2 out every 1000 infants (US statistics).

About 2% to 3% of babies are born with significant congenital birth defects. Birth defects involving the brain are the largest group at 10 per 1000 live births), compared to heart at 8 per 1000, kidneys at 4 per 1000, and limbs at 2 per 1000. All other defects have a combined incidence of 6 per 1000 live births. Birth defects of the heart are the most common birth defect leading to death in infancy, accounting for 28% of infant deaths due to birth defects, while chromosomal abnormalities and respiratory abnormalities each account for 15%, and brain defects about 12%.

About 25% of birth defects result from genetic abnormalities. About 5% involve abnormalities of large portions or entire chromosomes. Some congenital disorders arise from inheritance of abnormal genes from the parents, but some can arise from a new mutation in one of the germ cells that contributed to the fetus.

Influences of the intrauterine environment of the fetus that can cause birth defects are referred to as teratogenic. These can include dietary deficiencies, toxic effects, or infections. For example, dietary deficiency of maternal folic acid is associated with spina bifida. Exposure to harmful substances during pregnancy (e.g, mercury) or maternal drugs (e.g., phenytoin) can cause recognizable combinations of birth defects. Many infections which a mother can contract during pregnancy, such as rubella and syphilis, can also affect the baby.

A significant proportion of birth defects cannot be attributed to either genetic abnormalities or environmental exposures. These are referred to as sporadic birth defects, a term that implies an unknown cause, random occurrence, and a low recurrence risk for future children.

Terminology
Congenital disorders is a broad category that includes a variety of conditions. The following terms are used for various subsets of congenital disorders:


 * Birth defect is a widely-used term for a structural malformation of a body part, recognizable at birth, which is significant enough to be considered a problem.


 * A congenital physical anomaly is a difference, 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), 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.


 * A congenital malformation is a deleterious physical anomaly, a structural defect perceived as a problem. A recognizable combination of malformations or problems affecting more than one body part is referred to as a malformation syndrome.


 * Genetic diseases or disorders 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.

Congenital conditions can be referred to as diseases, defects, disorders, anomalies, or simply genetic differences. The usage overlaps, but also involves a value judgement as to the harmfulness of the condition. In particular, people may disagree as to whether a specific condition should be considered a birth defect or a minor congenital anomaly. See human variability and disease for more on the occasional difficulties of drawing these distinctions.
 * 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.

Risk of birth defects throughout gestation
The greatest risk of a malformation occurring is between the third and eighth week of gestation. Before this time, any damage to the embryo is likely to result in fatality, and the baby will not be born. After eight weeks, the fetus and its organs are more developed and less sensitive to teratogenic incidents.

The type of birth defect is also related to the time of exposure to a teratogen. For instance the heart is susceptible from three to eight weeks, but the ear is susceptible from a slightly later time to about twelve weeks.

Infection in the mother early in the third week may damage the heart. An infection in the eleventh week is less likely to damage the heart, but the baby may be born deaf.

Reference

 * The National Library of Medicine or MEDLINE/PubMed MeSH (medical subject heading) term