Imperforate anus is a rare inborn abnormality characterized by the absence or abnormal localization of the anus. The rectum or the colon may be connected to the vagina or the bladder by a tunnel (fistula). With surgical correction, normal elimination can become possible.
Imperforate anus is an abnormality present at birth, and characterized by the absence of the normal opening of the anus. Elimination of feces may not be possible until surgery is performed. In some cases the rectum opens into the lower part of the vagina in females, or close to the scrotum in males.
Imperforate anus is a birth defect that usually appears to occur randomly for unknown reasons (sporadically). Less commonly, the condition may be familial, suggesting autosomal dominant, autosomal recessive, or X-linked recessive inheritance. In addition, some researchers suggest that the condition may be caused by changes (mutations) of one or more genes, possibly in association with particular environmental factors (multifactorial inheritance).
Human traits, including the classic genetic diseases, are the product of the interaction of two genes for that condition, one received from the father and one from the mother.
In autosomal dominant disorders, a single copy of the disease gene (received from either the mother or father) will be expressed “dominating” the other normal gene and resulting in the appearance of the disease. The risk of transmitting the disorder from affected parent to offspring is 50 percent for each pregnancy regardless of the sex of the resulting child. The risk is the same for each pregnancy.
In autosomal recessive disorders, the condition does not appear unless a person inherits the same defective gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease but usually will not show symptoms. The risk of transmitting the disease to the children of a couple, both of whom are carriers for a recessive disorder, is 25 percent. Fifty percent of their children risk being carriers of the disease but generally will not show symptoms of the disorder. Twenty-five percent of their children may receive both normal genes, one from each parent, and will be genetically normal (for that particular trait). The risk is the same for each pregnancy.
X-linked recessive disorders are conditions that are coded on the X chromosome. Females have two X chromosomes, but males have one X chromosome and one Y chromosome. Therefore, in females, disease traits on the X chromosome may be “masked” by the normal gene on the other X chromosome (random X chromosome inactivation). Since males only have one X chromosome, if they inherit a gene for a disease present on the X, it will be expressed. Men with X-linked disorders transmit the gene to all their daughters, who are carriers, but never to their sons. Women who are carriers of an X-linked disorder have a 50 percent risk of transmitting the carrier condition to their daughters and a 50 percent risk of transmitting the disease to their sons. Thus, in summary, when a disorder is inherited as an X-linked recessive trait, the condition is usually fully expressed in males only.
In some cases, imperforate anus occurs as part of a malformation syndrome, such as VACTERL association, a rare disorder that may be characterized by a spectrum of birth defects, including anal, skeletal, kidney (renal), heart (cardiac), and/or other abnormalities. (For further information on this disorder, please choose “VACTERL” as your search term in the Rare Disease Database.)
Imperforate anus and other related abnormalities of the anus and rectum (anorectal malformations) occur in approximately one in 4,000 to 5,000 newborns in the United States. Reported instances of imperforate anus include affected individuals in whom the condition appeared to occur sporadically, members of certain multigenerational families (kindreds), and individuals in whom the condition occurred in association with other birth defects or malformation syndromes (e.g., VACTERL association).
Imperforate anus is surgically corrected by dilating, enlarging or repositioning the external opening, or other ways of providing an adequate rectal opening. Genetic counseling may be of benefit to patients and their families.
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Analysis of 1,992 Patients with Anorectal Malformations over the Past Two Decades in Japan. Steering Committee of Japanese Study Group of Anorectal Anomalies. E. Endo et al.; J Pediatr Surg (Mar 1999; 34(3)). Pp. 435-41.
FROM THE INTERNET
Online Mendelian Inheritance in Man (OMIM). Victor A. McKusick, Editor; Johns Hopkins University. Entry Number 301800, Last Edit Date 4/25/94; Entry Number 207500, Last Edit Date 11/12/95; Entry Number 107100, Last Edit Date 11/12/97.
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