Asplenia Health Article

Definition

The term "asplenia" literally means absent spleen. However, in the condition asplenia, the spleen is not always absent. Sometimes the spleen is present, but not fully developed (hypoplastic). In asplenia, the spleen is typically not the only organ affected. Individuals with this condition often have problems with other organs and organ systems. A related condition is polysplenia. The term "polysplenia" literally means multiple spleens. Both of these conditions affect the placement and development of the organs inside the body. There is controversy over whether asplenia and the other syndromes, like polysplenia, that affect the position of the internal organs are actually different aspects of the same condition, referred to as Heterotaxy syndrome, or separate and distinct syndromes. This issue has not been resolved.

Asplenia is just one of the names used to refer to this condition. Other names include Ivemark syndrome, right isomerism sequence, bilateral right-sideness sequence, splenic agenesis syndrome, and asplenia with cardiovascular anomalies.

Description

The human body can be viewed as having a right side and a left side. Normally, inside the human body, the right side and the left side are different with respect to the presence of certain organs. Several organs inside the body are placed asymmetrically, meaning that one organ may be located on one side of the body, but not the other. Furthermore, there are some organs that are found on both sides of the body, but have differences that distinguish the right organ from its partner on the left side. In asplenia, the position, location, appearance, and performance of some of the internal organs are altered. Organs can often be found on the wrong side of the body and/or have structural defects. Furthermore, in most people the right and left organs are different; in people with asplenia, both organs may appear to be structured the same.

Genetic profile

In most families, asplenia is believed to occur sporadically. In other words, it occurs for the first time in a family and has no known or identifiable pattern of inheritance.

There have been several couples described in the medical literature who have more than one child diagnosed with asplenia. In several of these families, the parents were related to each other. Individuals who are related to each other are more likely to carry some of the same non-working genes. Therefore, these families illustrate the possibility that asplenia can be inherited in an autosomal recessive manner. Individuals who have an autosomal recessive condition have both genes in a pair that do not work as expected or are missing, thereby causing the disease. One non-working gene is inherited from the mother and the other is inherited from the father. These parents are called carriers of that condition. When two people are known carriers for an autosomal recessive condition, they have a 25% chance with each pregnancy of having a child affected with the disease.

There are a few families where asplenia appears to be inherited in an autosomal dominant or X-linked manner. In autosomal dominant inheritance, only one gene in the pair needs to be abnormal to cause symptoms of the condition. In families where asplenia appears to be inherited in an autosomal dominant manner, family members who carry the same non-working gene can have different symptoms and the severity of the condition may vary. In autosomal dominant inheritance, if an individual carries the non-working gene, he or she has a 50% chance of passing the gene on with each pregnancy.

In families where asplenia appears to be inherited in a X-linked manner, the gene causing the condition is located on the X chromosome. Since women have two X chromosomes, if a woman inherits the nonworking gene on one of her X chromosomes, typically she will not have any symptoms of asplenia or will have a milder form of the condition. A woman who carries the X-linked form of asplenia will have a 50% chance of passing that non-working gene on with each pregnancy.

Since men tend to have one Y chromosome and one X chromosome, if it is a son that inherits the non-working gene, he will be affected with the condition. Men who have a X-linked form of asplenia will always pass their X chromosome containing the non-working gene on to all of their daughters, who would be carriers of the condition. In these families, asplenia will never be passed from the father to the son, since men give their sons a Y chromosome. If a woman who carries a X-linked condition passes the X chromosome containing the non-working gene to a daughter, then that daughter will be a carrier like her mother.

The pattern of inheritance of asplenia in a family is usually not obvious when there is only one individual diagnosed with the condition. Based on the families and studies performed on asplenia, the chance of a couple who have one child with asplenia having another child with the condition is approximately 5% or less. This chance may be higher if it is determined that asplenia is part of Heterotaxy syndrome, since there are a wider range of symptoms associated with that condition. Furthermore, if more than one family member has the diagnosis of asplenia, the chance of it occurring again in the family is based on the pattern of inheritance that the condition appears to be following.

Since asplenia appears to be inherited in different ways, it is theorized that there may be several different genes that could cause asplenia. This means that some families may have asplenia caused by one specific nonworking gene, but in other families, a different nonworking gene could cause the same condition to occur. The exact genes involved in causing asplenia have not been identified. However, there is ongoing research to identify the genes involved with this condition.