Congenital Sucrase-Isomaltase Deficiency (CSID) has been classically defined as an autosomal recessive disease of the small intestine characterized by a failure to absorb sucrose. According to the United States National Institutes of Health, an autosomal recessive disorder is one in which “two copies of an abnormal gene must be present in order for the disease…to develop.” CSID was originally defined by undetectable sucrase activity, a decrease of maltase activity to up to nearly one third of the normal level, and a varying degree of isomaltase activity. All CSID patients lack functional sucrase activity, but the degree of isomaltase activity may vary, suggesting that the disease is not a consequence of a complete lack of SI gene expression.
The term “congenital” means present at birth, and most congenital disorders are discovered in infancy or early childhood. In recent years, some patients with CSID have been diagnosed in their teens or adulthood. Because CSID shares the same symptoms as many other gastrointestinal disorders, such patients may have been misdiagnosed simply because the testing for CSID was not conducted. Since the term “congenital” points to an early childhood diagnosis, it may be more precise to call a disorder involving deficient sucrase activity Genetic Sucrase-Isomaltase Deficiency. This is particularly true since commercial genetic testing is now available to confirm the presence of Sucrase-Isomaltase mutations or variants leading to Genetic Sucrase-Isomaltase Deficiency.
As such, the term Genetic Sucrase-Isomaltase Deficiency is inclusive of patients who are diagnosed in infancy, in childhood, in teenage years, or in adulthood. This term also includes the many genetic variants or mutations that have been found to cause dysfunctional sucrase-isomaltase activity. Recent research also suggests that some patients with only one genetic mutation or variant have significant symptoms.
Therefore, the understanding of the science of Congenital Sucrase-Isomaltase Deficiency (CSID) is evolving. Early studies suggested that single mutations or variants inherited from both mother and father cause CSID. This is known as homozygous inheritance. More recent studies have shown that a child could inherit a mutation from one parent and a different mutation from another parent and still display dysfunctional sucrase activity. This is compound heterozygous inheritance. The most recent scientific studies seem to indicate that a patient can have only one mutation or variant and display symptoms of sucrase insufficiency. This evolution of understanding indicates that sucrase deficiency of genetic origin may be more common than previously thought.
Since the science is evolving, it may be useful to use a more general, all-encompassing term such as Genetic Sucrase-Isomaltase Deficiency.