Congenital sucrase-isomaltase deficiency (CSID) is a genetic disorder that affects a person’s ability to digest certain sugars. People with this condition cannot break down the sugars sucrose and maltose, and other compounds made from simple sugar molecules (carbohydrates), which can cause diarrhea, abdominal pain, and other symptoms.

Sucrose (a sugar found in fruits, and also known as table sugar) and maltose (the sugar found in grains) are called disaccharides because they are made of two simple sugars. Disaccharides are broken down into simple sugars during digestion. Sucrose is broken down into glucose and another simple sugar called fructose, and maltose is broken down into two glucose molecules.

Other names for CSID include genetic sucrase-isomaltase deficiency (GSID), congenital sucrose intolerance, congenital sucrose-isomaltose malabsorption, disaccharide intolerance I, SI deficiency, or sucrase-isomaltase deficiency.

Symptoms of CSID

Congenital sucrase-isomaltase deficiency usually becomes apparent after an infant is weaned and starts to consume fruits, juices, and grains and often is diagnosed under the age of 18 months. However, it may sometimes be diagnosed in older children or adults.

After ingestion of sucrose or maltose, an affected person will typically experience watery diarrhea, bloating, excess gas production, abdominal pain ("stomach ache"), and malabsorption of other nutrients. Other symptoms may include nausea, vomiting, or reflux-like symptoms.

In some children, these digestive problems can lead to failure to gain weight and grow at the expected rate (failure to thrive) and malnutrition. Most affected children are better able to tolerate sucrose and maltose as they get older.

Symptoms can vary for a variety of reasons including the timing of introduction of sucrose into a person's diet, and the amount of sugar and starch consumed. Infants who are breast-fed or fed lactose-containing formula will often not have symptoms of CSID until they ingest juices, solid foods, or medications that are sweetened by sucrose.

In some people symptoms may be milder than typically expected. Those with milder symptoms may not be diagnosed until later in childhood or in adulthood. In others with CSID the symptoms may mistakenly be thought due to something else, such as a functional GI disorder like irritable bowel syndrome with diarrhea (IBS-D) or dyspepsia.

How common is CSID

The prevalence of CSID is still unknown and a subject of study and debate. Estimates of prevalence in people of European descent generally range from 1 in 500 to 1 in 2,000, and fewer African Americans are thought to be affected. The condition is much more prevalent in the indigenous populations of Greenland, Alaska, and Canada, where as many as 1 in 10 to 1 in 30 people may be affected.

Nevertheless, more recent studies suggest that CSID may be more common than currently estimated. It is possible that some people remain undiagnosed and that the incidence is higher.

Causes of CSID

Mutations in a gene (the SI gene) cause congenital sucrase-isomaltase deficiency. The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is found in the small intestine and is involved in the digestion of sugar and starch. It is responsible for breaking down sucrose and maltose into their simple sugar components. These simple sugars are then absorbed by the small intestine.

In addition to genetic variations, other factors including dietary, gut motility, and nutritional interactions can affect the severity of symptoms.

Diagnosis of CSID

The standard method of confirming a diagnosis of suspected CSID is with an endoscopic biopsy, a tissue sample, taken from the small intestine for laboratory analysis. A sucrose breath hydrogen test has also been used, which is less invasive but more susceptible to error and can provoke GI symptoms due to the amount of sucrose that must be ingested to perform the study.

Developed more recently, a carbon-13 (13C-sucrose) breath test is noninvasive with a high degree of accuracy and avoids provoking symptoms. A genetic test of more common mutations in the CSID gene offers another less invasive simpler alternative.

In the absence of an early diagnosis, people with CSID may go for years without an accurate diagnosis. Usually they will exhibit normal growth, and often report persistent diarrhea. The diagnosis of CSID may be delayed while more common causes or disorders are investigated or diagnosed.

The newer genetic tests and more accurate noninvasive breath tests may lead to more accurate prevalence studies and diagnosis of less typical cases.

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