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Congenital enzyme defects resulting in chronic diarrhea and failure to thrive. Affected infants develop symptoms soon after ingesting sucrose, either from modified milk formulas, fruits, or starches. The clinical presentation consists of explosive, watery diarrhea, dehydration, abdominal distension, and pain.
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Congenital Sucrase-Isomaltase Deficiency; CSID; Disaccharidase Deficiency Type I; Congenital Sucrose Intolerance.
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Varies with ethnic group. It affects males and females in equal numbers. The prevalence in North America among Caucasians is approximately 1:2500 live births in the general population. It is well established that there is a higher frequency among Inuit populations from Greenland and Canada where the incidence is reported at 10%. Native from Alaska have an incidence of 1:33 people.
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It is an autosomal recessive disorder. The gene encoding for sucrase-isomaltase deficiency is localized on the long arm of chromosome 3 (3q25-q26).
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The absence or severe reduction in sucrase and isomaltase activity in the brush-border membrane of the small intestine is responsible for malabsorption of dietary disaccharides and starch. When these carbohydrates are introduced into the diet, they generate an osmotic pressure gradient in the intestinal lumen, attracting large volumes of isotonic fluid with normal sodium concentration. The capacity for colonic bacteria to ferment malabsorbed carbohydrates is rapidly overwhelmed, and osmotic diarrhea ensues.
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Starch is a mixture of the two polysaccharides, amylopectin and amylose. Hydrolysis of amylopectin and amylose yields mainly maltose, maltotriose, and glucose. Clinical presentation, oral tolerance test with the corresponding disaccharides, sucrose breath hydrogen test, differential urinary disaccharide excretion, and measurement of intestinal disaccharidase activity in a small intestine biopsy lead to the diagnosis. Disaccharidase deficiency is defined as an enzyme activity of at least two standard deviations below the normal mean value. A jejunal biopsy is the gold standard for the diagnosis because the disaccharidase levels are normally the highest here; however, the specimen is difficult to obtain. Furthermore, the circadian rhythm of enzyme activity must be considered.
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The clinical presentation varies and depends on the introduction of sucrose and starch to the diet. Once the diet ceases to consist exclusively of breast milk and lactose-free formulas, the introduction of sucrose-containing juices, solid food, or even medications causes chronic osmotic fermentative diarrhea and, occasionally, failure to thrive. Decreased duodenal-ileal transit time may compromise fat absorption. Empiric avoidance of dietary sucrose load may delay the diagnosis up to the toddler age (children show relatively normal growth but suffer from intermittent diarrhea with meteorism and abdominal cramps). A minority of patients requires hospitalization for severe dehydration, malnutrition, and muscle wasting. Lifelong elimination of sucrose and reduction of starch content in the diet leads to complete recovery.