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Congenital enzyme defect resulting in chronic diarrhea and failure to thrive.

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Congenital Sucrase-Isomaltase Deficiency; Disaccharidase Deficiency Type I.

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Varies with ethnic group (whites: 1:2500, native Alaskans: 1:33, native Canadians: 4-7:100).

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Autosomal recessive. The gene encoding for sucrase-isomaltase deficiency is localized on the long arm of chromosome 3.

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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.

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Obtain a full history of gastrointestinal symptoms (severity of diarrhea and degree of malnutrition) causing failure to thrive. In rare cases, TPN is required. Evaluate volume status clinically and check electrolytes, blood urea nitrogen, and creatinine. If the patient is on TPN, also check serum concentrations of albumin, glucose, phosphate, calcium, magnesium, transaminases, alkaline phosphatase, and bilirubin. Optimize volume status preoperatively by intravenous rehydration.

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Consider repeated intraoperative blood glucose monitoring if the patient is on TPN.

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In the presence of severe malnutrition (rare) or depressed renal and/or hepatic function secondary to complications from TPN, highly protein-bound drugs must be titrated carefully secondary to low protein levels (in particular α1-acid glycoprotein and albumin), which may result in decreased drug binding and increased free drug levels.

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