A 10-month-old infant is noted by his mother to have had more frequent, watery stools over the past 2 days. On examination vital signs include T 37.2 C, P 103/min, RR 12/min, and BP 85/45 mm Hg. The child is listless and has poor skin turgor. The child is at the 45th percentile for weight and the 55th percentile for height. No congenital abnormalities are noted. Laboratory studies show Hgb 16.2 g/dL, Hct 49.2%, MCV 95 fL, platelet count 188,700/microliter, and WBC count 7770/microliter. Serum electrolytes include sodium 159 mmol/L, potassium 3.6 mmol/L, chloride 125 mmol/L, CO2 18 mmol/L, glucose 62 mg/dL, creatinine 0.4 mg/dL, and bilirubin 0.7 mg/dL.

Questions:

6.1 What is suggested by these findings?

The diarrhea has led to dehydration. This can quickly become life-threatening in a child, as in this case in which there is marked hypernatremia.

6.2 What are possible etiologies for this condition?

Infantile diarrhea is often infectious. A watery diarrhea suggests a viral cause, not a bacillary dysentery. Most cases of gastroenteritis in infants and children are due to viruses. Rotavirus, calciviruses, astroviruses, and Norwalk virus.

6.3 What therapies can be employed?

Electrolyte and fluid replacement with commercial preparations such as Pedialyte ® or Lytren ® or the WHO formula are given.

6.4 Describe the physiology of gastrointestinal water and electrolyte absorption in adults.

About 2L of fluid are ingested each day. In the process of digestion, about 7 L of fluid is secreted via salivary glands (1 L), the stomach (2 L), pancreas (2 L), biliary tract (0.5 L), and small intestine (1.5 L). Of this 9 L, 98% is absorbed, leaving only about 200 mL in the stool. Most of the water is reabsorbed in the jejunum (5.5 L), with ileum accounting for less (2 L). Though the colon reabsorbs only 1.3 L, this is where the stool is concentrated, since there is no significant secretion into the colon.

The Na+K+2Cl- cotransporter can move sodium, potassium, and chloride into enterocytes. There is an Na+K+ATPase and a Na+/glucose cotransporter in enterocytes. Chloride can be actively secreted via chloride channels with activation by cyclic AMP.

Ordinarily, electrolytes and water are absorbed from the intestine until the lumenal osmolality is similar to plasma.

Thus, pathologic processes interfering with small intestinal absorption often lead to a "high volume" diarrhea, while diseases affecting the colon lead to a "low volume" diarrhea.

Lactase deficiency leads to increased lumenal lactose, which takes water with it, producing an "osmotic" diarrhea.

Cholera toxin can bind to an enterocyte receptor that leads to stimulation of cyclic AMP production, pumping out chloride, followed by sodium and water, leading to a severe watery diarrhea, a form of "secretory" diarrhea. Treatment with solutions of sodium chloride and glucose bypass this process and make use of the Na+K+ATPase and the Na+/glucose cotransporter.

Magnesium is poorly absorbed, so that administration of Epsom salts (MgSO4), or milk of magnesia (MgCOH2), leads to an osmotic catharsis.

Calcium is better absorbed. Both calcium and magnesium absorption is promoted by dietary protein.

6.5 Describe the acid-base disturbances that result from: 1) prolonged vomiting; 2) prolonged diarrhea.

Prolonged vomiting leads to loss of chloride ion (HCl from the stomach) with extracellular fluid volume depletion, increased renin, increased aldosterone, potassium depletion and retention of bicarbonate. This leads to metabolic alkalosis. Acid-base status would be reflected as follows:

Prolonged diarrhea leads to loss of bicarbonate from the lower GI tract with extracellular fluid volume depletion, potassium depletion, and hyperchloremia.