Causes of Chronic Diarrhea
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《新英格兰医药杂志》
Diarrhea can be classified in several ways. It is both a symptom and a sign. As a symptom, it is whatever the patient says it is: a decrease in consistency, an increase in the number or volume of bowel movements, or any combination thereof. As a sign, diarrhea is an increase in stool weight (or volume) of more than 200 g (or ml) per 24 hours in a person on a Western diet. The distinction between chronic and acute diarrhea is arbitrary: it is determined by duration. Diarrhea is generally considered acute when it lasts less than two or three weeks. Such diarrhea is frequently caused by an infectious agent, is usually self-limiting, and often resolves without treatment. Diarrhea is categorized as chronic when it lasts more than two or three weeks. Chronic diarrhea has multiple causes. During the past three decades, studies have delineated several ion-transport mechanisms that may be disturbed in one or more diarrheal disorders. The identification of five specific congenital diarrheal disorders (see table) has confirmed the importance of certain ion-transport mechanisms in health and disease. The report by Wang et al. in this issue of the Journal (pages 270–280) implicates a new cause of congenital diarrheal disorders: the absence of enteric hormones.
Congenital Diarrheal Disorders.
Diarrhea is often classified as osmotic or secretory.1 It is considered osmotic if luminal substances are responsible for the induction of the fluid secretion, and it is considered secretory if endogenous substances (often referred to as "secretagogues") induce fluid secretion that persists even when the patient is fasting. Textbooks frequently state that one can distinguish osmotic from secretory diarrhea by determining the electrolyte concentration in the stool: if there is an osmotic gap — that is, a substantial difference between the stool osmolality and twice the concentrations of sodium and potassium in the stool — the diarrhea is osmotic, and if not, it is secretory. The assessment of stool electrolytes, however, is more useful for teaching purposes than at the bedside, because data on what constitutes a substantial osmotic gap are scant. Moreover, many diarrheal disorders, such as celiac sprue, have more than one pathogenic mechanism, some of which may be secretory and others of which may be osmotic. I find it more helpful to assess the effect of a fast on stool output: when diarrhea ceases with fasting, a dietary nutrient is likely to be the cause; if diarrhea persists unabatedly with fasting, a dietary nutrient is not likely to be the cause.
In the former case, malabsorption of carbohydrates, fats, or both is probably to blame. The most common cause of carbohydrate malabsorption is lactose intolerance, secondary to primary lactase deficiency or lactase nonpersistence. In the absence of lactase deficiency, there are several uncommon defects in carbohydrate absorption — including sucrase–isomaltase deficiency — that may be responsible. In the absence of carbohydrate malabsorption in a patient with osmotic diarrhea, it is essential to determine whether steatorrhea is present. Although diarrhea alone may be responsible for an increase in fat excretion of up to 11 g per day (normally, less than 7 g of fat per day is excreted by persons consuming 100 g of fat per day), greater rates of fat malabsorption can be explained only by one or more defects in fat digestion and absorption, which consists of five well-characterized steps.2 Successful therapy will depend on the identification of the specific defect or defects in fat digestion and absorption. For example, the administration of pancreatic enzymes will reduce steatorrhea in patients with chronic pancreatitis or cystic fibrosis whose steatorrhea results from the reduced hydrolysis of dietary lipids by pancreatic lipase.
Central to almost all diarrheal disorders is the induction of fluid and electrolyte secretion in one or more segments of the small intestine, the large intestine, or both. In secretory diarrhea, secretagogues affect ion transport in the intestine both by stimulating chloride secretion through the activation of the cystic fibrosis transmembrane regulator and by inhibiting sodium and chloride absorption. In steatorrhea, diarrhea is caused by the induction of fluid and electrolyte secretion in the colon by nonabsorbed fatty acids.
What are the mechanisms underlying the diarrhea of the patients described by Wang et al.? These patients have mutant neurogenin-3, a transcription factor that is pivotal to the development of the pancreatic beta cell and, it would now seem, the enteroendocrine cell. The salient observations include the facts that diarrhea ceased when the patients fasted and was induced when anything other than water was ingested; that the mucosa of the small intestine was essentially normal, except for the absence of enteroendocrine cells; and that there was no inflammatory component. Although diarrhea was induced by orally administered glucose, it is not known whether a glucose-tolerance test would have disclosed normal glucose absorption.
The absence of intestinal endocrine cells and enteric hormones is likely to be central to the diarrhea in these patients; enteric hormones may increase sodium and chloride absorption or induce anion secretion. Hitherto, there has been little to implicate enteric hormones in causing diarrhea: a single patient with autoimmune polyglandular syndrome type I had a defect in cholecystokinin that presumably led to reduced pancreatic-enzyme secretion and steatorrhea.3 A similar mechanism, however, could not explain the diarrhea of the patients described by Wang et al., which persisted even in the absence of dietary fat.
Overall intestinal function involves the coordinated interaction of intestinal ion transport and motor activity, which are functionally linked through the enteric nervous system. Enteric hormones would presumably affect ion transport through a paracrine mechanism, either directly — by regulating absorptive processes, secretory processes, or both in intestinal epithelial cells (see figure) — or indirectly, through the enteric nervous system, which is regulated by several enteric peptides and neurotransmitters. The mechanisms that are responsible for diarrhea in patients with mutant neurogenin-3 remain unclear; their delineation should shed welcome light on the physiology of enteric hormones.
Regulation of Absorptive and Secretory Processes in the Intestine.
Epithelial cells in the small intestine originate in the crypt, have a spatial distribution along the crypt–villous axis, and then migrate to the tip of the villus, where they slough into the lumen. Secretory processes generally occur in crypt cells, whereas absorptive processes are located in villous cells. Enteroendocrine cells are sparse but are present in the crypt. Almost all diarrheal disorders are associated with net fluid secretion. This secretion is most often secondary to the stimulation of active chloride secretion and to the inhibition of active absorption of sodium and chloride (by messengers such as cyclic AMP) which involves the coupling of sodium–hydrogen exchange and chloride–bicarbonate exchange. Absorptive and secretory processes are regulated by both the enteric nervous system and enteric hormones, although little is known about the role of enteric hormones in vivo.
Source Information
Dr. Binder is a professor of medicine and cellular and molecular physiology at Yale University School of Medicine, New Haven, Conn.
References
Schiller LR, Sellin JH. Diarrhea. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's gastrointestinal and liver disease: pathophysiology, diagnosis, management. 8th ed. Philadelphia: Saunders/Elsevier (in press).
Binder HJ. Disorders of absorption. In: Kasper DL, Braunwald E, Fauci A, Hauser S, Longo D, Jameson L, eds. Harrison's principles of internal medicine. 16th ed. New York: McGraw-Hill, 2004:1763-76.
H?genauer C, Meyer RL, Netto GJ, et al. Malabsorption due to cholecystokinin deficiency in a patient with autoimmune polyglandular syndrome type I. N Engl J Med 2001;344:270-274.(Henry J. Binder, M.D.)
Congenital Diarrheal Disorders.
Diarrhea is often classified as osmotic or secretory.1 It is considered osmotic if luminal substances are responsible for the induction of the fluid secretion, and it is considered secretory if endogenous substances (often referred to as "secretagogues") induce fluid secretion that persists even when the patient is fasting. Textbooks frequently state that one can distinguish osmotic from secretory diarrhea by determining the electrolyte concentration in the stool: if there is an osmotic gap — that is, a substantial difference between the stool osmolality and twice the concentrations of sodium and potassium in the stool — the diarrhea is osmotic, and if not, it is secretory. The assessment of stool electrolytes, however, is more useful for teaching purposes than at the bedside, because data on what constitutes a substantial osmotic gap are scant. Moreover, many diarrheal disorders, such as celiac sprue, have more than one pathogenic mechanism, some of which may be secretory and others of which may be osmotic. I find it more helpful to assess the effect of a fast on stool output: when diarrhea ceases with fasting, a dietary nutrient is likely to be the cause; if diarrhea persists unabatedly with fasting, a dietary nutrient is not likely to be the cause.
In the former case, malabsorption of carbohydrates, fats, or both is probably to blame. The most common cause of carbohydrate malabsorption is lactose intolerance, secondary to primary lactase deficiency or lactase nonpersistence. In the absence of lactase deficiency, there are several uncommon defects in carbohydrate absorption — including sucrase–isomaltase deficiency — that may be responsible. In the absence of carbohydrate malabsorption in a patient with osmotic diarrhea, it is essential to determine whether steatorrhea is present. Although diarrhea alone may be responsible for an increase in fat excretion of up to 11 g per day (normally, less than 7 g of fat per day is excreted by persons consuming 100 g of fat per day), greater rates of fat malabsorption can be explained only by one or more defects in fat digestion and absorption, which consists of five well-characterized steps.2 Successful therapy will depend on the identification of the specific defect or defects in fat digestion and absorption. For example, the administration of pancreatic enzymes will reduce steatorrhea in patients with chronic pancreatitis or cystic fibrosis whose steatorrhea results from the reduced hydrolysis of dietary lipids by pancreatic lipase.
Central to almost all diarrheal disorders is the induction of fluid and electrolyte secretion in one or more segments of the small intestine, the large intestine, or both. In secretory diarrhea, secretagogues affect ion transport in the intestine both by stimulating chloride secretion through the activation of the cystic fibrosis transmembrane regulator and by inhibiting sodium and chloride absorption. In steatorrhea, diarrhea is caused by the induction of fluid and electrolyte secretion in the colon by nonabsorbed fatty acids.
What are the mechanisms underlying the diarrhea of the patients described by Wang et al.? These patients have mutant neurogenin-3, a transcription factor that is pivotal to the development of the pancreatic beta cell and, it would now seem, the enteroendocrine cell. The salient observations include the facts that diarrhea ceased when the patients fasted and was induced when anything other than water was ingested; that the mucosa of the small intestine was essentially normal, except for the absence of enteroendocrine cells; and that there was no inflammatory component. Although diarrhea was induced by orally administered glucose, it is not known whether a glucose-tolerance test would have disclosed normal glucose absorption.
The absence of intestinal endocrine cells and enteric hormones is likely to be central to the diarrhea in these patients; enteric hormones may increase sodium and chloride absorption or induce anion secretion. Hitherto, there has been little to implicate enteric hormones in causing diarrhea: a single patient with autoimmune polyglandular syndrome type I had a defect in cholecystokinin that presumably led to reduced pancreatic-enzyme secretion and steatorrhea.3 A similar mechanism, however, could not explain the diarrhea of the patients described by Wang et al., which persisted even in the absence of dietary fat.
Overall intestinal function involves the coordinated interaction of intestinal ion transport and motor activity, which are functionally linked through the enteric nervous system. Enteric hormones would presumably affect ion transport through a paracrine mechanism, either directly — by regulating absorptive processes, secretory processes, or both in intestinal epithelial cells (see figure) — or indirectly, through the enteric nervous system, which is regulated by several enteric peptides and neurotransmitters. The mechanisms that are responsible for diarrhea in patients with mutant neurogenin-3 remain unclear; their delineation should shed welcome light on the physiology of enteric hormones.
Regulation of Absorptive and Secretory Processes in the Intestine.
Epithelial cells in the small intestine originate in the crypt, have a spatial distribution along the crypt–villous axis, and then migrate to the tip of the villus, where they slough into the lumen. Secretory processes generally occur in crypt cells, whereas absorptive processes are located in villous cells. Enteroendocrine cells are sparse but are present in the crypt. Almost all diarrheal disorders are associated with net fluid secretion. This secretion is most often secondary to the stimulation of active chloride secretion and to the inhibition of active absorption of sodium and chloride (by messengers such as cyclic AMP) which involves the coupling of sodium–hydrogen exchange and chloride–bicarbonate exchange. Absorptive and secretory processes are regulated by both the enteric nervous system and enteric hormones, although little is known about the role of enteric hormones in vivo.
Source Information
Dr. Binder is a professor of medicine and cellular and molecular physiology at Yale University School of Medicine, New Haven, Conn.
References
Schiller LR, Sellin JH. Diarrhea. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's gastrointestinal and liver disease: pathophysiology, diagnosis, management. 8th ed. Philadelphia: Saunders/Elsevier (in press).
Binder HJ. Disorders of absorption. In: Kasper DL, Braunwald E, Fauci A, Hauser S, Longo D, Jameson L, eds. Harrison's principles of internal medicine. 16th ed. New York: McGraw-Hill, 2004:1763-76.
H?genauer C, Meyer RL, Netto GJ, et al. Malabsorption due to cholecystokinin deficiency in a patient with autoimmune polyglandular syndrome type I. N Engl J Med 2001;344:270-274.(Henry J. Binder, M.D.)