A 28-year-old man comes to the office due to chronic diarrhea. He reports having 5 or 6 nonbloody, liquid bowel movements daily that also sometimes awaken him at night. The patient has lost 2 kg (4.4 lb) and says he has diarrhea even when not eating. He has occasional bloating sensations but no abdominal pain, nausea, or vomiting. Several years ago, the patient was robbed at gunpoint and shot in the abdomen. He underwent multiple surgeries and developed posttraumatic stress disorder. Temperature is 37.1 C (98.8 F), blood pressure is 118/70 mm Hg, and pulse is 78/min. BMI is 20 kg/m². The abdomen is soft, nondistended, and nontender. Bowel sounds are normoactive in all 4 quadrants. On rectal examination, brown stool is present. Occult blood test is negative. Serum electrolytes are normal. The stool osmotic gap is low. Which of the following is the most likely mechanism of this patient's diarrhea?

Question

Quizplus · Accepted Answer

Explanation: This patient with a history of multiple abdominal surgeries has chronic, nonbloody diarrhea that occurs even during times of fasting. In association with the low stool osmotic gap, this presentation suggests secretory diarrhea due to unabsorbed bile acids. Stool is normally nearly isoosmolar with the serum (usually ~290 mOsm/kg). Therefore, the stool osmotic gap (SOG), which is normally between 50 and 125 mOsm/kg, can help determine the etiology of watery diarrhea. stool osmotic gap (SOG) Osmotic diarrhea occurs due to the presence of a nonabsorbed, osmotically active solute (eg, polyethylene glycol, sorbitol, lactose), which inhibits water resorption and results in a high SOG (>125 mOsm/kg). The diarrhea occurs after ingestion of the causative substance (eg, milk in a patient with lactose intolerance) and does not occur during fasting (Choice D). Secretory diarrhea occurs due to toxins (eg, produced by Vibrio cholerae), hormones (eg, produced by VIPomas), congenital disorders of ion transport (eg, cystic fibrosis), or bile acids (in postsurgical patients). It is caused by secretion of electrolytes and water into the intestine, resulting in a low SOG (<50 mOsm/kg). The diarrhea is typically large in volume and persists while fasting and at night. Secretory diarrhea can occur after bowel resection or cholecystectomy, when unabsorbed bile acids reach the colon and result in the direct stimulation of luminal ion channels. Resection of the ileocecal area additionally reduces the ability of the intestines to actively absorb sodium ions against the electrochemical gradient. (Choice A) Factitious diarrhea (eg, due to laxative abuse) is typically associated with psychiatric disorders and is most commonly seen (>90% of cases) in women with a history of health care work. In addition, it is often associated with signs of volume depletion (eg, lightheadedness, tachycardia, hypotension) and enhanced gastrointestinal mobility (eg, hyperactive bowel sounds), which are not seen in this patient. Depending on the laxative abused, the SOG can be high (eg, lactulose) or low (eg, senna). (Choice B) Inflammatory diarrhea is usually accompanied by either grossly bloody stools or positive occult blood testing. Systemic symptoms (eg, fever, fatigue) are also typical. (Choice C) Intestinal dysmotility disorders (eg, chronic intestinal pseudoobstruction) are often associated with nausea and vomiting. Diarrhea during fasting is unexpected. Educational objective: Secretory diarrhea is characterized by watery diarrhea that occurs even during fasting or sleep. Common etiologies include toxins (eg, produced by Vibrio cholerae), hormones (eg, produced by VIPomas), congenital disorders of ion transport (eg, cystic fibrosis), or unabsorbed bile acids (eg, due to postsurgical changes). A low stool osmotic gap (<50 mOsm/kg) is typical. References: Evaluation of chronic diarrhea. (http://www.ncbi.nlm.nih.gov/pubmed/22085666) Secretory diarrhoea: mechanisms and emerging therapies. (http://www.ncbi.nlm.nih.gov/pubmed/26122478)

A 28-Year-Old Man Comes to the Office Due to Chronic