Chapter 25: Gastric, Functional & Inflammatory Bowel Disorders

Gastric, Functional & Inflammatory Bowel Disorders details the pathophysiology, diagnostic approach, and pharmacological management of gastric, functional, and inflammatory bowel disorders. Nausea and vomiting (N/V) are common complaints stemming from the activation of the medullary emetic complex (EC), which receives noxious stimuli from multiple sensory centers including the chemoreceptor trigger zone (CTZ) and the vestibular system. Treatment for N/V aims to alleviate subjective discomfort and objective vomiting while preventing complications like dehydration and metabolic abnormalities. Pharmacologic agents are selected based on the etiology, targeting specific neurotransmitter receptors (dopamine, serotonin, histamine, acetylcholine) found in the vomiting pathways, utilizing classes such as phenothiazines, serotonin antagonists (5-HT3 blockers), antihistamines/anticholinergics, benzodiazepines, metoclopramide, corticosteroids, and cannabinoids. Functional bowel disorders, particularly Chronic Idiopathic Constipation (CIC) and Irritable Bowel Syndrome (IBS), along with acute diarrhea, are addressed through tailored therapy focusing first on lifestyle changes, such as modifying diet and increasing fiber intake. Constipation treatment utilizes various laxative classes—bulk-forming, hyperosmotic, saline, stimulant, and surfactant agents—to increase fecal water content and promote comfortable defecation. For CIC or Opioid-Induced Constipation (OIC), newer secretagogues like lubiprostone, guanylate cyclase-C agonists (linaclotide), or peripherally acting mu-opioid receptor antagonists (naloxegol) may be employed. Diarrhea management requires prompt rehydration and identifying the cause, often relying on antimotility agents (loperamide), adsorbents, or antibiotics (rifaximin). IBS, defined by the Rome IV criteria based on recurrent abdominal pain related to defecation and changes in stool form, is managed according to its subtype (IBS-C or IBS-D), incorporating osmotic laxatives, antidiarrheals, antispasmodics (e.g., dicyclomine), or low-dose antidepressants. Finally, Inflammatory Bowel Disease (IBD), encompassing Crohn’s disease (CD) and Ulcerative Colitis (UC), is treated based on the location and severity of inflammation. Initial IBD therapy involves aminosalicylates, progressing to corticosteroids for acute exacerbations, immunosuppressive agents (azathioprine, 6-mercaptopurine) for maintenance, and biological agents (TNF-alpha inhibitors, selective adhesion molecule inhibitors, ustekinumab) for severe or refractory disease. Throughout the management of all these GI disorders, meticulous monitoring for drug toxicities, adjusting therapy for special populations (pediatric, geriatric, pregnant women), and integrating nonpharmacologic interventions are crucial components of patient care.