Clinical meaning
Microscopic colitis is a chronic inflammatory bowel disease characterized by chronic, non-bloody, watery diarrhea in the setting of a grossly normal-appearing colonic mucosa on colonoscopy. The diagnosis can only be confirmed through microscopic examination of colonic mucosal biopsies, which is the distinguishing feature that gives the condition its name. There are two histological subtypes: collagenous colitis and lymphocytic colitis. In collagenous colitis, there is a thickened subepithelial collagen band (greater than 10 micrometers, compared to the normal thickness of less than 5 micrometers) beneath the surface epithelium of the colon, accompanied by an inflammatory infiltrate in the lamina propria. This collagen band disrupts normal fluid and electrolyte absorption across the colonic mucosa, leading to secretory diarrhea. In lymphocytic colitis, the subepithelial collagen band is normal in thickness, but there is a significant increase in intraepithelial lymphocytes (greater than 20 lymphocytes per 100 epithelial cells, compared to the normal value of fewer than 5 per 100). These intraepithelial lymphocytes are predominantly CD8-positive T cells that damage surface epithelial cells and impair their absorptive function. Both subtypes share a common pathogenesis involving an abnormal mucosal immune response to luminal antigens in genetically susceptible individuals. The exact triggering antigens remain unclear, but the leading hypotheses include medications (particularly NSAIDs, proton pump inhibitors, SSRIs, and statins), bile acid malabsorption, bacterial antigens, and dietary components. The inflammatory cascade involves activation of mucosal T cells, increased production of pro-inflammatory cytokines (interferon-gamma, TNF-alpha, interleukin-1), and disruption of tight junctions between colonocytes, resulting in increased mucosal permeability and impaired water and electrolyte absorption. Microscopic colitis predominantly affects middle-aged and older adults, with peak incidence between ages 60 and 70, and has a female predominance (particularly collagenous colitis, which is 3-4 times more common in females). The condition is increasingly recognized as a common cause of chronic diarrhea, accounting for 10-20 percent of cases investigated by colonoscopy for chronic watery diarrhea. Patients typically present with 4-9 watery, non-bloody bowel movements per day, often accompanied by abdominal cramping, urgency, fecal incontinence (particularly nocturnal), weight loss, and fatigue. Importantly, the diarrhea may be severe enough to cause dehydration and electrolyte imbalances, particularly hypokalemia and metabolic acidosis from bicarbonate loss in stool. There is a strong association with autoimmune conditions, including celiac disease, thyroid disorders, rheumatoid arthritis, and type 1 diabetes mellitus. Budesonide, a topical corticosteroid with high first-pass hepatic metabolism, is the first-line pharmacological treatment and is effective in inducing remission in 80 percent of patients. However, relapse is common after discontinuation (60-80 percent), and many patients require long-term maintenance therapy. The practical nurse must monitor fluid and electrolyte status, assess stool frequency and characteristics, administer medications as prescribed, and provide patient education regarding dietary modifications and medication adherence.