A 67-year-old woman presents to her physician with 4 months of non-bloody watery diarrhea. She reports 6–8 loose bowel movements daily and notes that symptoms occasionally awaken her from sleep. She denies hematochezia, fever, or significant weight loss. Her medical history includes osteoarthritis, gastroesophageal reflux disease, and hypertension. Medications include ibuprofen, omeprazole, and amlodipine. Vital signs are within normal limits. Physical examination is normal. Laboratory studies, including a complete blood count and metabolic panel, are within normal limits. Colonoscopy reveals normal-appearing colonic mucosa.
Which of the following is the most likely diagnosis?
The correct answer is:
C) Microscopic colitis
This patient has classic features of microscopic colitis. The key clues are chronic watery diarrhea in an older woman, nocturnal symptoms, normal laboratory studies, and a completely normal colonoscopic appearance. Many students incorrectly assume that a normal colonoscopy excludes inflammatory disease; however, microscopic colitis is diagnosed by biopsy because the mucosa often appears normal during endoscopy.
Several risk factors are present in this vignette. Microscopic colitis is more common in older women and has been associated with proton pump inhibitors, NSAIDs, selective serotonin reuptake inhibitors, and smoking. Histologically, the disease is divided into collagenous colitis and lymphocytic colitis. Both produce similar clinical manifestations and are managed similarly.
An important Step 2 distinction is differentiating microscopic colitis from irritable bowel syndrome. IBS often causes abdominal pain associated with bowel movements and rarely produces nocturnal symptoms. Patients with microscopic colitis frequently awaken from sleep due to diarrhea because the disease reflects true mucosal inflammation rather than altered gut sensitivity. First-line treatment typically includes removal of offending medications when possible and budesonide for persistent symptoms.
Answer choice A: Crohn disease, is incorrect.
Crohn disease can cause chronic diarrhea but usually produces abdominal pain, weight loss, anemia, or endoscopic evidence of inflammation. A completely normal colonoscopy makes Crohn disease much less likely.
Answer choice B: Irritable bowel syndrome, is incorrect.
This is the most tempting distractor. Both disorders may cause chronic diarrhea and have relatively normal laboratory findings. However, IBS rarely causes nocturnal symptoms and is a functional disorder without mucosal inflammation.
Answer choice D: Ulcerative colitis, is incorrect.
Ulcerative colitis usually causes bloody diarrhea and continuous mucosal inflammation visible during colonoscopy. Normal mucosal appearance argues strongly against this diagnosis.
Answer choice E: Villous adenoma, is incorrect.
Large villous adenomas can cause secretory diarrhea and electrolyte abnormalities. However, they would be directly visualized during colonoscopy rather than producing a normal examination.
Key Learning Point
Microscopic colitis causes chronic watery diarrhea, often with nocturnal symptoms, despite a normal colonoscopic appearance; diagnosis requires colonic biopsy.
