Gastroenterology · Inflammatory Bowel Disease
The facts most likely to be tested
Microscopic colitis presents clinically as chronic, non-bloody, watery diarrhea in middle-aged to older adults.
The diagnosis is established by histopathologic examination of colonic biopsies, as the gross appearance of the mucosa is normal on colonoscopy.
Lymphocytic colitis is characterized by an increase in intraepithelial lymphocytes with a normal collagen layer.
Collagenous colitis is defined by a thickened subepithelial collagen band greater than 10 micrometers.
Commonly implicated trigger medications include NSAIDs, proton pump inhibitors (PPIs), sertraline, and aspirin.
The first-line treatment for symptomatic microscopic colitis is budesonide, a glucocorticoid with high first-pass metabolism.
Patients who fail initial therapy or have refractory disease may require immunomodulators such as azathioprine or biologic agents.
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A 68-year-old woman presents with a 4-month history of chronic, watery, non-bloody diarrhea occurring 6-8 times daily. She reports no weight loss, fever, or abdominal pain. Her medical history is significant for hypertension and GERD, for which she takes lisinopril and omeprazole. Physical examination is unremarkable, and laboratory studies including celiac serology and fecal calprotectin are normal. A colonoscopy is performed and reveals a completely normal-appearing colonic mucosa.
What is the most likely diagnosis?
Microscopic colitis
The patient's presentation of chronic watery diarrhea with a normal-appearing colonoscopy is classic for microscopic colitis, which requires biopsy for diagnosis and is often associated with PPI use.
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Etiology / Epidemiology
Common in middle-aged women; strongly associated with NSAID use and smoking.
Clinical Manifestations
Presents with chronic, non-bloody, watery diarrhea; nocturnal diarrhea is a classic feature.
Diagnosis
Requires colonoscopy with biopsy showing normal gross mucosa but abnormal histology.
Treatment
Budesonide is the first-line therapy; avoid NSAIDs to prevent exacerbation.
Prognosis
Excellent prognosis with 90% response rate to steroids; rarely progresses to malignancy.
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Epidemiology & Etiology
Predominantly affects women aged 50-70. Strongest environmental triggers include NSAIDs, lansoprazole, and sertraline. Smoking is a significant risk factor that often leads to an earlier age of onset.
Pertinent Anatomy
The disease involves the entire colon but spares the small intestine. Gross appearance is normal during endoscopy, which is why biopsy is mandatory.
Pathophysiology
Characterized by chronic inflammation of the colonic mucosa. Two main subtypes exist: collagenous colitis (thickened subepithelial collagen band >10μm) and lymphocytic colitis (increased intraepithelial lymphocytes >20 per 100 epithelial cells). Both result in impaired water and electrolyte absorption.
Clinical Manifestations
Patients present with chronic, watery, non-bloody diarrhea often accompanied by fecal urgency and nocturnal diarrhea. Weight loss and abdominal pain are less common but possible. Symptoms are often intermittent and may mimic irritable bowel syndrome.
Diagnosis
The colonoscopy with biopsy is the gold standard. Histology is required for diagnosis as the mucosa appears normal to the naked eye. Biopsies must be taken from both the right and left colon to avoid sampling error.
Treatment
Initial management involves discontinuing offending agents like NSAIDs. Budesonide is the first-line pharmacologic treatment for induction of remission. Avoid long-term systemic steroids due to side effects; use budesonide for its high first-pass hepatic metabolism.
Prognosis
Most patients achieve clinical remission with budesonide. Relapse is common upon cessation, often requiring maintenance therapy at the lowest effective dose. There is no increased risk of colorectal cancer.
Differential Diagnosis
IBS-D: normal histology and no nocturnal symptoms
Celiac disease: positive serology and small bowel involvement
IBD: gross mucosal abnormalities on endoscopy
Infectious colitis: acute onset with positive stool cultures
Lactose intolerance: symptoms correlate strictly with dairy intake