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Difficulty: MediumPathology and Biopsy Interpretation

A 64-year-old woman comes to the clinic due to a 4-month history of frequent, watery, non-bloody diarrhea. She reports having up to 8 loose bowel movements per day, which occasionally wake her from sleep. She has no fever, abdominal pain, nausea, or unintended weight loss. Her past medical history is notable for osteoarthritis, for which she takes daily ibuprofen, and hypertension managed with lisinopril. Vital signs are within normal limits. Abdominal examination is unremarkable. Laboratory studies, including a complete blood count, comprehensive metabolic panel, and thyroid function tests, are normal. Stool studies are negative for infectious etiologies. A colonoscopy reveals an entirely normal-appearing colonic mucosa throughout. Random biopsies are obtained from the right and left colon. Histopathologic examination of the biopsy specimens shows a preserved crypt architecture, an increased number of intraepithelial lymphocytes, chronic inflammation in the lamina propria, and a notably thickened subepithelial eosinophilic band measuring 15 micrometers (normal is typically less than 5 micrometers). Which of the following is the most likely diagnosis based on these histopathologic findings?

  1. Collagenous colitisAnswer
  2. B
    Lymphocytic colitis
  3. C
    Ulcerative colitis
  4. D
    Pseudomembranous colitis
  5. E
    Celiac disease

Answer

Collagenous colitis
The patient's clinical presentation of chronic watery diarrhea with a macroscopically normal colonoscopy and specific biopsy findings is characteristic of collagenous colitis. Microscopic colitis has two main subtypes: collagenous colitis and lymphocytic colitis. Both present similarly (frequent in older women, often associated with NSAID or PPI use) and have normal gross endoscopic appearances. The distinguishing feature is purely histopathological. The biopsy in this patient reveals a thickened subepithelial eosinophilic (collagen) band measuring >10 micrometers, which is the pathognomonic diagnostic criterion for collagenous colitis.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient is an older adult female presenting with chronic, watery, non-bloody diarrhea and a history of daily NSAID use.
Identifying the demographic and chronicity of diarrhea narrows the differential diagnosis to functional, medication-induced, or inflammatory causes.
2
Evaluate the endoscopic findings.
The colonoscopy demonstrates an entirely normal-appearing colonic mucosa.
A macroscopically normal colon essentially rules out classic inflammatory bowel disease (like Crohn disease or ulcerative colitis) and points toward microscopic colitis.
3
Interpret the biopsy results.
Histopathology shows preserved crypt architecture, increased intraepithelial lymphocytes, and a thickened subepithelial eosinophilic band (15 micrometers).
The thickened subepithelial collagen band is the pathognomonic finding that differentiates collagenous colitis from lymphocytic colitis.

Key Concept

Microscopic colitis variants are distinguished histopathologically; collagenous colitis is defined by a thickened subepithelial collagen band, whereas lymphocytic colitis lacks this feature despite both showing intraepithelial lymphocytosis.
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Topics

Pathology and Biopsy Interpretation

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