Collagenous Colitis

Differential Diagnosis

Artifacts

Tangential Sectioning	Collagenous Colitis
Present only where adjacent crypts cut as cross-sections	Present even when adjacent crypts are cut longitudinally
Indistinct edges but no entrapped elements	Jagged edges with entrapped capillaries and inflammatory cells
Lacks chronic inflammatory infiltrate	Prominent chronic inflammatory infiltrate in lamina propria

Epithelial cell nuclei occasionally line up in the center of the cytoplasm leaving a uniform eosinophilic band of cytoplasm that may be confused with a thickened collagen layer
- Usually resolvable by close inspection
- Lacks irregular, spiculated base
- Trichrome stain will show lack of collagen

Chronic colitis is a term best not used as a pathologic diagnosis
- Moderate numbers of lymphocytes and plasma cells are normal in the lamina propria of the large intestine
- The chronic changes associated with inflammatory bowel disease are better diagnosed as “crypt distortion” with a comment that it is consistent with chronic inflammatory bowel disease
- An increased chronic inflammatory infiltrate in the absence of an increased collagen layer may be diagnosed as lymphocytic colitis

Lymphocytic colitis and collagenous colitis are distinguished by the presence of a thickened collagen layer only in the latter
- They both exhibit a prominent intramucosal lymphocytic infiltrate
- The thickened collagen layer may be variably distributed
  - Multiple biopsies may need to be examined

Systemic Sclerosis	Collagenous Colitis
Fibrosis surrounds crypts	Collagen restricted to apical zone
Fibrosis may extend into muscularis mucosae through to the serosa	Collagen restricted to sub-basement membrane region

Amyloid	Collagenous Colitis
Perivascular distribution usually present	Restricted to sub-basement membrane zone
Congo red positive	Congo red negative
Trichrome variable	Trichrome stains blue

Ischemic Colitis and Radiation Damage	Collagenous Colitis
Fibrosis surrounds crypts	Collagen restricted to apical zone
Crypt distortion frequent	Crypt distortion generally not presenet