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Microscopic colitis is
used to describe both lymphocytic colitis (LC) and collagenous
colitis (CC), and these conditions should be considered in any
patient with unexplained nonbloody diarrhea. Patients undergoing
either sigmoidoscopy or colonoscopy for unexplained diarrhea who
have normal endoscopic findings should have biopsy samples taken
to diagnose or rule out either form of microscopic colitis.
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LC and CC are relatively rare conditions that
are diagnosed when a patient with chronic watery nonbloody
diarrhea has an endoscopically or radiographically normal colon,
but colonic biopsies show unique inflammatory changes. Because
the mucosa is not ulcerated or otherwise disrupted, the diarrhea
generally does not contain blood or pus.
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No definite etiology
has been determined. Many drugs may cause diarrhea as an adverse
effect, nonsteroidal anti-inflammatory drugs (NSAIDs) show a
strong trend (p=0.057) toward increasing the risk of CC.
Antidepressant selective serotonin reuptake inhibitors (SSRIs)
as a group increase the risk of CC, but, among this class of
medications, sertraline alone significantly increases the risk
of LC. HLA-A1 is more frequent in patients with LC (67%)
than in controls (28%) or in patients with CC (26%).
Approximately 50% of both patients with CC and patients with LC
may have circulating autoantibodies, especially antiparietal
cell, antithyroglobulin, and antimicrosomal antibodies.
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The characteristic
feature of LC is an infiltration of lymphocytes into the colonic
epithelium. CC shares the histologic features of LC but
additionally demonstrates a thickened subepithelial collagen
layer (usually >10 µm) in the lamina propria. LC and CC have
been suggested to represent different phases of a single
pathophysiologic process, with LC possibly being a precursor or
earlier phase of CC. Surface epithelium shows a chronic
inflammatory infiltrate of plasma cells, lymphocytes, and
eosinophils in the lamina propria. Intraepithelial lymphocytosis,
with greater than 20 lymphocytes per 100 epithelial cells, is
pathognomonic of the diagnosis of LC. Epithelial cell damage is
demonstrated by cell flattening, subepithelial blebs, and
denuded epithelium. Crypts typically have normal architecture,
unlike Crohn disease or ulcerative colitis.
The
Johns Hopkins Division of Gastroenterology and Hepatology
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