Focal active colitis (FAC) is a histologic pattern of injury, not a specific diagnosis. From a morphologic standpoint, FAC is defined either as a single focus of neutrophilic crypt injury (cryptitis) or as multiple separate foci of cryptitis occurring in different pieces of tissue within the same biopsy specimen. The crypt injury may be accompanied by increased mononuclear cells in the lamina propria; however, no features of chronic injury, such as architectural distortion, basal plasmacytosis, or Paneth cell metaplasia are present.
So what does FAC mean clinically? Well, it depends on the clinical and endoscopic findings. In an asymptomatic patient undergoing a routine screening colonoscopy, the finding of FAC is most likely related to bowel preparation and therefore of no clinical significance. Oral sodium phosphate preparations are reported to cause focal active colitis in up to 3% of patients. In this scenario, the finding of FAC can typically be safely ignored.
In symptomatic patients, studies have shown the most common underlying clinical condition associated with FAC was found to be infection, including cases of so-called acute “selflimited colitis” diarrheal illness in which no infectious organism is ever found, as well as cases in which a specific etiologic agent is identified by laboratory methods. Most infections showing only FAC represent either mild or resolving infectious colitis.
In the absence of an infectious etiology, what else can cause focal active colitis? The same studies implicated ischemia as the cause of a focal active colitis pattern of injury in 5-10% of cases. Non-steroidal anti-inflammatory drugs (NSAIDs) and other medications are increasingly recognized causative agents in the setting of focal active colitis. Diverticular disease-associated segmental colitis comprises a spectrum of morphologic changes, including FAC, and this fact emphasizes how important endoscopic-pathologic correlation can be in arriving at a diagnosis: if the pathologist is aware that the biopsy was taken from an area affected by diverticular disease, then it can be suggested with a high degree of certainty that the focal active colitis is related to underlying diverticula.
How does focal active colitis relate to idiopathic inflammatory bowel disease (IBD)? Certainly, the finding of FAC would not be unusual in a patient with a known history of IBD, but what if the patient has no known history of IBD? In one study of non-IBD patients with focal active colitis as the only histopathologic finding, no adult patients developed IBD; however, in another study, 13% of adult patients were later diagnosed with Crohn’s disease after a histologic finding of FAC. (Volk, Mod Pathol 1998; 11:789-794) It is important to note, though, that patients in the latter study also had clinical and endoscopic findings of Crohn’s disease. Therefore, it is reasonable to conclude that an isolated pathologic finding of FAC in adult patients is rarely a harbinger of incipient IBD. Children represent an important exception: in one study, 27.6% of children with an isolated finding of focal active colitis were later diagnosed with Crohn’s disease (Bihlmeyer, Diagnostic pathology, gastrointestinal, Salt Lake City: Amirsys; 2010, 5-68—5-69.). These data show that FAC must be interpreted differently in younger patients.
Unfortunately, in some cases of focal active colitis, the underlying etiology may never be determined. Such “incidental FAC” cases may comprise up to 25% of cases, underscoring the importance of endoscopic-pathologic correlation so that a specific diagnosis can at least be suggested in as many cases as possible.
Bihlmeyer, SK: Focal active colitis. In: Greenson JK, ed. Diagnostic pathology, gastrointestinal, Salt Lake City: Amirsys; 2010, 5-68—5-69.
Greenson JK, Odze RD: Inflammatory disorders of the large intestine. In Odze RD and Goldblum, eds. Surgical pathology of the GI tract, liver, biliary tract, and pancreas, Philadelphia: Saunders; 2009:355—394.
Greenson JK, Stern RA, Carpenter SL, et al: The clinical significance of focal active colitis. Hum Pathol 1997; 28:729-733.
Volk EE, Shapiro BD, Easley KA, et al: The clinical significance of a biopsy-based diagnosis of focal active colitis: A clinicopathologic study of 31 cases. Mod Pathol 1998; 11:789-794