Microscopic colitis (MC) is an inflammatory condition of the colon that can only be identified by examining tissue under a microscope, distinguishing it from other inflammatory bowel diseases like Crohn’s disease or ulcerative colitis. MC is a collective term for two similar conditions, lymphocytic colitis and collagenous colitis, both causing chronic, watery diarrhea. Confirmation requires a structured process, beginning with a review of symptoms and medical history, proceeding through laboratory tests to rule out other causes, and culminating in a specialized procedure to obtain tissue for definitive analysis.
Initial Assessment and Symptom Review
The investigation for microscopic colitis typically begins when a patient reports persistent, watery, and non-bloody diarrhea. This symptom is the primary clinical feature that prompts a doctor to consider the diagnosis. Patients often experience significant urgency and sometimes nocturnal stools.
A detailed medical history focuses on any recent or ongoing medication use. Certain drugs have been associated with MC or can mimic its symptoms. Non-steroidal anti-inflammatory drugs (NSAIDs), proton pump inhibitors (PPIs) used for acid reflux, and selective serotonin reuptake inhibitors (SSRIs) are among the most commonly implicated medications. Identifying and potentially discontinuing these agents is an early step, helping the physician narrow the field of possible diagnoses.
Preliminary Laboratory Screening
A series of non-invasive laboratory tests are performed to exclude other common causes of chronic diarrhea. Stool samples are routinely analyzed to check for infectious pathogens, such as Clostridium difficile or other bacteria and parasites, which can produce similar symptoms. Ruling out an infection is a standard prerequisite in the diagnostic pathway.
Blood tests are also commonly ordered, including a complete blood count (CBC) to check for anemia or signs of infection. Inflammatory markers like C-reactive protein (CRP) or fecal calprotectin are often measured to help distinguish MC from other forms of inflammatory bowel disease. Unlike Crohn’s or ulcerative colitis, these markers are frequently normal or only minimally raised in microscopic colitis, which serves as an important diagnostic clue.
The Essential Diagnostic Procedure
The definitive diagnosis of microscopic colitis cannot be made without a tissue sample obtained through a procedure called a colonoscopy. During this procedure, a flexible tube with a camera is inserted to examine the entire lining of the large intestine. The challenge with MC is that the colon lining often appears completely normal, or near-normal, to the naked eye of the endoscopist.
Because the inflammation is not visually apparent, the physician must maintain a high index of suspicion and proactively take multiple tissue samples, known as biopsies. It is crucial to collect biopsies from various segments of the colon, including the right and left sides, as the inflammatory changes can be patchy. Taking four to eight random biopsies is often recommended to ensure that a diagnostic area is not missed.
Histopathological Confirmation
The final step involves sending the collected tissue samples to a pathologist for microscopic examination. This is the only way to confirm the diagnosis and is the reason the condition is called “microscopic colitis.” The pathologist analyzes the biopsies to identify specific cellular changes indicative of one of the two subtypes of MC.
Collagenous Colitis (CC)
One subtype is Collagenous Colitis (CC), defined by a thickened layer of collagen directly beneath the surface epithelium. A diagnosis of CC is made when this subepithelial collagen band is measured to be thicker than 10 micrometers. This thickened band can also contain entrapped inflammatory cells and red blood cells.
Lymphocytic Colitis (LC)
The second subtype is Lymphocytic Colitis (LC), characterized by an increased number of lymphocytes, a type of white blood cell, infiltrating the colon’s inner lining. Specifically, the diagnosis of LC requires the presence of 20 or more intraepithelial lymphocytes per 100 epithelial cells lining the surface. Both subtypes also show an increase in general inflammatory cells within the lamina propria.