Lymphocytic Colitis Evolving into Collagenous Colitis: A Long-Term Retrospective Cohort Study in a Tertiary Center

For decades, doctors have treated two specific causes of chronic, watery diarrhea as if they were entirely different diseases. One is called lymphocytic colitis, where the lining of the colon becomes crowded with immune cells that look like small white blood cells. The other is collagenous colitis, which shares that same immune crowding but adds a distinct, thick layer of tough, fibrous material just beneath the surface of the tissue. Because these two conditions look different under a microscope, they have historically been classified as separate entities, each with its own name and diagnostic box. However, the human body is rarely so neat. In recent years, a quiet question has emerged among specialists: could these two conditions actually be different chapters of the same story? Could a patient start with one pattern and slowly, over many years, shift into the other as the disease changes its shape?

A team of researchers at the University of California, Los Angeles, set out to answer this question by looking backward through time. They gathered the medical records of sixty-six adults who had been diagnosed with microscopic colitis at their hospital between 2016 and 2025. These were not patients with a single, one-time diagnosis. Instead, the researchers focused on people who had undergone multiple colon biopsies over the years, allowing them to see how the tissue changed from one visit to the next. The study was designed to see if the rigid lines drawn between lymphocytic and collagenous colitis held up when viewed over a long period, or if the disease was more fluid than previously thought.

The results of this long-term look suggest that the boundary between the two conditions is far more porous than medical textbooks have traditionally admitted. When the researchers examined the sixty-six patients, they found that the vast majority did not fit neatly into one category or the other. Instead of having a clear-cut case of lymphocytic colitis that suddenly became a clear-cut case of collagenous colitis, most patients showed a mix of features. Their biopsy samples often displayed signs of both conditions at the same time, or showed a gradual evolution where the tissue slowly began to develop the thick fibrous layer characteristic of collagenous colitis after years of showing only the immune-cell crowding of lymphocytic colitis.

Among the small group of patients who did have two distinct diagnoses at different times, the change did not happen overnight. On average, it took nearly seven years for a patient diagnosed with the first subtype to later show clear signs of the second. This long gap suggests that if the disease is changing, it is doing so through a slow, gradual remodeling of the tissue rather than a sudden switch. The researchers noted that in some cases, the biopsies showed a middle ground, where the fibrous layer was present but not yet thick enough to meet the strict definition of the second condition. This finding supports the idea that these two diagnoses might represent different points along a single, continuous spectrum of disease, rather than two separate illnesses.

The people in this study were typical of those who suffer from microscopic colitis: they were mostly women, with an average age of about sixty-three, and many had other autoimmune conditions, such as thyroid issues or celiac disease. Almost all of them suffered from chronic diarrhea, which was the main reason they sought medical help. The study also confirmed that certain common medications, including pain relievers, antidepressants, and acid-reflux drugs, were frequently used by these patients, a pattern that aligns with what is already known about factors that can trigger or worsen the condition.

Perhaps most reassuringly, the study found that even when the disease appeared to be changing its microscopic appearance, the patients did not necessarily get worse in terms of how they felt or how hard it was to treat. The majority of patients responded well to a standard steroid treatment called budesonide, and none of them required the powerful, complex drugs used for more aggressive autoimmune diseases. This suggests that whether the tissue looks like one subtype or the other, or even a mix of both, the clinical approach to managing the patient remains largely the same.

The researchers acknowledge that their study has limits. Because they looked back at old records, they could not control exactly when biopsies were taken or ensure that every single patient was followed perfectly. It is possible that some of the apparent changes were simply due to the fact that a biopsy only samples a tiny piece of the colon, and the disease might have been patchy rather than uniform. However, the fact that so many patients showed a consistent pattern of overlapping features over many years makes it unlikely that this is just a sampling error.

Ultimately, this work challenges the old way of thinking about microscopic colitis. It suggests that doctors and pathologists should view lymphocytic and collagenous colitis not as two separate islands, but as neighboring lands that can merge and shift over time. For the patients, this means that a diagnosis is not necessarily a permanent label, but a snapshot of a disease that may be slowly evolving. While the study does not prove exactly why the tissue changes or how to stop it, it provides a clearer picture of the disease's natural history, showing that the human body often follows a path of gradual change that defies simple, static categories.