A woman who had diarrhea up to 40 times a day was misdiagnosed with IBS. It was really an inflammatory bowel disease called microscopic colitis.

• For over a decade, a UK mom experienced increasing diarrhea, bathroom urgency, and fatigue.
• She was finally diagnosed with microscopic colitis, which requires a biopsy of the colon, in 2022.

Victoria Rennison was working at a tennis tournament in London over a decade ago when she was struck with severe stomach cramps and the need to go to the bathroom — urgently. Then 21, she ended up quitting the job because it didn't allow her to spontaneously leave the court to use the bathroom.

But after many tests at the emergency room, doctors told Rennison it was "just irritable bowel syndrome," or IBS, she told the charity Guts UK.

A few years later, Rennison's symptoms worsened. "I became scared of eating in case it triggered symptoms," she told the London-based charity. By 2020, she'd developed "diarrhea-induced agoraphobia," or a fear of leaving the house and being unable to use the bathroom.

At her condition's worst, Rennison said, she used the toilet as many as 40 times a day. "I felt more tired than I'd ever felt in my life and completely hopeless," the finance worker and mom in South London said.

Rennison was diagnosed after getting a biopsy of her colon

Finally, in August 2022, Rennison booked an appointment with a gastroenterologist. While she'd seen doctors and undergone colonoscopies before, this time she asked for a biopsy of her colon too.

The tests revealed that she had microscopic colitis (MC), or an inflamed large intestine that causes "persistent watery diarrhea," per the Mayo Clinic. Unlike other inflammatory bowel diseases, like Crohn's disease and ulcerative colitis, the condition is typically only diagnosable via biopsy, where a colon tissue sample is examined under a microscope.

Rennison was specifically diagnosed with lymphocytic colitis, a subtype of MC in which white blood cells increase in colon tissue.

Though relieved to have an answer, Rennison said she was angered by how differently she was treated when people thought she "just" had IBS.

"I'd been gaslighted for years and made to feel that because my illness wasn't clinically worse, the severity and frequency of my symptoms and the impact on my quality of life didn't matter," she said.

Microscopic colitis is common, but forgotten by the public

MC is relatively common, affecting between and 1 and 25 per 100,000 people in the US each year, various estimates show.

Tamara Duker Freuman, a dietitian who specializes in gastroenterology, told Insider that American doctors frequently conduct biopsies during colonoscopies specifically to rule out MC in patients with abnormal stools.

Still, awareness lags in the general population, Rennison believes, compared to more well-known types of inflammatory bowl disease like ulcerative colitis and Crohn's disease. "It feels like the forgotten Inflammatory Bowel Disease," she said.

The main symptom is watery diarrhea, though patients can also experience abdominal pain, weight loss, fatigue, and fecal incontinence, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

MC is most often diagnosed in women in their 60s, and smoking and certain medications can raise the risk, the site says.

Duker Freuman, author of "Regular: The Ultimate Guide to Taming Unruly Bowels and Achieving Inner Peace," said the condition is usually treated with rectal steroids or anti-inflammatories.

The Mayo Clinic recommends a low-fat, low-fiber diet, as well as cutting out dairy and gluten, too.

Unlike Crohn's disease and ulcerative colitis, MC doesn't seem to raise the risk of colon cancer, the Mayo Clinic says. Still, as Rennison experienced, it can greatly affect quality of life. After two bouts of steroids, she said, she's grateful to have resumed "some semblance of normal."