Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder involving chronic symptoms related to how the gut works. It is defined by recurrent abdominal pain linked to changes in bowel movements. IBS is classified into distinct subtypes based on the predominant pattern of stool consistency: IBS with Constipation (IBS-C), IBS with Diarrhea (IBS-D), and Irritable Bowel Syndrome with Mixed bowel habits (IBS-M). IBS-M is a challenging form of the disorder, characterized by a fluctuating and unpredictable combination of both constipation and diarrhea.
Identifying Features of IBS-M
IBS-M is defined by the Rome IV diagnostic criteria, requiring patients to experience recurrent abdominal pain for at least one day per week over the last three months. This pain must be associated with defecation or a change in stool frequency or form. The “mixed” designation is applied when a patient’s bowel habits alternate significantly between constipation and diarrhea, involving a substantial proportion of both hard, lumpy stools and loose, watery stools.
For a diagnosis of IBS-M, at least 25% of all bowel movements must be of the hard or lumpy type (Bristol Stool Scale types 1 or 2). Simultaneously, at least 25% of all bowel movements must be of the loose or watery type (Bristol Stool Scale types 6 or 7). This fluctuating presentation sets IBS-M apart from the other subtypes.
The time frame for measuring these percentages is when the patient is not using medications designed to treat bowel habit abnormalities. This ensures the underlying, natural pattern of the disorder is correctly identified. The unique alternating pattern is the hallmark of IBS-M.
The Biological Basis of Mixed Symptoms
The erratic nature of IBS-M symptoms stems from a complex interplay of physiological factors, primarily involving miscommunication along the gut-brain axis. This axis is a bidirectional signaling system linking the central nervous system with the enteric nervous system of the gut. Disturbances in this system can lead to highly variable gastrointestinal function.
One key mechanism is gut motility dysfunction, which refers to the erratic movement of muscles in the intestines. In patients with IBS-M, this dysfunction can swing between periods of slow intestinal transit, leading to constipation, and periods of rapid transit, resulting in diarrhea. Conflicting signals can cause the muscles to contract too quickly or too slowly at different times.
Another significant contributor is visceral hypersensitivity, where the nerves in the gut become over-responsive to normal stimuli, such as gas or stool passing through the colon. This heightened pain perception means that even minor changes in gut movement or volume can be felt as intense pain or discomfort. The combination of erratic motility and increased sensitivity explains why patients feel intense pain regardless of whether they are constipated or experiencing diarrhea.
The gut-brain axis is sensitive to psychological stress, which directly influences gut motility and sensitivity. Stress-related signals can trigger the release of neurotransmitters that alter intestinal movement, contributing to the alternating pattern of rapid and slow transit. Alterations in the gut microbiome—the community of microorganisms living in the digestive tract—also influence this axis, shaping the diverse and fluctuating symptoms seen in IBS-M.
Targeted Management Strategies for IBS-M
Managing IBS-M requires an individualized, phased approach, as therapies must address both constipation and diarrhea without worsening the opposite condition. Because of the disorder’s fluctuating nature, patients often need to track their symptoms closely to determine the best course of action. Dietary adjustments are a common first-line strategy, with the Low-FODMAP (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) diet often recommended.
This elimination diet aims to reduce the intake of poorly absorbed short-chain carbohydrates that ferment in the colon and cause gas, bloating, and altered bowel habits. Fiber intake needs careful balancing; soluble fiber, such as psyllium, helps normalize stool consistency by adding bulk. However, high amounts of insoluble fiber may exacerbate gas and bloating, so the goal is to find a level that supports regular bowel movements without triggering discomfort.
Pharmacological interventions are used to target specific, current symptoms. For periods of diarrhea, medications like loperamide may be used to slow intestinal transit. Conversely, during periods of constipation, osmotic laxatives or prosecretory agents like linaclotide or lubiprostone may be prescribed to increase fluid secretion and speed up bowel movements.
Beyond diet and medication, lifestyle modifications are an important component of management, particularly those focused on the gut-brain connection. Techniques such as Cognitive Behavioral Therapy (CBT) or hypnotherapy can help patients manage the visceral hypersensitivity and stress that often drive symptom flares. Regular physical activity improves gut motility and overall well-being, providing a non-pharmacological means of regulating the unpredictable symptoms of IBS-M.