Irritable Bowel Syndrome (IBS) is a common, chronic functional gastrointestinal disorder involving a disturbance in how the gut and brain interact. It is primarily characterized by recurrent abdominal pain linked to changes in bowel habits. Unlike inflammatory bowel disease, IBS does not cause physical damage to the digestive tract, but it significantly affects quality of life. The change in bowel habits can manifest as diarrhea, constipation, or a pattern alternating between the two.
The Official IBS Subtypes
The medical community classifies Irritable Bowel Syndrome into distinct subtypes based on the patient’s predominant stool pattern. This classification guides the initial approach to treatment. The three primary categories are Irritable Bowel Syndrome with Constipation (IBS-C), Irritable Bowel Syndrome with Diarrhea (IBS-D), and Irritable Bowel Syndrome with Mixed bowel habits (IBS-M).
Diagnosis often utilizes the Rome IV criteria, requiring recurrent abdominal pain for at least one day per week over the last three months. Subtype determination relies on stool consistency, measured objectively using the Bristol Stool Scale. This scale categorizes stool into seven types, where Types 1 and 2 indicate constipation and Types 6 and 7 indicate diarrhea.
For a diagnosis of IBS-C or IBS-D, the constipated or diarrheal stool types must represent more than 25% of all bowel movements, while the non-predominant type must occur less than 25% of the time. This threshold standardizes categorization and ensures therapeutic strategies are tailored to the specific manifestation.
What Defines the Mixed Type (IBS-M)
The diagnostic category of Irritable Bowel Syndrome with Mixed bowel habits (IBS-M) addresses whether a person can have both IBS-C and IBS-D. This subtype is defined by the concurrent presence of both hard and loose stools, capturing the true inconsistency of bowel habits.
A diagnosis of IBS-M requires that constipated stools (Bristol Types 1-2) occur at least 25% of the time, and diarrheal stools (Bristol Types 6-7) also occur at least 25% of the time. This pattern reflects highly variable gut motility, where transit time fluctuates significantly.
IBS-M represents a currently mixed pattern, not simply a history of alternating between IBS-C and IBS-D over long stretches of time. This ongoing, simultaneous inconsistency often presents challenges for management.
Why Symptoms Can Shift Over Time
The subtype of IBS a person experiences can change over months or years, driven by physiological and environmental factors. Changes in the speed of gut motility—the movement of contents through the digestive tract—directly alter stool consistency.
Dietary triggers significantly impact stool volume and transit time. For instance, increasing fermentable carbohydrates (FODMAPs) can cause gas and diarrhea, shifting the pattern toward IBS-D. Conversely, low fiber intake or dehydration can slow transit, favoring a shift toward IBS-C.
The gut-brain axis, the communication pathway between the central nervous system and the bowel, is another influential factor. High levels of psychological stress or depression can disrupt this signaling, leading to unpredictable changes in gut function. Hormonal fluctuations and changes in the gut microbiome further contribute to the temporal instability of IBS symptoms.
Subtype-Specific Management Strategies
The specific IBS subtype dictates the management approach, as treatments must target the predominant symptom.
Managing IBS-C
For those with IBS-C, the focus is on softening stools and improving intestinal movement. This involves increasing soluble fiber intake or using osmotic laxatives like polyethylene glycol. Prescription medications such as prokinetics or chloride channel activators may also be used to stimulate bowel movements.
Managing IBS-D
Patients with IBS-D require strategies to slow down gut transit and reduce the frequency of loose stools. Management includes anti-diarrheal medications such as loperamide, or antibiotics like rifaximin to modify gut bacteria. Dietary adjustments often focus on reducing trigger foods and potentially following a low-FODMAP diet to minimize fermentation.
Managing IBS-M
The management of IBS-M is the most challenging because it requires balancing opposing symptoms. Treatment involves managing acute episodes of constipation and diarrhea separately while maintaining a stable, customized diet. This approach may necessitate the alternating use of motility-balancing medications, along with therapies targeting the underlying gut-brain interaction, such as hypnotherapy.