Irritable bowel syndrome (IBS) is a common disorder of the gut-brain interaction, affecting the large intestine and causing chronic discomfort and altered bowel function. It is a functional gastrointestinal disorder, meaning there are no visible signs of disease or damage in the digestive tract, unlike conditions such as inflammatory bowel disease. IBS is categorized into specific subtypes based on the predominant pattern of stool consistency.
Defining Irritable Bowel Syndrome Mixed
The designation IBS-M, or Irritable Bowel Syndrome with Mixed bowel habits, is applied when a patient experiences a fluctuating pattern of both constipation and diarrhea. This alternating nature is the defining characteristic that distinguishes it from other subtypes.
The most current diagnostic guidelines, known as the Rome IV criteria, provide a specific definition for IBS-M based on the patient’s bowel movements. To meet the criteria, the patient must report that at least 25% of their bowel movements are hard or lumpy, indicative of constipation. Simultaneously, the patient must report that at least 25% of their bowel movements are loose or watery, indicative of diarrhea.
The alternating symptoms experienced by individuals with IBS-M can make management complex, as treatments effective for one state may exacerbate the other.
Differentiating IBS Subtypes
IBS is divided into three main subtypes to guide treatment, all of which require recurrent abdominal pain that occurs at least one day per week for the last three months. These subtypes are defined by the consistency of the stool, which is standardized using the Bristol Stool Scale.
The constipation-predominant subtype, IBS-C, is diagnosed when more than 25% of BMs are hard or lumpy (Type 1 or 2 on the Bristol Stool Scale), and less than 25% are loose or watery (Type 6 or 7). Conversely, the diarrhea-predominant subtype, IBS-D, is characterized by having more than 25% of BMs as loose or watery, with less than 25% being hard or lumpy.
A fourth category, IBS-U (Unclassified), is used when the patient meets the general IBS criteria but the bowel habits do not fit the percentage cutoffs for C, D, or M.
Etiology and Contributing Factors
The precise cause of IBS-M, like other IBS subtypes, is not fully understood, but it is believed to result from a combination of interconnected factors. A primary mechanism involves a disruption in the gut-brain axis, which is the bidirectional communication pathway between the central nervous system and the enteric nervous system of the gut. This communication issue can lead to an abnormal regulation of gut motility, causing food to pass through the digestive tract too quickly during diarrheal phases and too slowly during constipated phases.
Another factor is visceral hypersensitivity, where the nerves in the gut wall become overly sensitive, causing normal amounts of gas or intestinal stretching to be perceived as painful or uncomfortable. Genetics may also play a part, as IBS tends to run in families, suggesting a possible inherited predisposition to the condition. Alterations in the gut microbiome, known as dysbiosis, are also implicated, with an imbalance in the types and quantities of bacteria potentially leading to inflammation and abnormal gut function.
Some cases of IBS-M may be triggered by a prior gastrointestinal infection, a condition referred to as post-infectious IBS (PI-IBS). The infection can cause long-term changes to the gut lining, immune activation, and nerve function, which persist long after the initial pathogen has been cleared. Stress and psychological factors, while not a direct cause, also strongly influence the severity of symptoms due to the intimate link of the gut-brain axis.
Diagnostic Process and Symptom Tracking
A diagnosis of IBS-M is a clinical one, primarily based on a thorough medical history and the application of the Rome IV criteria. The diagnosis is also one of exclusion, meaning the clinician must rule out other serious conditions that can mimic IBS symptoms.
To exclude other diseases, the physician looks for “red flag” symptoms that suggest a more serious pathology, such as Inflammatory Bowel Disease (IBD) or celiac disease. These alarm features include unexplained weight loss, gastrointestinal bleeding, new onset of symptoms after age 50, anemia, or pain that consistently wakes the patient from sleep. If any of these red flags are present, further testing, such as blood work, stool tests, or a colonoscopy, is required to investigate the underlying cause.
The most critical tool for confirming the IBS-M subtype is the patient’s own record of their bowel movements. Patients are often asked to track their stools for several weeks, using the Bristol Stool Scale to classify the consistency of each movement. This tracking allows the doctor to accurately calculate the percentage of hard/lumpy (Type 1-2) and loose/watery (Type 6-7) stools.
Tailored Management Strategies
Managing IBS-M presents a challenge because treatment must address both constipation and diarrhea without worsening the opposing symptom. The management plan is individualized and combines dietary adjustments, lifestyle changes, and targeted pharmacological therapies. A key first step is often dietary modification, with the Low FODMAP (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) diet showing efficacy for many IBS patients by reducing fermentable carbohydrates that contribute to gas and bloating.
However, the Low FODMAP diet requires careful implementation, often with the guidance of a registered dietitian, to ensure nutritional balance and to properly identify specific trigger foods. Lifestyle interventions, such as stress management techniques and regular physical activity, are also recommended. Techniques like cognitive behavioral therapy (CBT) or gut-directed hypnotherapy can help modulate the gut-brain axis, reducing visceral hypersensitivity and overall symptom severity.
Pharmacological treatment for IBS-M often involves alternating or combination therapy. During constipated phases, the patient may use osmotic laxatives, which draw water into the colon to soften stools. Conversely, anti-diarrheal agents like loperamide may be used on an as-needed basis during diarrheal phases to slow gut transit time. For patients with significant abdominal pain, low-dose tricyclic antidepressants (TCAs) are sometimes prescribed to reduce pain perception and help regulate overall gut motility.
Newer medications approved for IBS-C and IBS-D are not specifically licensed for IBS-M. However, certain agents that target underlying mechanisms, such as antispasmodics to reduce cramping, can be used to manage global symptoms.