Irritable Bowel Syndrome (IBS) is a common functional gastrointestinal disorder. This chronic condition is primarily characterized by recurrent abdominal pain or discomfort associated with changes in bowel habits. Symptoms can include cramping, bloating, gas, and an altered frequency or form of stool. IBS is classified as a disorder of gut-brain interaction, disrupting communication between the brain and the gut.
The Subtypes of Irritable Bowel Syndrome
A person can experience both constipation and diarrhea, and this pattern defines a distinct subtype of the condition. Irritable Bowel Syndrome is categorized into four primary subtypes based on the individual’s predominant bowel movement pattern. Classification is determined by analyzing the consistency of bowel movements on days when the bowels are abnormal, rather than on all days.
IBS with predominant constipation, or IBS-C, is diagnosed when hard or lumpy stools occur more than 25% of the time, and loose or watery stools occur less than 25% of the time. Conversely, IBS with predominant diarrhea, or IBS-D, is characterized by loose or watery stools more than 25% of the time, with hard or lumpy stools occurring less than 25% of the time.
The mixed subtype, known as IBS-M, is reserved for individuals who experience both hard/lumpy stools and loose/watery stools more than 25% of the time they have abnormal bowel movements.
A fourth classification, IBS-U or Unclassified, is used when a person meets the general diagnostic criteria for IBS but their stool consistency does not meet the specified percentage thresholds for C, D, or M. In the IBS-U classification, both hard/lumpy and loose/watery stools occur less than 25% of the time. Understanding the specific subtype is the first step toward creating a targeted management plan.
How Mixed Type is Diagnosed
Accurately classifying a patient as having IBS-M requires the clinical application of internationally recognized standards, specifically the Rome IV criteria. These criteria establish that the patient must have experienced recurrent abdominal pain on average at least one day per week in the last three months, with the pain associated with defecation or a change in stool frequency or form. The symptom onset must have occurred at least six months prior to the formal diagnosis.
To differentiate the subtypes, the Rome IV criteria heavily rely on a tool called the Bristol Stool Form Scale (BSFS). This scale categorizes human stool into seven types based on consistency and shape, with Type 1 being separate hard lumps and Type 7 being entirely liquid. Stool types 1 and 2 are considered constipated, while types 6 and 7 are considered diarrheal.
For an IBS-M diagnosis, the patient must track their bowel movements, and the physician must determine that more than 25% of all abnormal bowel movements are Type 1 or 2, and simultaneously more than 25% are Type 6 or 7. This specific threshold is necessary to distinguish IBS-M from subtypes where one symptom clearly dominates the other.
Navigating Treatment for Alternating Symptoms
The management of IBS-M presents a unique challenge because treatments for constipation and treatments for diarrhea often work in opposition to each other. A medication designed to speed up the gut, such as a laxative, may resolve a constipation episode but could easily trigger or worsen the subsequent diarrhea phase. Conversely, an anti-diarrheal agent, like loperamide, risks exacerbating the constipation phase.
For this reason, treatment for IBS-M often focuses on symptom-specific, as-needed interventions rather than a single daily medication. Patients might use a bulk-forming agent or a gentle laxative during a constipated phase and an anti-diarrheal only during periods of loose stools. Antispasmodics are sometimes prescribed to address the abdominal pain and cramping that can occur with both extremes of bowel function.
Dietary adjustments are highly personalized and must be flexible to reflect the current symptom presentation. For instance, a low-FODMAP diet, which restricts certain fermentable carbohydrates, is often effective for many IBS patients, but the specific foods to reintroduce or avoid may need to shift. Lifestyle factors such as regular exercise and stress management techniques like meditation are considered important components of a comprehensive treatment plan.