What is constipation?

Primary chronic constipation 

This is the most common and is included within gut-brain axis disorders. Although the mechanisms are not fully understood, several factors are considered to be involved: Visceral hypersensitivity (increased internal organ pain sensitivity), functional sensory and motor disturbances, slow colonic transit, disturbances in central perception (distortions in the way sensory information is interpreted), and functional defecation disorders.

Based on the predominant symptoms, primary chronic constipation is divided into three subtypes: functional chronic constipation, irritable bowel syndrome with constipation and functional defecation disorders.

The definition of functional chronic constipation includes a chronic course of symptoms and is based on clinical criteria after excluding secondary causes. It is due to primary functional disturbances of the colon and anorectum and is defined by the Rome IV criteria (Table). It can be divided into three subgroups: 

• Normal colonic transit constipation. This represents the most common group (60% of NTC cases). Stool transit time through the colon is normal and patients are thought to perceive bowel frequency badly.
• Slow colonic transit constipation (STC, also called colonic inertia). 20% of patients have a lower frequency of high-amplitude colonic contractions; i.e., those that propel the colonic contents over long distances. Thus, it takes longer for intestinal material to travel through the colon compared to normal. As intestinal contents remain in the colon longer, water and electrolyte absorption increases, decreasing their volume and making them harder. This effect leads to difficulty evacuating and the formation of faecaliths (accumulation of hard stool in the rectum or colon) in some cases. 
• Functional defecation disorders. Difficulty emptying the rectum is detected. Occasionally, this occurs in the context of neurological disorders, spinal cord injuries or anatomical abnormalities, such as rectocele (bulging of the rectum into the vaginal wall) or enterocele (descent of the small intestine into the pelvic cavity). However, in most cases, no structural or medical abnormality is found to justify it.