Visceral hypersensitivity
Visceral hypersensitivity = the gut is more sensitive to even normal sensations in the gut. Pain is felt when the sensation shouldn’t be painful, for example a normal amount of gas can cause pain or the feeling of bloating. Mild pain can also feel much worse than it truly is. The cause is not well understood, but likely involves inflammation and increased gut permeability, and sometimes gut infection.
Abdominal pain is a hallmark symptom of IBS. The reason for this is probably visceral hypersensitivity, which means that people who have IBS are more sensitive to the sensations in the gut.
In IBS, pain is visceral: pain happens in the hollow organs of the abdomen, such as the intestines, and it is typically felt in the midline of the body (upper abdomen, around the belly button, or in the lower abdomen) and it can actually be difficult to pinpoint where exactly the pain is located (3). What happens, is that the pain receptors in the gut respond to something going on in the gut and tell the brain that there is a problem, which is then felt as pain. The stimulus could be inflammation, irritants/chemicals, stretching (from food or gas), or contractions of the gut. In IBS this stimulus is often something that wouldn’t normally cause pain.
It is suggested that if pain starts within 5-30 minutes after eating, it is because of signaling to the large intestine to increase contractions. The gastrocolic reflex is an example. If the pain happens 4-6 hours after eating, it is more likely to be a direct effect of food stretching the large intestinal walls, as in this time food from the meal is entering the large intestine. If the pain happens 30 minutes-4 hours after eating, the cause could be in the small intestine (4). It might be hard to know sometimes which meal the gut is responding to, since ideally we should eat about every 4 hours.
In visceral hypersensitivity, stimulation of pain receptors in the gut that normally wouldn’t cause pain, is felt as pain. Pain may be felt more easily (a lower pain threshold), the pain may feel worse than it should, and the pain sensation may last longer than normal. Why this happens in IBS is not well understood, but it is thought that hypersensitivity follows inflammation, infection or greater gut permeability, with mast cells and stress potentially being part of the mechanism (both IBS-C and IBS-D). (5).
In IBS this hypersensitivity continues after the original cause of pain has already gone away, one possible reason being stress-based inflammation (6) that keeps the nerve-endings that sense pain activated. In addition to abdominal pain, visceral hypersensitivity can also lead to bloating and the feeling of urgency in IBS. (7)
The central nervous system (brain and spinal cord), where pain signals are carried to and interpreted, also plays a role in causing pain to become chronic. This is called central sensitization – the brain thinks that a normal sensation is pain, or the pain signal gets interpreted as much worse than it is. Central desensitization can also increase anxiety, and stress and poor sleep can lower pain threshold (7). So, the pain receptors in both the gut and the brain/spinal cord can become overly sensitive.
A remedy for hypersensitivity is to calm down the receptors of the gut and the brain. This can be achieved by utilizing stress release methods and calming the vagus nerve (the information highway between the gut and the brain).