Pain is a sensation of discomfort that can vary from being mild to moderate or severe. Apart from the suffering of the patient, pain is a warning signal that some part of the body is undergoing damage and appropriate action, where possible, needs to be taken. Therefore pain is a protective mechanism. Pain itself is a complex process but can be divided into nociceptive and non-nociceptive pain. Nociceptive pain is due to stimulation of nocicpetors (pain receptors) which are actually free nerve endings of a special type of nerve known as nociceptive neurons. These pain receptors can be stimulated by mechanical, thermal and chemical damage. Non-nociceptive pain does not arise specifically from these pain receptors and may be associated with some other injury, abnormality or dysfunction of nerves.
What is Visceral Pain?
Visceral pain is pain that originates from an organ within the body. The organs in the body lack other types of receptors for the different stimuli like touch and heat. This allows the organs to continue with various involuntary activities without the person being aware of it. However, pain receptors are present in almost every organ besides the brain, deep tissue of the liver and lungs.
Visceral pain is often more gradual in onset, progresses in severity over time and tends to last longer. This is simply known as slow pain which is contrast to the rapid onset, excruciating pain that starts within seconds of injury in parietal and somatic pain described below (fast pain). This is possibly due to the type of pain nerve fibers in these organs. Generally visceral pain is described as dull and aching in contrast to the sharp and severe pain with parietal ad somatic pain.
Visceral Pain vs Parietal Pain
Many organs in the body also have an outer lining or covering that is not part of the organ itself, or at least one layer when multi-layered, not being part attached closely to the organ. For example, the lining around the heart (pericardium), lungs (pleura), brain (meninges) and certain abdominal organs (peritoneum). These outer linings are extremely sensitive to pain and this type of pain is known as parietal pain. Despite this differentiation, parietal pain is often considered under visceral pain and pain emanating within the organ itself is sometimes commonly referred to as “true” visceral pain.
Parietal pain is very intense and easy to localize when compared to “true” visceral pain. As explained above the difference may lie in the type of nerve fiber and parietal pain is therefore a fast pain. The difficulty in locating visceral pain may be due to the transmission of pain signals from within the organ through the autonomic nerve bundles often leading to referred pain to surface areas of the body. With parietal pain transmission, the signals are sent directly into the local spinal nerves.
Visceral Pain vs Somatic Pain
The term somatic pain refers to pain from the skin, muscles, joints and bones. It can be divided into deep and superficial somatic pain, it is easier to locate and usually more intense than visceral pain. Most of the time the pain that we perceive in everyday life is somatic pain, often associated with superficial injury to the skin surface, muscle strain and impact on the joints. Generally a person is more accustomed to somatic pain and can easily identify its origin and take appropriate action where possible to remove the offending agent or at least seek medical treatment and report the location and nature of the pain to a doctor.
This, however, differs with visceral pain. It may sometimes not be as obvious a sensation as somatic pain, is difficult to isolate and sometimes mistaken for other sensations for which the organ does not have the appropriate receptors. It may be referred to other sites and prove confusing both to the doctor and patient, especially if there are no other clinical features that indicate the origin of the abnormality. The exception though is with parietal pain which is well localized due to the transmission of signals directly into local spinal nerves.
Causes of Visceral Pain
As previously mentioned, pain itself is complex process based on several theories, the most widely accepted of which has been integrated with more modern findings. Simply this gate control theory of pain proposes that the transmission of pain signals is constantly blocked until signals from pain receptors can disable the blocking mechanism and allow pain signals to be relayed to the brain. These signals then activate certain centers in the cerebral cortex and the sensation of pain is perceived.
Visceral pain is associated with tissue damage within the organ. Pain is a key feature of inflammation. Certain chemical mediators of inflammation may irritate nerve endings and the swelling seen with inflammation also compresses the surrounding tissue. Injury therefore has to be significant enough to elicit inflammation for pain to be experienced. This may be related to :
- Stretching or distension of the organ, particularly hollow cavities like the gastrointestinal tract. Normally these organs can stretch, often significantly compared to its smallest size, but will lead to pain if overdistended. This can be further compounded if the distended organ compresses surrounding organs, nerves, blood vessels and other structures. Distension may also collapse the blood vessels supplying the organs thereby starving the tissue of oxygenated blood and further contributing to tissue damage (ischemia).
- Ischemia is the damage of tissue associated with an interruption in its blood supply. Disruption of the oxygen supply changes the normal biochemical pathways and some of the metabolites from these processes may cause inflammation or directly irritate nerve endings.
- Cramping is due to spasm of the smooth muscle within an organ. Various organs throughout the body have smooth muscle in its walls. Pain associated with spasm may be due to compression of the nerve endings itself or disruption of the blood supply thereby leading to ischemic pain.
- Chemical injury is more often associated with the gastrointestinal tract and its digestive enzymes. If these enzymes pass into a portion of the gut that is not equipped to deal with it, or if it leaks out of the gut, it can cause significant damage to the tissue. Visceral pain may also arise in other hollow organs if noxious substances are delivered to the site, either by ingestion, injection or through other invasive procedures.
- Mechanical injury rarely arises in an organ without first penetrating the outer layers and thereby eliciting somatic and/or parietal pain. It may happen with calculi (stones), malignancies that invade surrounding tissue and other disturbances in growth and structure but this usually arises internally. However, with modern diagnostic techniques using sound and electromagnetism and surgery, it is possible to reach deeper sites from the exterior and cause organ damage without eliciting pain superficially.