Pain affecting our 'soft' organs and body tissues, or viscera, is extremely common and can be agonizing. Injury and inflammation can be particularly problematic, as organs become highly sensitive to any kind of stimulation, as in inflammatory bowel disease and other disorders.
All of us have experienced, at one time or another, pain from our internal organs, from the mild discomfort of indigestion to the agony of a renal colic. Many forms of visceral pain are particularly prevalent in women and are associated with their reproductive life (period pains, labour pain or postmenopausal pelvic pain) and for both men and women, pain of internal origin is the number one reason to consult a doctor.
Only a minority of people will suffer from neuropathic or even post-traumatic pain but all of us will endure throughout our lives a great deal of visceral pain.
Until recently visceral pain was not considered to be a major problem by the very specialists that dealt with it. Obstetricians, gynaecologists, cardiologists, gastroenterologists and urologists were mainly concerned with the diagnosis and treatment of the underlying disease, and their approach was to assume that if the disease went away so would the pain. Only recently, and mainly because of popular pressure, has pain become a subject that can be treated directly and independently of the accompanying disease as doctors realize that this 'symptom' is often the very centre of the problem.
A strange pain
On the other hand, relatively minor lesions in viscera such as the stomach, the bladder or the ureters can produce excruciating pain. There is no close relationship between damage and pain like that seen when the lesions affect a somatic organ.
The reasons for this strange situation lie with the innervation of the internal organs. Some viscera are innervated by sensory neurons that signal harmful events (nociceptors) but other internal organs lack this form of sensor, so that injuries or lesions to these organs cannot be translated into signals that the brain would perceive as painful.
The internal organs with nociceptors are mostly the hollow viscera (the gut, the bladder, the uterus) and it is from these organs that we get most of our visceral pain sensations. The insides of these organs are, in effect, an extension of the external environment so these organs are in contact with potentially harmful agents. They therefore need to be protected by pain mechanisms.
Visceral nociceptors are very similar to those that innervate the skin or muscle. They respond not only to intense mechanical stimuli (distension and overstretching) but also to irritant chemicals and specially to the products of inflammation. Some visceral nociceptors become active only after inflammation of the mucosa of the organs that they innervate. They are particularly important in signalling pain from inflamed and sensitized viscera.
Many people know that cardiac ischaemia produces pain in the left part of the chest and even in the left arm and hand. This is referred cardiac pain, a sensation felt in an otherwise normal part of the body but that it is due to a poor oxygen supply to the heart.
Similar patterns of referred pain can be detected in diseases of the gut, the bladder or the internal genital organs, where the pain is felt in the abdomen, the pelvic region or the back, with the patient not being able to locate the pain very accurately.
The reason for the 'referral' of visceral pain is the lack of a dedicated sensory pathway in the brain for information concerning the internal organs. The sensory neurons from the viscera connect within the brain with sensory pathways that carry information from the skin and muscles, and the brain interprets the signals that originate from internal organs as coming from the overlying skin or muscles. This is known as 'viscero-somatic convergence' and it is thought to be the neural basis for referred visceral pain.
However, recent studies using brain imaging have shown that the areas of the brain activated by painful visceral stimuli are not exactly coincidental with those turned on during somatic pain. Although viscero-somatic convergence may underlie referred pain, there are also other factors involved in the integration of sensory information from internal organs. [see also Mapping pain in the brain]
The increased sensitivity of the viscera after inflammation has two causes:
Both processes are known as 'sensitization' either peripherally (in the viscera) or centrally (in the brain) and are thought to be responsible not only for the pain produced by the inflammatory disease but also for hyperalgesic sensations that can occur in the absence of an identifiable cause, such as pain in conditions like irritable bowel syndrome.
This process of sensitization is currently the subject of a great deal of research, to identify its molecular basis and to find ways to restore normal sensitivity to the distorted system. The aim is to reduce hyperalgesic sensations caused by the regular functioning of internal organs without interfering with the normal sensitivity of the viscera or with the digestive, secretory or reproductive functions of the organ.
Unfortunately, we have very few specific painkillers for visceral pain, and the therapies commonly used are extensions of those used for pain in general. Because of the prevalence of visceral pain, there is a great need for therapies aimed specifically at the conditions that cause the pain. This is particularly the case for diseases characterized by visceral hypersensitivity (such as irritable bowel syndrome), in which the therapeutic aim should be to reduce the increased sensations felt from the bowel without damping sensation in general or impairing the ability of the patient to live a normal life.
We may not be able to have a completely pain-free world, but we are trying to reduce the suffering of many men and women who every day face pains that come not from outside but from inside their own bodies.
Professor Fernando Cervero is Director of the Anesthesia Research Unit, McGill University, Montreal, Quebec H3G 1Y6, Canada. E-mail: firstname.lastname@example.org
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