Pain is a perception that signals the individual that tissue damage has occurred or may be occurring. It is subjective and very complex. The processes in the body that are involved in the perception of pain are called "nociception." Basic and clinical research during the past 50 years has confirmed that there are many mechanisms involved in nociception.
Pain can be "acute" or "chronic." Acute pain lasts a short time, or is expected to be over soon. The time frame may be as brief as seconds or as long as weeks.
Chronic pain may be defined as pain that lasts beyond the healing of an injury, continues for a period of several months or longer, or occurs frequently for at least months.
To develop the best treatment strategies, health care professionals also classify pain based on its characteristics, its cause, or the mechanisms in the body or the mind that are probably involved in sustaining it. One common classification based on mechanisms distinguishes pain into categories called "nociceptive," "neuropathic," and "psychogenic."
Nociceptive Pain: Nociceptive pain is believed to be caused by the ongoing activation of pain receptors in either the surface or deep tissues of the body. There are two types: "somatic" pain and " visceral" pain.
"Somatic" pain is caused by injury to skin, muscles, bone, joint, and connective tissues. Deep somatic pain is usually described as dull or aching, and localized in one area. Somatic pain from injury to the skin or the tissues just below it often is sharper and may have a burning or pricking quality.
Somatic pain often involves inflammation of injured tissue. Although inflammation is a normal response of the body to injury, and is essential for healing, inflammation that does not disappear with time can result in a chronically painful disease. The joint pain caused by rheumatoid arthritis may be considered an example of this type of somatic nociceptive pain.
"Visceral" pain refers to pain that originates from ongoing injury to the internal organs or the tissues that support them. When the injured tissue is a hollow structure, like the intestine or the gall bladder, the pain often is poorly localized and cramping. When the injured structure is not a hollow organ, the pain may be pressure-like, deep, and stabbing.
Neuropathic Pain: Neuropathic pain is believed to be caused by changes in the nervous system that sustain pain even after an injury heals. In most cases, the injury that starts the pain involves the peripheral nerves or the central nervous system itself. It can be associated with trauma or with many different types of diseases, such as diabetes. There are many neuropathic pain syndromes, such as diabetic neuropathy, trigeminal neuralgia, postherpetic neuralgia ("shingles"), post-stroke pain, and complex regional pain syndromes (also called reflex sympathetic dystrophy or "RSD" and causalgia). Some patients who get neuropathic pain describe it as bizarre, unfamiliar pain, which may be burning or like electricity. The pain may be associated with sensitivity of the skin.
Psychogenic Pain: Most patients with chronic pain have some degree of psychological disturbance. Patients may be anxious or depressed, or have trouble coping. Psychological distress may not only be a consequence of the pain, but may also contribute to the pain itself. "Psychogenic" pain is a simple label for all kinds of pain that can be best explained by psychological problems.
This close relationship between pain and psychological distress means that all patients with chronic pain should have an assessment of these psychological factors, and psychological treatments should be considered an important aspect of pain therapy. In some cases, psychological problems appear to be a main cause of the pain. This does not mean that the person is not actually experiencing the pain. Rather, the patient is truly suffering but the main cause somehow relates to the emotions, or to learning, or to some other psychological process. Although doctors sometimes encounter patients who pretend to be in pain (some can be called malingerers), this appears to be a rare occurrence. Most patients with pain that appears to be determined primarily by psychological processes are hurting just like those who have pain associated with a clear injury to the body.
Sometimes, psychogenic pain occurs in the absence of any identifiable disease in the body. More often, there is a physical problem but the psychological cause for the pain is believed to be the major cause for the pain. The various types of psychogenic pain can be diagnosed using a classification developed by psychiatrists (the Diagnostic and Statistical Manual of the American Psychiatric Association).