Understanding pain

The International Association for the Study of Pain (IASP) classification system describes pain according to five categories: duration and severity, anatomical location, body system involved, cause, and temporal characteristics (intermittent, constant, etc.).This system has been criticized by Woolf and others as inadequate for guiding research and treatment,] and an additional category based on neurochemical mechanism has been proposed.
Duration
Main article: Chronic pain

Pain is usually transitory, lasting only until the noxious stimulus is removed or the underlying damage or pathology has healed, but some painful conditions, such as rheumatoid arthritis, peripheral neuropathy, cancer and idiopathic pain, may persist for years. Pain that lasts a long time is called chronic, and pain that resolves quickly is called acute. Traditionally, the distinction between acute and chronic pain has relied upon an arbitrary interval of time from onset; the two most commonly used markers being 3 months and 6 months since the onset of pain, though some theorists and researchers have placed the transition from acute to chronic pain at 12 months.Others apply acute to pain that lasts less than 30 days, chronic to pain of more than six months duration, and subacute to pain that lasts from one to six months.A popular alternative definition of chronic pain, involving no arbitrarily fixed durations is "pain that extends beyond the expected period of healing."[10] Chronic pain may be classified as "malignant" (caused by cancer) or "benign" (non-malignant).
Region and system

Pain can be classed according to its location in the body, as in headache, low back pain and pelvic pain; or according to the body system involved, such as myofascial pain (emanating from skeletal muscles or the fibrous sheath surrounding them), rheumatic pain (emanating from the joints and surrounding tissue), neuropathic pain (caused by damage or illness affecting the somatosensory system), or vascular (pain from blood vessels).
Cause

The crudest example of classification by cause simply distinguishes "somatogenic" pain (arising from a perturbation of the body) from psychogenic pain (arising from a perturbation of the mind: when a thorough physical exam, imaging, and laboratory tests fail to detect the cause of pain, it is assumed to be the product of psychic conflict or psychopathology).Somatogenic pain is divided into "nociceptive" and "neuropathic".
Nociceptive

Nociceptive pain is caused by stimulation of peripheral nerve fibers that respond only to stimuli approaching or exceeding harmful intensity (nociceptors), and may be classified according to the mode of noxious stimulation; the most common categories being "thermal" (heat or cold), "mechanical" (crushing, tearing, etc.) and "chemical" (iodine in a cut, chili powder in the eyes).

Nociceptive pain may also be divided into "visceral," "deep somatic" and "superficial somatic" pain. Visceral pain originates in the viscera (organs) and often is extremely difficult to locate, and nociception from some visceral regions may produce "referred" pain, where the sensation is located in an area distant from the site of the stimulus. Deep somatic pain is initiated by stimulation of nociceptors in ligaments, tendons, bones, blood vessels, fasciae and muscles, and is dull, aching, poorly-localized pain. Examples include sprains and broken bones. Superficial pain is initiated by activation of nociceptors in the skin or other superficial tissue, and is sharp, well-defined and clearly located. Examples of injuries that produce superficial somatic pain include minor wounds and minor (first degree) burns
Neuropathic
Main article: Neuropathic pain

Neuropathic pain is caused by damage or disease affecting the central or peripheral portions of the nervous system involved in bodily feelings (the somatosensory system).Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.”Bumping the "funny bone" elicits peripheral neuropathic pain.
Phantom
Main article: Phantom pain

Phantom pain is pain from a part of the body that has been lost or from which the brain no longer receives signals. It is a type of neuropathic pain. Phantom limb pain is a common experience of amputees.

The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%.One study found that eight days after amputation, 72 percent of patients had phantom limb pain, and six months later, 65 percent reported it.Some amputees experience continuous pain that varies in intensity or quality; others experience several bouts a day, or it may occur only once every week or two. It is often described as shooting, crushing, burning or cramping. If the pain is continuous for a long period, parts of the intact body may become sensitized, so that touching them evokes pain in the phantom limb, or phantom limb pain may accompany urination or defecation

Local anesthetic injections into the nerves or sensitive areas of the stump may relieve pain for days, weeks or, sometimes permanently, despite the drug wearing off in a matter of hours; and small injections of hypertonic saline into the soft tissue between vertebrae produces local pain that radiates into the phantom limb for ten minutes or so and may be followed by hours, weeks or even longer of partial or total relief from phantom pain. Vigorous vibration or electrical stimulation of the stump, or current from electrodes surgically implanted onto the spinal cord all produce relief in some patients.

Work by Vilayanur S. Ramachandran using mirror box therapy allows for illusions of movement and touch in a phantom limb which in turn cause a reduction in pain.

Paraplegia, the loss of sensation and voluntary motor control after serious spinal cord damage, may be accompanied by girdle pain at the level of the spinal cord damage, visceral pain evoked by a filling bladder or bowel, or, in five to ten per cent of paraplegics, phantom body pain in areas of complete sensory loss. This phantom body pain is initially described as burning or tingling but may evolve into severe crushing or pinching pain, fire running down the legs, or a knife twisting in the flesh. Onset may be immediate or may not occur until years after the disabling injury. Surgical treatment rarely provides lasting relief.
Psychogenic
Main article: Psychogenic pain

Psychogenic pain, also called psychalgia or somatoform pain, is pain caused, increased, or prolonged by mental, emotional, or behavioral factors.[21] Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic.[22] Sufferers are often stigmatized, because both medical professionals and the general public tend to think that pain from a psychological source is not "real". However, specialists consider that it is no less actual or hurtful than pain from any other source.

People with long term pain frequently display psychological disturbance, with elevated scores on the Minnesota Multiphasic Personality Inventory scales of hysteria, depression and hypochondriasis (the "neurotic triad"). Some investigators have argued that it is this neuroticism that causes acute injuries to turn chronic, but clinical evidence points the other way, to chronic pain causing neuroticism. When long term pain is relieved by therapeutic intervention, scores on the neurotic triad and anxiety fall, often to normal levels. Self-esteem, often low in chronic pain patients, also shows improvement once pain has resolved.

“The term ‘psychogenic’ assumes that medical diagnosis is so perfect that all organic causes of pain can be detected; regrettably, we are far from such infallibility… All too often, the diagnosis of neurosis as the cause of pain hides our ignorance of many aspects of pain medicine.”
— Ronald Melzack, 1996.