pain

Pain is an unpleasent sensation. It is defined by the International Association for the Study of Pain (IASP) as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”.
Nociception[1] (sometimes also called nociperception[2]) is a measurable physiological event of a type usually associated with pain.

Scientifically, pain (a subjective experience) is separate and distinct from nociception, the system which carries information about inflammation, damage or near-damage in tissue, to the spinal cord and brain. Nociception frequently occurs without pain being felt and can convey information without conscious awareness. Conversely, but less frequently, a sensation of pain can exist in the absence of nociception.

Pain is part of the body's defense system: it triggers mental problem-solving strategies that seek to end the painful experience, and it promotes learning, making repetition of the painful situation less likely. The nociceptive system transmits signals that usually trigger the sensation of pain, it is a critical component of the body's ability to react to damaging stimuli and it is part of a rapid-warning relay instructing the central nervous system to initiate reactions for minimizing injury.

Intensity
Pain may range in intensity from slight through severe to agonizing. It is experienced as having qualities such as sharp, throbbing, dull, nauseating, burning and shooting. It often has both an emotional quality and a sensed bodily location. Medical professionals will sometimes ask patients to rate their pain on a scale of zero through ten, where ten is consistent with screaming and thrashing about.

Localization

This subjective reality of the localisation of pain to an area of the body is the basis for speaking of pain receptor, neck pain, referred pain, cutaneous pain, as well as pain in my foot, kidney pain, or the painful uterine contractions occurring during childbirth. This common usage of pain is not entirely consistent with the scientists' model of pain being a subjective experience.

Insensitivity to pain
Inability to experience pain, as in the rare condition congenital insensitivity to pain or congenital analgesia, can cause various health problems.

Types of pain

Pain can be classified as acute or chronic. The distinction between acute and chronic pain is not based on its duration of sensation, but rather the nature of the pain itself. In general, physicians are more comfortable treating acute pain, which has as its source soft tissue damage, infection and/or inflammation. It can be modulated and removed by treating its cause and through combined strategies using analgesics to treat the pain and antibiotics to treat the infection. In general, while it is uncomfortable to experience, it is easy to treat; is distinguished by having a specific cause and purpose, and generally produces no persistent psychological reaction. Physicians are more likely to prescribe medications to treat acute pain, particularly in those situations when they are satisfied that they understand the pain's origin and believe the pain will be short in duration. This is why a patient might leave the hospital with two weeks' worth of adequate pain medicine, but the same medications may not be readily prescribed if the patient's pain lasts beyond an expected period of time. It is not the pain itself that is short in duration: it is the diagnosis of "acute pain" and the expectation that it will be short in nature that continues to confuse both the medical establishment and those who experience pain.
The primary distinction is this: acute pain serves to protect one after an injury. Chronic pain does not serve this or any other purpose. Acute pain is the symptom of pain. Chronic pain is the disease of pain.

Chronic Pain

American pain associations estimate that 40-80 million Americans live with chronic pain.[citation needed] At the same time, there are only 8,000 qualified pain management specialists. Many physicians faced with patients who live with chronic pain have had no professional training in pain management. It is not regularly taught in medical school, and even recent legislation in some states to ensure that physicians receive continuing education in pain medicine and end-of-life care do not guarantee proper training in pain. In many states, there remains no legislation ensuring that licensed physicians, even those who work in hospital emergency rooms, have any pain management training whatsoever.
Chronic pain has no time limit, often has no apparent cause and serves no apparent biological purpose. Chronic pain can trigger multiple psychological problems that confound both patient and health care provider, leading to feelings of helplessness and hopelessness. The most common causes of chronic pain include low-back pain, headache, recurrent facial pain, cancer pain, and arthritic pain. Sometimes chronic pain can have a psychosomatic or psychogenic cause.

Chronic pain was originally defined as pain that has lasted 6 months or longer. It is now defined as "the disease of pain." Its origin, duration, intensity, and specific symptoms vary. The one consistent fact of chronic pain is that, as a disease, it cannot be understood in the same terms as acute pain, and the failure to make this distinction (particularly in those who suffer chronic pain) has been and continues to be the cause of multi-dimensional suffering, depression, social isolation, and helplessness. The failure to recognize chronic pain as substantially different from acute pain cannot be blamed on the medical profession: it is a societal lapse.
Chronic pain, no matter how debilitating it is in one's life, continues to be considered by most insurance carriers as a 3-17% disability.

There have been some theories that not treating acute pain properly can lead to chronic pain.[4]
The experience of physiological pain can be grouped according to the source and related nociceptors (pain-detecting neurons).

Cutaneous pain is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a well-defined, localized pain of short duration. Examples of injuries that produce cutaneous pain include paper cuts, minor cuts, minor (first degree) burns and lacerations.
Somatic pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves. It is detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, poorly-localized pain of longer duration than cutaneous pain; examples include sprains and broken bones. Myofascial pain usually is caused by trigger points in muscles, tendons and fascia, and may be local or referred.

Visceral pain originates from body's viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching and of a longer duration than somatic pain. Visceral pain is extremely difficult to localize, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localized to an area completely unrelated to the site of injury. Myocardial ischaemia (the loss of blood flow to a part of the heart muscle tissue) is possibly the best known example of referred pain; the sensation can occur in the upper chest as a restricted feeling, or as an ache in the left shoulder, arm or even hand. "The brain freeze" is another example of referred pain, in which the vagus nerve is cooled by cold inside the throat. Referred pain can be explained by the findings that pain receptors in the viscera also excite spinal cord neurons that are excited by cutaneous tissue. Since the brain normally associates firing of these spinal cord neurons with stimulation of somatic tissues in skin or muscle, pain signals arising from the viscera are interpreted by the brain as originating from the skin. The theory that visceral and somatic pain receptors converge and form synapses on the same spinal cord pain-transmitting neurons is called "Ruch's Hypothesis".

Phantom limb pain, a type of referred pain, is the sensation of pain from a limb that has been lost or from which a person no longer receives physical signals. It is an experience almost universally reported by amputees and quadriplegics.
Neuropathic pain, can occur as a result of injury or disease to the nerve tissue itself. This can disrupt the ability of the sensory nerves to transmit correct information to the thalamus, and hence the brain interprets painful stimuli even though there is no obvious or known physiologic cause for the pain. Neuropathic pain is, as stated above, the disease of pain. It is not the sole definition for chronic pain, but does meet its criteria.

Selected common and serious causes of pain by region
Visceral pain sensation is often referred by the CNS to a dermatome region which may be far away from the originating organ. These correlate to the position of the organ in the embryo. Examples of this include the heart which originates in the neck, thus producing the classical pain and arm pain experienced during acute cardiac pain.

Head and neck
Jaw - Temporal arteritis (serious), trauma
Ear - otitis media (very common esp. in children), otitis externa, trauma
Eye - glaucoma, trauma
Head - migraine, tension headache, cluster headache, cancer, cerebral aneurysm, sinusitis, meningitis
Neck pain - MI (atypical), trauma

Thorax
Back - cancer, also see joints section
Breast - perimenstrual, cancer, trauma
Chest - MI (common and sometimes fatal), GERD (very common), pancreatitis, hiatal hernia, aortic dissection (rare), pulmonary embolism (more frequently asymptomatic), Costochondritis
Shoulder - cholecystitis (right side), MSK

Abdomen
Abdominal
Left and right upper quadrant - peptic ulcer disease, gastroenteritis, hepatitis, pancreatitis, cholecystitis, MI (atypical), abdominal aortic aneurysm, gastric cancer (usually asymptomatic)
Left and right lower quadrant - appendicitis (serious), ectopic pregnancy (serious/women only), endometriosis (women only), pelvic inflammatory disease (women only), diverticulitis (common in the elderly), urolithiasis, pyelonephritis, cancer (colorectal cancer most common)

Back
Back - MSK (muscle strain), cancer, spinal disc herniation, degenerative disc disease, coccyx (coccydynia), also see joints section

Limbs
Arm - myocardial infarction (classically the left arm, sometimes bilateral), musculoskeletal
Leg - deep vein thrombosis, peripheral vascular disease (claudication), musculoskeletal, spinal disc herniation, sciatica

Joints
Classically small joints - osteoarthritis (common in the elderly), rheumatoid arthritis, systemic lupus erythematosis, gout, pseudogout
Classically large joints (hip, knee) - osteoarthritis (common in the elderly), septic arthritis, hemarthrosis, osteonecrosis, trauma
Classically back - ankylosing spondylitis, inflammatory bowel disease
Other - psoriatic arthritis, Reiter's syndrome