When pain lasts for a long time, it is considered to be chronic pain. Many physicians consider pain to be chronic when it has lasted for six months or longer.
Others say that pain is chronic when it lasts one month longer than would generally be expected considering the injury, surgery, or disease that is causing it.
To understand chronic pain, it can be helpful to categorize pain in general. Basically, there are two types of pain: nociceptive and neuropathic.
Nociceptive pain is caused when special nerve endings—called nociceptors—are activated. This type of pain results from an injury to the body (such as a cut or burn), surgery, or a disease that is not a part of the nervous system (like arthritis or cancer). Pain from the activation of nociceptors depends on the parts of the body involved. It can be felt as a localized sharp, aching, or throbbing pain that is constant, or it can be a generalized deep, aching pain that comes and goes.
Neuropathic pain is caused by a malfunction of the nervous system due to injury, disease, or trauma. It can be sharp, intense, and constant, usually felt as a burning, shooting, or tingling pain; it can also be sporadic and felt as a dull, aching, and throbbing pain (for example, the chronic pain that people experience in their lower backs, upper backs, and legs is usually of this type). Neuropathic pain is divided into two categories: simple and complex. Simple neuropathic pain usually involves a single extremity such as an arm or leg, while complex neuropathic pain usually involves multiple extremities and has the possibility of spreading.
Because chronic pain can arise spontaneously without a known cause and can vary widely in intensity, location, and response to therapy, treating it successfully can be a major challenge. Not treating it or under-treating it, however, can be devastating.
Source: St. Jude Medical