Pain can be defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Pain can be classified according to the following four characteristics:
- Duration: how long does the pain persist? Is the pain sudden in onset (acute pain e.g. after a bone fracture or a wound) or recurrent (e.g. aspecific low backpain) or continuous over a long period (e.g. chronic pain in cancer or arthritis)?
- Localisation: where is the pain localised? Is it restricted to one specific area (e.g. a small cut in the finger) or is it a regional (e.g. abdominal pain), radiating (pain radiating into one leg in hernia nuclei pulposi) or is it widespread (e.g. fibromyalgia).
- Source: what is the origin of the pain? (The physician himself should try to answer this question.) Is the pain restricted to muscles, joints or skin (e.g. a "somatic" cause like skin burn), or is it possible that the pain originates from organs and tissues (e.g. a "visceral" cause like inflammation of the kidney). The term 'referred pain' describes the situation in which visceral pain is perceived as somatic pain in an embryologically related part of the body (e.g. pain in the left arm during a heart attack).
- Sensation: how does the patient experience the pain? Is the pain sharp, intense and well localised (fast fiber pain) or is the pain deep-aching and poorly localised (slow fiber pain)?
Four phenomena are involved in the final perception of pain:
- Transduction describes the recognition of the stimulus by the nociceptors.
- Transmission is the transport of the signal (action potential) from the nociceptors to the central nervous system.
- Modulation is the changing of the signal by the central nervous system and other activities in the body.
- Perception of pain occurs when the signal is experienced in the brain as pain.
Pain perception occurs in the spinal cord.
Chronic pain can be defined as pain that persists longer than the expected repair time of the original tissue damage.