​The IASP (International Association for the Study of Pain) defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage …”. Unlike acute pain that serves as a “fifth vital sign” to warn of an injury or a disease, chronic or persistent pain has no purpose. There is no collective agreement on the definition of chronic pain but it commonly refers to pain that serves no biological value, lasts longer than the usual healing time, has no or minimal response to specific treatments or remedies, and is of a duration greater than 6 months.

Chronic pain can be severe and may bring about psychological distress, job loss, social isolation, and is highly comorbid with depression and anxiety. Additionally, in a general hospital setting, many medically unexplained pains can now be better understood to involve an interplay between neurophysiological mechanisms and psychological factors that may have gone awry.

SSD or Somatic Symptoms Disorder is a single diagnostic entity that has now replaced Pain Disorder, previously found in the DSM-IV (Diagnostic and Statistical Manual, 4th edition) somatoform disorders category where other diagnoses such as somatization disorder, undifferentiated somatoform disorder, hypochondriasis are grouped.