Low back pain: Where does the pain come from?

The pain mechanisms in patients with low back pain are unclear. Different theories exist about the mechanisms behind the experienced pain, and different originating tissues are suggested to be of importance. The mechanisms in acute low back pain with recovery within a few days are believed to be totally different compared to those in more long-standing (usually referred to as chronic) low back pain. In acute pain, the muscle spasms are, to a high degree, believed to participate in the pain experience; however, it is not clear if the muscle response is a primary or a secondary event. In patients with long-standing low back pain, degeneration of the intervertebral disc is considered to be the most important source of pain. Disc deterioration/ degeneration may influence the nervous system by stimulation of nociceptors in the annulus fibrosus causing nociceptive pain, which is often referred to as discogenic pain. This stimulation of nociceptors may be of mechanical or inflammatory origin. Deterioration of a disc with loss of normal structure and weight bearing properties may lead to abnormal motions causing mechanical stimulation. This theory is supported by the common patient experience of increased pain with weight bearing and certain other movements. In addition, an ingrowth of nerve fibres into deeper layers of the annulus fibrosus has been demonstrated in degenerated discs. A large number of inflammatory and signalling substances have also been suggested to play a role in the development of back pain, for example, tumour necrosis factor (TNF), interleukins (IL-1B, IL-6, IL-8) and nitric oxide. Other spinal structures such as facet joints, ligaments and muscles are also possible pain generators. Disc deterioration can influence these structures negatively, and may thus be responsible for the development of chronic low back pain without being the actual pain focus. In response to stimulation of the nociceptors in the spinal structure, the somatosensory system may increase its sensitivity resulting in a nonfunctional response - normally innocuous stimuli result in an amplified response (peripheral sensitization). In addition, pain can be modulated at higher centres, both at the spinal and the supraspinal level (central sensitization). The altered magnitude of perceived pain is often referred to as neural plasticity and is considered to play a critical role in the evolution of chronic pain. Furthermore, the pain experience in patients with low back pain may also be influenced by psychological aspects. © 2010 Springer-Verlag Berlin Heidelberg.