Corticosteroid injections by epidural or root blocks are commonly used to treat leg and back pain and may indicate that in some cases there may be an inflammatory contribution to the pain. In disc material removed from humans during operation has been found heightened levels of inflammatory chemicals which could be affected by steroid use. These inflammatory chemicals may increase degeneration of discs and sensitise the fibres of the outer disc walls. Some chemicals may also sensitise the discs by elevating reactivity of sensory nerves, inhibit the creation of new proteoglycans which hold water in the discs and may contribute to disc degeneration.
The nerve structures which are part of the lumbar motion segment can be sensitised and so become hyper reactive, leading to mechanical stresses which normally would be painless becoming painful. Nerve root or radicular pain is the pain caused by compression, irritation or inflammation of one of the spinal nerves at or close to the area they emerge from the spinal cord. It is not clear exactly what is the pathology underlying severe sciatica, although hypotheses include compression plus dysfunction of the axons, ischaemia, inflammation and biochemical factors. Spinal nerve roots may be more susceptible to compression injuries and to swelling formation inside their structure than other nerves.
Compression of the nerve root may allow increased inflow of fluids to the nerve, resulting in oedema of the nerve which gives a higher internal fluid pressure. This can compromise the flow through local capillaries and cause the nerve to become fibrosed. Since around sixty percent of the nutrition of the spinal nerve roots is provided by the cerebrospinal fluid, any fibrosis of the nerves can reduce this and make them more liable to suffer symptoms of compression due to their increased sensitivity. Pain could be generated by the nerve alterations caused by slowing or interruption of the blood throughput, rather than the sensory and motor loss following peripheral nerve compression.
If the nerve and its vascular supply are compromised quickly then nerve root symptoms are more likely than with a slow onset, perhaps giving a clue to why older people do not suffer such symptoms despite compromise of the nerve exits. Disc material has been experimentally placed in the epidural space of dogs and found to generate a significant inflammatory reaction and may cause damage to nerves when in contact with them. If TNF (tumour necrosis factor) is injected into a nerve it causes similar changes to those which ensue after nerve root compromise. An auto-immune reaction to the internal disc material, which is not normally accessible, has also been proposed but it is likely that many aspects are relevant.
Facet shaped articular surfaces from a vertebra above and one below form the spinal facet joints, which are synovial joints like many others. Our facet joints respond to injury and damage in the same way as our other joints and they can complain of loss of motion, generate muscle spasm, cause pain and stiffness and ultimately suffer degenerative changes. The facets follow the degenerative cascade which initially occurs in the intervertebral disc joints. The capsules of the joints are fibrous bags with a rich nerve supply and in research have been shown to respond to injections into them by referring pains to the legs, arms or upper back depending on their position. Studies have shown significant contributions of these joints to thoracic, lumbar and neck pain.
The sacroiliac joint, a complex joint between the sacrum and the ilium, is known to refer pain to areas over the joint, in the buttock, the lower lumbar spine, the leg and the groin. Experimental work with patients with chronic back pain has shown a variable involvement of the sacroiliac joint in from 2 to 30% of cases. Muscles also have pain receptors which are sensitive to pressure and stretching amongst other stimuli. If they are overloaded either in one instance or repeatedly then injury can occur with pain, shortening and increased tension.
A described muscle pain problem is myofascial pain syndrome, in which muscles are regularly used incorrectly or have been damaged develop tightness and trigger points. Trigger points can be palpated in the muscles as tender and firm tissue areas which then refer pain out into well known nearby regions.
