Spinal disc herniation
Spinal disc herniation pressing on one of the lumbar or sacral nerve roots is the primary cause of sciatica, being present in about 90% of cases.

Sciatica caused by pressure from a disc herniation and swelling of surrounding tissue can spontaneously subside if the tear in the disc heals and the pulposus extrusion and inflammation cease.
Spinal stenosis
Other compressive spinal causes include lumbar spinal stenosis, a condition in which the spinal canal (the spaces through which the spinal cord runs) narrows and compresses the spinal cord, cauda equina, or sciatic nerve roots. This narrowing can be caused by bone spurs, spondylolisthesis, inflammation, or herniated disc, which decreases available space for the spinal cord, thus pinching and irritating nerves from the spinal cord that travel to the sciatic nerves.
Piriformis syndrome
Piriformis syndrome is a controversial condition that, depending on the analysis, varies from a “very rare” cause to contributing to up to 8% of low back or buttock pain.[7] In 15% of the population, the sciatic nerve runs through, or under the piriformis muscle rather than beneath it. When the muscle shortens or spasms due to trauma or overuse, it’s posited that this causes compression of the sciatic nerve. It has colloquially been referred to as “wallet sciatica” since a wallet carried in a rear hip pocket will compress the muscles of the buttocks and sciatic nerve when the bearer sits down. Piriformis syndrome may be a cause of sciatica when the nerve root is normal.
Pregnancy
Sciatica may also occur during pregnancy as a result of the weight of the fetus pressing on the sciatic nerve during sitting or during leg spasms. While most cases do not directly harm the fetus or the mother, indirect harm may come from the numbing effect on the legs which can cause loss of balance and falling. There is no standard treatment for pregnancy induced sciatica.
Other
Sciatica can also be caused by tumours impinging on the spinal cord or the nerve roots.[5] Severe back pain extending to the hips and feet, loss of bladder or bowel control, or muscle weakness may result from spinal tumours or cauda equina syndrome. Trauma to the spine, such as from a car accident, may also lead to sciatica.

Pathophysiology

Sciatica is generally caused by the compression of lumbar nerves L3, L4, or L5 or sacral nerves S1, S2, or S3, or by compression of the sciatic nerve itself. When sciatica is caused by compression of a dorsal nerve root (radix), it is considered a lumbar radiculopathy (or radiculitis when accompanied with an inflammatory response). This can occur as a result of a spinal disk bulge or spinal disc herniation (a herniated intervertebral disc), or from roughening, enlarging, or misalignment (spondylolisthesis) of the vertebrae, or as a result of degenerated discs that can reduce the diameter of the lateral foramen (natural hole) through which nerve roots exit the spine. The intervertebral discs consist of an annulus fibrosus, which forms a ring surrounding the inner nucleus pulposus. When there is a tear in the annulus fibrosus, the nucleus pulposus (pulp) may extrude through the tear and press against spinal nerves within the spinal cord, cauda equina, or exiting nerve roots, causing inflammation, numbness, or excruciating pain. Inflammation in the spinal canal can also spread to adjacent facet joints and cause lower back pain and/or referred pain in the posterior thigh(s). Pseudosciatic pain can also be caused by compression of peripheral sections of the nerve, usually from soft tissue tension in the piriformis or related muscles.
The spinal discs are composed of a tough spongiform ring of cartilage (“annulus fibrosus”) with a more malleable center (“nucleus pulposus”). The discs separate the vertebrae, thereby allowing room for the nerve roots to properly exit through the spaces between the vertebrae. The discs cushion the spine from compressive forces, but are weak to pressure applied during rotational movements. That is why a person who bends to one side, at a bad angle to pick something up, may more likely herniate a spinal disc than a person jumping from a ladder and landing on his or her feet.
Herniation of a disc occurs when the liquid center of the disc bulges outwards, tearing the external ring of fibers, extrudes into the spinal canal, and compresses a nerve root against the lamina or pedicle of a vertebra, thus causing sciatica. This extruded liquid from the “nucleus pulposus” may cause inflammation and swelling of surrounding tissue, which may cause further compression of the nerve root in the confined space in the spinal canal.

Diagnosis

Sciatica is typically diagnosed by physical examination, and the history of the symptoms. Generally if a person reports the typical radiating pain in one leg as well as one or more neurological indications of nerve root tension or neurological deficit, sciatica can be diagnosed.
The most applied diagnostic test is the straight leg raise to produce Lasègue’s sign, which is considered positive if pain in the distribution of the sciatic nerve is reproduced with between 30 and 70 degrees passive flexion of the straight leg.[12] While this test is positive in about 90% of people with sciatica, approximately 75% of people with a positive test do not have sciatica.
Imaging tests such as computerised tomography or magnetic resonance imaging can help with the diagnosis of lumbar disc herniation. The utility of MR neurography in the diagnoses of piriformis syndrome is controversial.

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