Sciatica – Causes
The Sciatic nerve is formed by the collection of nerve roots L4, L5, S1, S2, and S3. The nerve roots arise from the distal part of the spinal cord. The roots exit the vertebral column through neural foramen.
The dural sac containing all the nerve roots from cauda equina lie inside the central canal. The central canal is bound by the vertebral body and the intervening intervertebral disc in the front and the lamina in the back. The facet joints are present near the neural foramen on the sides of the vertebral canal.
Any pathology leading to compression or irritation of the nerve roots L4 to S3 may cause symptoms of Sciatica. The symptoms may be acute and may last for a few days to weeks or may progressively worsen.
Herniated Intervertebral Disc
The herniation of intervertebral discs is the most common cause of sciatica symptoms. A bulging or herniated disc may press the nerve roots forming the Sciatic nerve.
The herniated disc may protrude and cause compression. The protrusion happens when the inner soft gel material is contained within the outer covering. In some cases there may be disc extrusion or sequestration. In disc extrusion, the inner nucleus pulposus herniates through the outer annulus but is still contained inside the disc space. In disc sequestration, the herniated disc material loses contact with the parent disc.
Furthermore, the symptoms depend upon the location of the herniated disc. In central disc herniation, the entire dural sac containing the nerve roots may be compressed. In cases where the herniation is paracentral or foraminal the disc material presses upon the traversing and the exiting nerve roots respectively.
Disc herniations may either be caused by age-related wear and tear of the spine known as degenerative disc disease. In the younger age group, the herniation may be a result of sudden trauma which may be sustained in a fall from height or in a motor vehicle accident. Disc herniations may also occur in patients involved in repetitive motion in the lower back.
The degenerative disc disease is caused due to loss of water content of the disc with growing age. The loss of water makes the disc less flexible and more prone to tears with day to day activities. Patients who are overweight, obese, smokers, and those participating in track and field sports are especially vulnerable.
Lumbar canal stenosis
The degenerative disc disease may lead to accelerated degeneration of the facet joints. There may be development of bone spurs and ossification of the ligaments in the vertebral column. The overgrowth of bone and calcification of ligaments lead to decreased space in the already precarious space available for nerves causing lumbar canal stenosis.
The symptoms get worse on bending backward or walking downhill as the ligament being the dural sac buckles to press on the sac. The neural foramen may also be stenosed causing nerve root compression as they exit the foramen.
Trauma and Tumors
Direct trauma to the nerve roots from accidental falls and motor vehicle accidents may cause Sciatica. There may be injury to the nerve roots from traumatic hip dislocations or inadvertently during surgery.
Rarely, tumors of the vertebral column and the spine in the lumbar region may encroach upon the nerve roots forming the Sciatic nerve. The compression of nerve roots by the tumor growth causes symptoms of Sciatica.
Malignant tumors of organs such as the prostate or breast may metastasize to the lumbar vertebrae and cause compression of the nerve roots. Primary spine tumors in the lumbar region such as bone cysts and osteoid osteoma may rarely cause Sciatica.
Spondylolysis is the break in the pars interarticularis that may cause the vertebrae to displace. The malpositioned vertebrae may narrow the space available for the dural sac and the nerve roots and cause symptoms of Sciatica.
Spondylolisthesis may result from degenerative changes in the spine, a traumatic or repetitive activity causing break of pars, or rarely a patient may be born with spondylolisthesis.
A number of different conditions may cause Sciatica. The nerve may get entrapped in there piriform muscle as it exits the pelvic region. The nerve roots may also be irritated by infection of any structures close to them. Additionally pregnant females may experience Sciatica as the fetus grows or shifts. Inadvertent gluteal injections related to Sciatic nerve injury may also result in the symptoms of Sciatica.