Health

Disc Herniation

By far, the most common cause of sciatica is lumbar disc herniation.

Lumbar intervertebral discs are susceptible to herniation because they are subject to great forces during the following activities:

• Lifting
• Flexion combined with extension of the trunk
• Twisting.

Because discs lie in close proximity to nerves as they exit the spinal cord, disc herniations can cause compression of nerve roots. Over 95% of lumbar disc herniations occur at L4-5 and L5-S1 levels. Discs usually herniate posteriorly and laterally (i.e., to the back and side) because this is where the annulus fibrosus is weakest.

There are different types of disc herniations. Disc protrusion describes a herniation in which the disc fragments are contained by the annulus fibrosus, which contains an incomplete defect. In disc extrusion, the herniation is not contained by the annulus fibrosus due to a complete defect in the annulus. A sequestered disc is a disc from which extruded nucleus pulposus has lost contact with the non-displaced nucleus pulposus.

The nerve roots do not tolerate compression well. They lack two of the protective coverings that nerves elsewhere in the body have, called epineurium and perineurium. They are also tethered to the vertebral body, such that they may become stretched if pushed upon by a herniated disc. Nerve compression alone may not be sufficient to cause symptoms of sciatica. "Bioactive molecules" also appear to play a role in creating the discomfort associated with sciatica. Bioactive molecules are proteins that generate inflammation in the body. Examples include interleukins, TNFα, phospholipase A2, nitric oxide, and prostaglandin E. Inflammatory agents such as these may be necessary to sensitize the nerve root to compression by a herniated disc, resulting in symptoms of sciatica.           

Risk factors for developing a herniated disc include a sedentary lifestyle, frequent driving, chronic cough, pregnancy, smoking, and frequent heavy lifting. Participation in some sports, including dancing, golf, football, and weight lifting, has also been associated with disc herniation.  

Piriformis Syndrome

A less common cause of sciatic symptoms is called piriformis syndrome. The piriformis is a small muscle that lies deep in the buttocks, under the gluteal muscles. It works to turn the leg outward. As stated above, the sciatic nerve is formed by nerve roots that join together in the lumbosacral plexus. As the sciatic nerve dives down the buttock to the leg, it usually passes below the piriformis muscle. In approximately 20% of people, the sciatic nerve actually pierces the piriformis muscle. It is this close association that seems to make the sciatic nerve susceptible to irritation by the piriformis muscle. The pain associated with piriformis syndrome is poorly understood but may be related to nerve irritation or muscle spasm. Piriformis syndrome causing sciatic nerve pain is relatively rare.

Other causes of Sciatica

Rarely, other processes can cause symptoms of sciatica. Bony abnormalities can also impinge upon nerve roots, as can tumors, abscesses, cysts, and hematomas. Some illnesses can cause sciatica symptoms, e.g., HIV/AIDS and Lyme disease.  For this reason it is important that some patients seek medical attention if symptoms of sciatica develop. This includes patients who:

• are older than 50
• have a fever
• have a history of significant trauma
• carry a history of cancer
• have pain while lying down
• have unexplained weight loss
• have a history of alcohol and drug abuse.