Sciatica may be caused by the compression of a nerve root in the lower (lumbar) spine, a condition known as radiculopathy. The most common cause of such a compression is a herniated disc in the lower back.

The intervertebral discs are pads made up of cartilage that separate the spinal bones (vertebrae). Each disc contains a jelly-like substance surrounded by a tough, fibrous covering. When this covering tears because of injury, illness or deterioration due to aging, it can allow the jelly-like interior to seep out or rupture. This substance can then cause pressure and swelling of the nerve root, causing back pain or leg pain that is often excruciating.

When a damaged disc is located in the lower part of the back, it may cause numbness, tingling or weakness in the buttock, leg or foot.

In addition to herniated discs, other conditions can also pressure the sciatic nerve. These include:

• Spinal stenosis. A condition in which one or more areas of the spinal canal narrow, causing pressure to build on the spinal cord or on the roots of nerves that branch off from the spinal cord. When this narrowing occurs in the lower spine, the lumbar and sacral nerve roots are sometimes affected, potentially resulting in sciatica.
  
  
• Degenerative disc disease. Age-related condition in which the intervertebral discs wear down, causing compression of sciatic or other spinal nerve roots.
  
  
• Spondylolisthesis. Occurs when a vertebra slips slightly forward over another vertebra. The displaced bone may pinch a sciatic nerve at the point where it leaves the spine. Causes of spondylolisthesis include degenerative disc disease and spondylolysis, a type of stress fracture in a lumbar vertebra that is common in school-age athletes.
  
  
• Pregnancy. The growing fetus can press against one or both of the mother’s sciatic nerves.
  
  
• Piriformis syndrome. Disorder where the piriformis muscle (which runs directly above the sciatic nerve) becomes tight or spasms, putting pressure on the sciatic nerve. Runners and walkers most often develop the condition, which can also be caused by prolonged sitting, car accidents and falls.
  
  
• Trauma. Accidents, falls or blows to the spine can injure lumbar or sacral nerve roots, resulting in sciatica. Other sources of trauma that may cause sciatica include pelvic fractures; nerve damage accidentally inflicted during injections to the buttocks, surgery or other medical treatments; gunshot wounds; or other trauma to the buttocks or thighs. Another potential cause of pelvic or spinal fractures is osteoporosis.

• Arthritis in the back, such as spinal osteoarthritis (spondylosis) or ankylosing spondylitis.
  
  
• Spinal tumors. Tumors – masses of abnormal cells – can occur inside the spinal cord, within the membranes that cover the spinal cord or in the space between the spinal cord and the vertebrae. Benign (noncancerous) or malignant tumors can compress the spinal cord itself or the nerve roots, resulting in severe back pain, hip pain, leg pain or foot pain. Other symptoms include muscle weakness or loss of sensation, difficulty walking (gait disturbances) and loss of bladder or bowel function.
  
  
• Impaired circulation. Conditions that decrease the blood supply (e.g., peripheral arterial disease) to the sciatic nerves can spark sciatica.
  
  
• Spinal abnormalities. Anatomical irregularities such as scoliosis (a sideways bending of the spine) may cause impingement of a sciatic nerve.
  
  
• Sciatic nerve tumor or injury. In rare situations, the sciatic nerve itself may be affected by a tumor or trauma. This can result in sciatic pain.
  
  
• Other causes. In some cases, it may be difficult to find the source of sciatic pain. Problems affecting the bones, joints and muscles can all potentially cause sciatic pain.

In addition, certain factors may raise the risk of sciatica for some people. These include:

• Age. Most people have some deterioration in their intervertebral discs by the time they are 30, and herniated discs are most likely to occur in a person’s 40s or 50s. One of the leading causes of sciatica – spinal stenosis – primarily occurs in people who are 50 and older.
  
  
• Job. People who work in occupations that require them to twist their backs, carry heavy loads or drive for hours at a time are at increased risk for sciatica.
  
  
• Sedentary lifestyle or some forms of exercise. People who sit for long periods of time or who live sedentary lifestyles are more vulnerable to sciatica than those who exercise regularly. Prolonged sitting or lying with pressure on the buttocks is particularly likely to trigger symptoms. Some forms of exercise, such as jogging, may increase the risk of sciatica for some people. Patients should consult their physician about the most appropriate exercise regimen.
  
  
• Genetic factors. Genetics may predispose some people to having disc problems, which can lead to sciatica.
  
  
• Diabetes.  Patients who have this condition, a disorder in how the body uses glucose (blood sugar), are more likely to develop the type of nerve damage and circulatory problems that can lead to sciatica.

• Other conditions that can cause peripheral neuropathy. For example, alcohol abuse or uremia (a toxic syndrome resulting from kidney failure) can cause sciatica-like symptoms.