What Causes Sciatica?

Sciatica, which affects 40 percent of adults during their lifetime, can occur when spinal stenosis or a herniated disc irritates some part of either sciatic nerve. The sciatic nerve is formed by nerve roots that emerge from the lower spine, join together in the hip region and run down the back of each thigh. Near the knee, the sciatic nerves branch into smaller nerves that extend into the calves, ankles, feet and toes.

Sciatica results from irritation of one of the nerve roots in the lower back, most often those emanating from the fourth lumbar vertebra (L4), fifth lumbar vertebra (L5) or first sacral vertebra (S1). Pain or numbness develops along the path of the sciatic nerve; the precise location of symptoms depends on which spinal nerve root is affected. Compression of the sciatic nerves can also diminish leg strength.

After age 50, sciatica is more commonly caused by spinal stenosis than disk herniation. Most of the time, sciatica from a herniated disc resolves within six weeks with little or no treatment. In up to 50 percent of cases, sciatica from spinal stenosis resolves by itself within a month, but 25 percent may have pain for as long as four months.

Compression of the cauda equina (the bundle of nerves at the end of the spinal cord) may result in sudden onset of impaired bowel and bladder function (usually an inability to empty the bowel or bladder or, conversely, incontinence); loss of sensation in the groin, buttocks and legs; and severe weakness or paralysis in the legs.

These symptoms—known as cauda equina syndrome—indicate an emergency situation that requires immediate surgical attention. People who cannot reach their doctor quickly should go to the emergency room of a nearby hospital for treatment.

Sciatica or cauda equina syndrome is usually due to one of the major disorders of the spine, such as disc herniation, degenerative changes of the spinal discs, spinal stenosis or vertebral fracture.