Causes of sciatica 

Sciatica occurs when the sciatic nerve, which runs from your back to your feet, becomes compressed or irritated.

There are many reasons why this may happen, although the vast majority of cases are caused by a herniated or "slipped" disc.

Herniated ('slipped') disc

Your spine is made up of vertebrae, discs and nerves. Vertebrae are the blocks of bone that make up the structure of your spine and protect the nerves.

The vertebrae are supported and cushioned by discs. These discs are made from a tough, fibrous case that contains a softer, gel-like substance. A herniated disc occurs when the outer part of the disc ruptures (splits), allowing the gel inside to bulge and protrude outwards between the vertebrae. When this presses against the sciatic nerve, it can cause sciatica.

It is not always clear what causes a disc to break down, although age is a common factor. As you get older, your spinal discs start to lose their water content, making them less flexible and more likely to rupture  this is sometimes called "degenerative disc disease".

Spinal stenosis

Spinal stenosis is the narrowing of the passage where the spinal cord travels down the spine.

It occurs when the ligaments become overgrown, but bulging of spinal discs and bony spurs from the vertebrae can also contribute. The sciatic nerve is only occasionally affected, meaning that the symptoms tend to be pain in a person's lower back, with heaviness of the legs, a stooped posture and difficulty walking.

Spinal stenosis is usually caused by age-related changes in the spine, but can also be down to degenerative disease of the spinal joints, which causes the vertebrae to lose their correct alignment.

Spondylolisthesis

Spondylolisthesis occurs when a vertebra slips out of position. If the slipped vertebra compresses the sciatic nerve, it can cause sciatica.

Spondylolisthesis is most commonly caused by age-related or degenerative wear of the spinal joints. However, in younger people, it can be caused by spinal fractures and repeated excessive bending of the spine.

Other causes

Less commonly, sciatica may be caused by:

  • an infection in the spine
  • an injury to the spine, or the surrounding muscles and ligaments
  • a growth within the spine, such as a tumour
  • cauda equina syndrome (see below)

Cauda equina syndrome

Cauda equina syndrome is a rare but serious condition that can cause sciatica.

The cauda equina is the bundle of nerves that lead out from the end of the spinal cord. Cauda equina syndrome occurs when these nerves are compressed and damaged. It can eventually lead to paralysis if left untreated.

One of the warning signs of cauda equina syndrome is suddenly losing control of your bladder or bowels. If this happens, you should call 999 for an ambulance immediately.

Sciatica: animation

Sciatica causes pain that radiates out from the lower back, down the buttocks and into one or both of the legs, right down to the calf. This animation explains what sciatica is and what causes it.

Media last reviewed: 18/01/2013

Next review due: 18/01/2015

Spondylolisthesis

Everything you need to know about this lower back condition

Page last reviewed: 26/08/2014

Next review due: 26/08/2016