What are the main causes of sciatic nerve pain ?

Sciatica simply means an irritation of the sciatic nerve. Sciatica symptoms normally include leg pain, weakness, numbness or tingling. Confusingly, it is often used as a layman’s terms to describe any lower back pain.

What is the Sciatic Nerve and where does it come from?

The spine is made up of large bones called vertebra and these are separated from one another by spinal discs. In the lower back these discs are large and they act as shock absorbers between the vertebrae.

Your spinal cord runs in a bony channel from your brain to your tailbone. The channel is formed by the vertebrae which encircle it and protect it from injury.


spinal cord

The spinal cord encircled by the bony channel of the vertebrae

As it passes down the bony channel the spinal cord branches out to form nerve roots. These nerve roots are in pairs and they leave the spinal canal, on on each side, in a small bony gap between each of the vertebrae called the foramen.

sciatica causes and symptoms

Nerve roots leaving the spinal cord through the gap called the foramen

These nerve roots bundle together to make up the nerves in our arms and legs. They are called peripheral nerves.

origin of the sciatic and femoral nerves
The femoral and sciatic nerves are formed by
nerve roots bundling together. This shows which roots make
up the bigger nerves.

In the legs there are two main bundles of nerves.

  • The nerve running down the back of your thigh and calf is called the sciatic nerve

  • The one running down the front is called the femoral nerve.

diagram of the sciatic nerve in the back of the thigh
The sciatic nerve starts off at the spinal cord
and runs down the back of the thigh
into the calf and foot (image adapted from Grays Anatomy)

What causes sciatica?

Sciatica is caused by an irritation of a nerve root as it leaves the spine.

The medical name for it is lumbar radiculitis or radiculopathy - often people refer to it simply as a pinched nerve in the back.

There are several different things that can irritate this nerve:

  • Disc Bulges and Herniations- this is the most common cause - I have made a slideshow about disc bulges that shows you what they are.
  • Spondylolisthesis- This is a movement of one vertebra on the other which can irritate the sciatic nerve.
  • Spinal stenosis - this is a condition where the spinal canal becomes narrowed and this can lead to nerve root irritation.
  • Spinal disc disease - as we age the disc becomes dehydrated and thinner.
  • More serious problems- These are very unusual but include cancer and infection. Read this section on red flags and serious back problems.

 

Which tests are useful to diagnose sciatica?

Plain X-rays rarely show anything useful but an MRI scan may show disc bulges and prolapses and will show if a nerve is being compressed.

Tests for the nervous system such as reflexes, sensation and muscle power can be performed by your doctor or physio and may point to sciatic nerve problems if they are abnormal.

There are also other clinical tests that your doctor of physical therapist can do, such as a straight leg raise test, which may help with diagnosis. If this test is negative its very unlikely that you have sciatic nerve pain although a positive test does not automatically mean that you have.

In practice the findings in a clinical examination are taken together with your history, symptoms and how the symptoms behave to help your doctor make a diagnosis.

References

1.Sciatica Guidelines

Van, R.M. et al., 2012. Computed tomography for the diagnosis of lumbar spinal pathology in adult patients with low back pain or sciatica: A diagnostic systematic review. European Spine Journal, 21(2), pp.228–239.


Wassenaar, M. et al., 2012. Magnetic resonance imaging for diagnosing lumbar spinal pathology in adult patients with low back pain or sciatica: A diagnostic systematic review. European Spine Journal, 21(2), pp.220–227.

 

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23-Sep-2013