Medical information for patients


Sciatica is the term given to pain down the leg, which is caused by irritation of the main nerve into the leg, the sciatic nerve. This pain tends to be caused where the nerves pass through and emerge from the lower bones of the spine (lumbar vertebrae). Much of the advice regarding this condition is the same as that for low back pain.


In sciatica, there is a pain down into the leg, which travels below the knee and may involve the foot. There may be numbness and there may be weakness of the lower leg muscles.

These symptoms may come on their own, but are often combined with low back pain. The order in which the symptoms appear may vary. Sometimes the back pain comes before the sciatica and sometimes it will follow.

Red Flags

As with low back pain, there is a group of features (the Red Flags) which doctors use to highlight the need to act more quickly, because they may suggest a problem which is not as straightforward as simple back pain or sciatica. If you fit into one of these groups or are concerned, you should see your doctor soon:

Anyway, it is probably wise to see your doctor if you develop sciatica. Certainly you should consult him or her if you are not able to adequately control the pain with simple painkillers or anti-inflammatory drugs, or if the pain carries on for more than two weeks.


The most common cause of true sciatica is a "slipped disc". The discs (or inter-vertebral discs) are the cushions which separate the bones of the spine (vertebrae). Your doctor may refer to a slipped disc as a prolapsed intervertebral disc (PID) or a herniated nucleus pulposus.

The intervertebral disc looks a bit like a draughts piece in shape. The ring around the outside is tough and fibrous, and the centre is rather more like the consistency of a chewy sweet. These discs allow for some flexibility between the bones of the spine and also act as shock absorbers.

The pressures within the discs can reach high levels when we bend or twist, even without carrying a heavy load. If we add to that a heavy load, especially held out at arms' length, the pressures rise even higher.

If part of the fibrous outer ring of the disc is rather weaker than the rest, the softer centre (nucleus pulposus) may push its way through, bulging outwards. If this bulge presses against a nerve which is running from the main, central nervous system to one of the legs, it causes symptoms in that leg.

Sciatica occurs when the herniated disc presses against the nerves which go to make up the sciatic nerve. This is more likely to cause problems when the nerve is squashed, usually between the disc and an adjacent bone.

Nerves have many functions, transmitting messages around the body rather like a telephone system. They allow us to feel things that happen to us, such as things we touch or that touch us, hot and cold, and pain, and they cause our body to do things, for example making your leg muscles contract when you want to walk. Thus, when a nerve is squashed, it may malfunction, and we might feel pain, numbness, or pins and needles, and we might find our limbs do not work in the way they should.

Other things can cause irritation of or pressure on a nerve in the spine. Sometimes this may be a rough and enlarged part of one of the vertebrae, brought about by ageing, and sometimes rarer conditions, infections and tumours are to blame. Most times the cause is nothing too serious, but one of the reasons for seeing your doctor if the pain persists is to make sure that serious and treatable causes have been ruled out.


If you still have sciatica after a few weeks, it is likely that your doctor will arrange for you to have a CT (Computerized Tomography) or MRI (Magnetic Resonance Imaging) scan. These show up the soft structures in the spine, as well as the bones, and will thus show whether a nerve is being squeezed by a disc, or whether something else is causing irritation of the nerve.

Really the main reason for doing such a scan is to see whether an operation will help to cure the sciatica.


The key points about most back pain and sciatica are:

Further information


BMA Guide to Back Care, by British Medical Association
A useful quick guide to back pain and care from the British Medical Association.
More books
Search for more books on Sciatica

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