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Sometimes it's difficult for a doctor to diagnose the cause of low back pain and sciatica.

Your doctor will ask you questions about your back problems and your symptoms.¹ If your back pain doesn't go away, you may need an MRI scan or a CT scan. These help your doctor see what's happening in your back.

Your doctor will ask you some questions about how and when your symptoms started. And you'll probably have a physical examination.² This helps to rule out other conditions that can cause back pain and sciatica.

Your doctor will ask you about the pain and other feelings you have in different parts of your legs, feet and toes.³ To check that your sense of touch is working properly, your doctor may give you a pinprick on the back of your leg or on your foot.⁴

Your doctor may test the strength of your muscles by asking you to push against his or her hand. He or she will also test the reflexes in your knees and ankles, by gently tapping you with a special hammer just below your kneecaps and just above your heels. A slipped disk can cause you to lose a reflex in the leg that is affected by sciatica.

The straight leg raising test is one that doctors often use to help diagnose a slipped disk. Your doctor will ask you to lie on your back and then raise the painful leg without bending your knee. Most people with a slipped disk get sciatica pain before their leg is lifted two-thirds of the way up. This suggests a slipped disk, but it isn't proof.²

There are more tests you can have. If you've had back pain for only a few weeks, your doctor may say that it's too early to go in for further tests. If your back pain hasn't gotten better after a month or so, your doctor may order further tests, or refer you to a back specialist.

Having a scan of your spine is especially important if you are considering surgery for your back pain. Surgery won't help if the cause of your low back pain is a strained muscle or ligament. A scan can help doctors find out if a slipped disk is causing your back pain.⁵ It can also be used to rule out more serious problems such as cancer.

A scan gives doctors a detailed picture of your spine. Two kinds of scans can be used: an MRI scan and a CT scan.

When you have an MRI scan, you lie inside a machine that uses a magnetic field and radio waves to make a very clear picture of all the parts of your back. You lie on a table that slides into a compartment that's actually a big magnet. Some people should not have an MRI, like those who have pacemakers. You will be asked some questions to be sure an MRI is safe for you.

You're in a small space while you have the scan. This makes some people feel very anxious and scared (claustrophobic). Often you can listen to the radio or to music while you're having the scan, so it doesn't seem so scary. But there is a "panic button" you can push if you feel too nervous. If you push it, the scan stops and the technician will take you out of the scanner.

Doctors now think this kind of scan is the best way to confirm you have a slipped disk.¹ That's because it gives such a clear picture of the structures inside the back, including the softer parts that don't show up well with a CT scan.

A CT scan uses a computer and X-rays to produce a picture of your back. Usually, you lie flat while the scanner moves around your back.

MRI and CT scans make it possible to see the disks of the lower back and to spot a slipped disk.

Finding a bulging disk or other disk problem on a scan such as an MRI is not ironclad proof that this is the cause of your back pain or sciatica.

About two-thirds of people who have this kind of scan find that they have disk problems (including a slipped disk), even though they don't have any pain in the lower back or any other symptoms.⁶ So if you have a scan, it could find a disk problem that isn't causing back pain. This could lead to surgery you don't need.^{7 8}

It's important to remember that most people with slipped disks get better without having surgery.²

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.