Sciatica: symptoms, causes and diagnosis
Sciatica is the name for a syndrome which is characterised by pain that radiates along the path of the sciatic nerve. The sciatic nerve is the longest and largest nerve in the body — it is as thick as one of your fingers where it arises in the lower spine. It then travels from your lower back through your hip and buttock and down your leg where it divides into two branches at the knee. Each leg has its own sciatic nerve.
Symptoms of sciatica
The pain from sciatica is felt along the path of the sciatic nerve and can be felt deep in the buttock, with pain that travels down the back of the leg, sometimes to the foot. The pain can be accompanied by tingling, ‘pins and needles’, or numbness, and sometimes by muscular weakness in the leg.
Sciatica is usually felt in only one leg at a time. Sometimes, a sensation like an electric shock can be felt along the nerve. The nerve pain can range from a mild ache to incapacitating pain. Sciatic nerve pain is often felt when you sneeze, cough, go to the toilet, or when you're sitting, and may be accompanied by lower back pain.
Most sciatica gets better within a few weeks and doesn't result in permanent damage. If your pain doesn't go away after a couple of weeks, see your doctor. If you lose control of your bladder or bowels or you have severe pain, weakness of your muscles, or numbness, seek medical attention immediately.
One of the most common causes of sciatica is a herniated intervertebral disc. Herniate means bulge or stick out. Intervertebral discs are the spongy discs that form a cushion between the vertebrae of the spine. The discs have a strong outer layer made of cartilage, with a soft gelatinous centre that acts as a shock absorber. If the outer layer of the disc becomes weak, through age or injury, the soft centre can bulge out through the weakness — this is what’s known as a herniated or ruptured disc.
Any of the 5 pairs of nerve roots that form the origin of the sciatic nerve can be compressed in the spine by a ruptured disc. Not only does a herniated disc press on the nerve root, but chemicals leaking from the soft centre of the disc irritate the nerve root, causing inflammation. This combination of compression and inflammation can give rise to pain down the sciatic nerve.
Bony growths (osteophytes)
Bone spurs or osteophytes are bony outgrowths on the edges of joints. Bone spurs form where cartilage is worn away (for example in osteoarthritis) and bone is rubbing on bone. In an effort to protect the body, new bone forms on the edge of joints. This is called a bone spur.
Bone spurs are not painful in themselves, but unfortunately, the bone spurs can rub on nearby bone or nerves, causing problems. In the spine, bone spurs can intrude into the space normally reserved for the nerves, thus causing sciatica.
Piriformis syndrome is an uncommon cause of sciatica. The piriformis is a muscle that lies deep underneath the gluteal muscles of the buttocks. The function of the piriformis muscle is to externally rotate and stabilise the hip. The sciatic nerve passes directly underneath the piriformis.
Piriformis syndrome occurs when the piriformis muscle becomes tight, goes into spasm, or swells. When this happens it compresses the sciatic nerve beneath it. In roughly one in 7 people, the sciatic nerve passes through the piriformis instead of underneath it — and these people may be prone to sciatic nerve problems.
Lumbar spinal canal stenosis
Stenosis means narrowing, and lumbar spinal canal stenosis is narrowing of the space available for nerves in the lumbar spine. Narrowing of the lumbar spinal canal can put pressure on the nerves, which may cause back pain, pain down the sciatic nerve, numbness and weakness of the legs.
If you have lumbar spinal canal stenosis you may have no symptoms. Or you may have low back pain that radiates to the buttocks and legs, or just leg pain. The pain often occurs when you stand or walk. Spinal stenosis tends to affect older people rather than younger people.
In this condition, one vertebra slips forward over the vertebra below it. This distorts the spinal canal and can result in the nerve roots of the sciatic nerve being compressed.
In adults, the fourth and fifth lumbar vertebrae (L4 and L5) are most commonly affected, with the L4 vertebra moving forward on the L5 vertebra. Spondylolisthesis can result from degenerative diseases of the spine, such as arthritis.
Sciatica can be caused by tumours impinging on the spinal cord or the nerve roots. Severe back pain extending to the hips and feet, loss of bladder or bowel control, or muscle weakness, may result from spinal tumours. Trauma to the spine, such as from a car accident, may also lead to sciatica.
Your doctor (GP - general practitioner) will take into account your medical history and will perform a physical examination, especially of the spine and legs. You may be asked to demonstrate several movements so that your doctor can check your muscle strength, flexibility and your reflexes.
X-rays aren’t used as a matter of course, because they don’t show up herniated discs or nerve damage, but they can show spondylolisthesis, bone spurs, narrowed disc spaces and other bone disease. When symptoms are severe, persistent or worsening, CT scans or MRI scans are often used to pinpoint the cause of the sciatica.
It’s important to note, however, that imaging results often don’t relate to symptoms. Problems such as disc abnormalities, bone spurs and spondylolisthesis frequently show up on scans even in people without back pain or other symptoms.
Depending on your symptoms and diagnosis, your doctor may refer you to a specialist (such as a spinal surgeon) for further assessment and treatment.
2. BMJ Best Practice. Assessment of back pain (updated Sep 2018; reviewed Nov 2018). https://bestpractice.bmj.com (accessed Dec 2018).
3. BMJ Best Practice. Discogenic low back pain (updated Mar 2018; reviewed Nov 2018). https://bestpractice.bmj.com (accessed Dec 2018).