What is sciatica?
It is important to make a distinction between back pain and sciatica, as the treatment is different, even though they may both be caused by similar conditions. Sciatica implies pain in the leg due to irritation of one of the nerve roots that make up the sciatic nerve. This pain usually radiates down the back or the side of the leg and extends below the knee, often into the foot. It is frequently associated with numbness or pins and needles and may also cause weakness in the leg. Back pain may be referred to the groin or thigh, in much the same way as a heart attack can cause pain in the arm, but this should not be confused with true sciatica.
What is a disc prolapse?
The disc is composed of two parts. The outer part, or annulus, is composed of many fibres that join the vertebrae together. In cross section this has a similar appearance to the rings of a tree trunk. In the middle is the nucleus, which is the soft part of the disc. This contains a large proportion of water and gradually dries out and shrinks with age, causing the disc space to narrow.
Small tears appear either between these layers or across layers. These can occur with minor injury or normal everyday life. If several of these small tears join together, or less commonly if there is a major injury causing a single large tear, the nucleus (central, soft part of the disc) may squeeze out through the tear. This is called a disc prolapse or herniation or “slipped disc”. In this situation, the disc prolapse (shown in black) may press on a nerve (red) and cause sciatica.
If the pain persists you will usually have some Xrays. Plain Xrays are of little use and most often the first test will be a CT scan. This will almost always show a disc prolapse if one is present (see below). Occasionally you will need an MRI scan (like the CT but done with a strong magnet) and this needs to be ordered by a specialist. The most common indications for an MRI are previous surgery or a good history for a disc prolapse but a “normal” CT.
Fortunately, the vast majority of disc prolapses get better with conservative treatment. The water is gradually absorbed and the disc fragment shrinks, relieving the pressure on the nerve. A few days of rest and then avoidance of prolonged sitting, bending and lifting will be necessary. Regular analgesics and possibly an antiinflammatory will help. The severe pain will usually ease after a week or two and then gradually disappear over the next few weeks. Once over the acute phase it is worth seeing a physio to work on an exercise program to reduce the risk of recurrence.
What if it doesn’t get better?
If the pain is not improving as expected, other treatments will be necessary. These include other medications, epidural steroid and surgery.
There are a variety of other medications that can be of value in sciatica. Apart from painkillers, there are two types of drugs that can sometimes help, antidepressants and anticonvulsants. In smaller doses than for their primary use, they can have an effect on chronic pain and reduce the need for analgesics.
Injection of steroid into the epidural space (the space around the nerves in the spinal canal) can sometimes be very beneficial for sciatica but is of no benefit for back pain. It is done under CT by a radiologist who will explain it to you beforehand. You will need to spend a few hours in hospital and should not drive yourself home. In most cases you will notice an improvement in your leg pain immediately from the local anaesthetic and the pain may then recur when this wears off. The steroid takes one to two weeks to take full effect.
There are some occasions where surgery to remove the prolapsed part of the disc is necessary. This is when the pain is so severe that it can no longer be controlled with analgesics (unusual), when there is significant weakness in the leg or when conservative treatment has been tried for six weeks with little or no benefit. The operation (microdiscectomy) involves removal of the disc prolapse and any loose pieces of disc material from within the disc space. The outer part of the disc (annulus), consisting of fibres holding the vertebrae together, is left intact. The chances of relieving the leg pain are excellent but back pain may persist. For further information see the Microdiscectomy Information Guide.
Rarely, the disc prolapse can be large enough to compress all of the spinal nerves rather than just one. It may then cause severe weakness in the legs and paralysis of bladder and bowel function. This is an emergency and needs urgent surgical treatment.
Other causes of sciatica
Sciatica may also be caused by narrowing of the space that the nerve passes through as it leaves the spine (called the foramen). This can be caused by arthritis in the facet joints or narrowing of the disc space, but is usually a combination of both. The top picture shows the red nerve in a normal foramen and the bottom picture shows how narrowing of the disc also narrows the foramen and compresses the nerve.
Another cause of sciatica is a disc that prolapses out to the side rather than directly backwards (far lateral disc prolapse). shown by the red arrows in the following picture with the normal nerve on the opposite side. The normal nerve is surrounded by fat (black) but on the affected side the fat has been displaced by the disc prolapse and the nerve is so compressed that it can not be seen.
These types of sciatica are treated a little differently and often respond to an injection of cortisone around the nerve. If this fails it is possible to remove a disc fragment surgically or drill away some of the bone to enlarge the foramen