Sciatica is the name given to the syndrome or set of symptoms that describes the pain which comes from the compression or irritation of the sciatic nerve.
The sciatic nerve runs from the lower back, down to the buttocks, and then travels down the back of both legs ending in the feet.
The compression or irritation of the sciatic nerve causes pain which radiates out from the lower back and travels down the leg along the path of the nerve, to the calf and feet. The pain from this can range from being mild to very painful.
Sciatica has a more formal medical name, lumbar radiculopathy, and is used to describe the cause of irritation, or compression of the sciatic nerve where it originates at one of 5 spinal nerve roots in the lumbar and sacral sections of the spine. Most commonly, sciatica occurs on just one side, right or left, but occasionally on both sides.
The sciatic nerve is the largest and longest nerve in the body. The path it follows starts at the 5 spinal roots. these 5 nerves combine into one, the sciatic nerve, in a muscle in the lower back, just above the gluteal (or buttock) muscle. From here the sciatic nerve travels down the back of the thigh of each leg branching out to provide motor and sensory function to nearly the whole of the skin of the leg, the muscles of the back of the thigh, and those of the entire leg and foot.
Sciatica presents in every case, first with back pain accompanied by restricted mobility. The symptoms show themselves when the inflammation that arises from the compression in one of the 5 spinal nerve roots has progressed far enough to cause neurological symptoms including pain, numbness and pins & needles, and difficulty or pain on moving the affected leg; a key sign to look for in the diagnosis of sciatica is moderate to severe difficulty and pain in raising the straightened affected leg whilst lying down. Sciatica will present as an acute or chronic condition. There are distinct differences in how each presents, mostly arising from how mild or severe the symptoms are, and their duration.
Acute sciatica will generally be mild and of short term duration. A diagnosis is not usually required for this reason. Chronic or persistent sciatica however is where the symptoms are more severe and or continue for a period greater than 6 weeks.
Acute, in the medical sense refers to a disease or symptoms which occur with a rapid onset and/or have a short course. It can also be used to refer to a phase of a condition when the symptoms are at their most severe and before any treatment strategy has had a chance to take effect.
During an acute phase of sciatica, the pain and sensation of inflammation radiates down the leg, following the path of the nerve. The symptoms are located in different areas of the legs and feet depending on which of the 5 nerve roots are involved. For example pain and pins & needles occur on the base of the foot when the 5th lumbar nerve root is involved; and pain will radiate down the back of the thigh when the compression is present at either the 5th lumbar or 1st sacral nerve roots.
When the acute phase has passed, the symptoms begin to subside starting with the portions of the nerves nearest the trunk of the body, and finishing when the symptoms in the feet subside also. This progression is one of the physiological consequences arising from how nerve impulses travel along the nerve fibres. The prognosis for sciatica can either be for the symptoms to occur only during short acute phases, or subside to a condition where the sensation of pins and needles, in the feet remains to a greater or lesser extent; this can be accompanied by some very short episodes of pain and/or inflammation further up the pathway of the nerve. This is known as the chronic phase.
Chronic, in the medical sense refers to a condition or disease that is persistent or long lasting, and mainly applies to symptoms which last for more than 6 weeks. Here a consultation with a GP will rule out a more serious condition and can assess whether any red flag symptoms are present, such as numbness in the bottom, particularly in the perineum, or in the lower back and leg, and loss of control of bladder and/or bowel function.
There are several common causes of the irritation or compression of the sacral nerves. The most common is called a spinal disc herniation, or slipped disc. Each vertebra is separated by a disc which consists of fibrous cartilage. They give the spine some slight mobility and act as a kind of 'shock absorber' on movement. When the disc slips it bulges out from between the vertebra, and in the case of sciatica the bulging disc impinges on the nerve, and it is this which causes the irritation and pain. Among the other causes are spinal stenosis, which is a narrowing of the spinal canal - the channel in which the spinal cord travels, and sciatica can occur during pregnancy. Here it is a result of the weight of the foetus pressing on the sciatic nerve when sitting, where it travels on its path to the leg, underneath the uterus.
Koes BW et al. Diagnosis and treatment of sciatica. BMJ 2007; 334: 1313-7.