Sciatica, pain of the sciatic nerve, is caused by compression or irritation of the sciatic nerve, at any level. The sciatic nerve is a large nerve that starts as a series of nerve roots exiting the lumbar (lower) spine. They join together to make a larger nerve traveling from the low back, through the buttock, down to the leg and foot. This nerve has motor (causes muscles to move) and sensory (senses pain/temperature, etc.) components, and thus both carries signals from the brain/spinal cord to the leg/foot and information from the leg/foot to the spinal cord/brain.
Experienced as low back, buttock and/or leg/foot pain, it may feel burning, electric, sharp-shooting, pins/needles, and/or numb/tingling. The pain may be worse in cold weather. Additionally, there may be muscular weakness causing one-sided listing while standing, or difficulty moving/controlling the leg. Laying down may provide some relief versus standing or sitting. Typically, symptoms occur on one side of the body. If prolonged, leg muscle weakness or atrophy, and rarely blood flow compromise may occur. The location of the sciatic nerve compromise producing the symptoms/findings is diagnosed by examination, possibly assisted by MRI, XRay or nerve studies (EMG).
The causes of sciatic nerve compromise/compression vary with the course of the nerve down the leg. Nerve compromise at the level of the spine is often due to a disk or bone issue. A disk herniation irritates nerves roots that join to produce the sciatic nerve. Arthritis and/or osteophytes (extra bone growths, as seen with bunions) can also compress the nerve roots as they leave the spine. This kind of arthritis, sometimes called facet joint syndrome, may be addressed by steroid injection into this joint (facet joint injection).
When inflammation or compression affecting the sciatic nerve occurs away from the spine, it is usually through irritation of the sacroiliac joint or piriformis muscle. Less commonly, branches of the sciatic nerve can be entrapped at the knee or ankle joints, producing similar symptoms. Conservative treatments include physical therapy, steroid injections, and relative rest.