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Sciatica is the experience of pain in the distribution of all or part of the sciatic nerve. Typically, pain is felt to varying degrees in the buttock, back of the thigh, back or side of the leg below the knee, and around the ankle or into the foot and toes.

There may be associated sensory disturbance, such as pins and needle or numbness, and less commonly, but potentially of more concern, weakness.

The exact distribution of the pain, sensory disturbance and weakness will depend on which nerve root or roots is compressed or irritated.

Uncommonly, but seriously, sciatica can be associated with sensory change around the anus and genitals, and disturbance of sphincter function (loss of control of urination or defaecation, impotence), which may indicate serious compression of the lower sacral nerve roots and constitutes a medical emergency.

Sciatica is most commonly caused by compression or irritation of the L5 or S1 nerve roots, typically by a herniated or protruded disc, but it can also be caused by synovial cysts arising within the spine from facet joints; by arthritic narrowing of the spinal canal; by nerve sheath tumours; and by compression outside the spine, e.g. piriformis syndrome, where a tense piriformis muscle irritates the passing sciatic nerve.

Initial treatment is usually conservative, and many patients will settle without intervention, as inflammation subsides and herniated disc material shrinks.

An early surgical opinion should be sought if there is severe pain or noticeable muscular weakness.

An urgent surgical opinion should be sought if there is disturbance of bowel or bladder function, or if sciatica is bilateral of sudden and recent onset.

Compression of nerve roots above L5 is less common, and present with characteristic distributions of pain, sensation disturbance, and muscle weakness. Assessment is along the same lines as for sciatica.