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Sciatica is simply the medical term for pain radiating down the leg due to a trapped nerve in the lumbar (lower) spine.   The pain may be continuous or intermittent and there may be certain positions which exacerbate the pain. The cause of the trapped nerve in the lower back that produces the sciatica is either a disc prolapse or more longstanding degenerate changes producing compression on the lumbar nerve root.

It may commonly present with a sudden onset of leg pain often but not invariably associated with back pain.  There are, therefore, effectively 2 groups of people who suffer from sciatica, the first with a sudden onset of leg pain with or without back pain and the second who have rather more long term (chronic) symptoms.  In the majority of cases, the sciatica radiates down the back or side of the leg, although the distribution does depend on which nerve is trapped in the lumbar spine and can occasionally be down the front of the thigh and indeed into the shin. 

Sciatica is not infrequently associated with other neurological symptoms in the leg but may be simply that of isolated pain.  Other neurological symptoms most commonly include paraesthesia (pins and needles) and less frequently, numbness.  On relatively rare occasions, the severity of the nerve root entrapment means that in addition to pain and sensory disturbance, there is loss of motor function (power to the leg).  Given that the cause of the sciatica is usually entrapment of a single nerve, the motor deficit tends to be equally precisely located and is most commonly manifest by a weakness either of standing on tiptoe or lifting up the foot.  In the most severe cases this can produce a foot drop such that the toes of the affected foot drag along the ground during walking as the sufferer is unable to lift the toes away from the floor.

As has been described, the cause of sciatica is usually a trapped nerve and is most commonly unilateral, in other words affecting one or other leg rather than both.  In rare cases, the underlying condition causing the trapped nerve is a large central disc prolapse rather than the more usual one sided disc prolapse.  Clinically, this manifests itself by the sufferer complaining not of unilateral (one sided) sciatica but rather bilateral (both sides) sciatica with pain radiating down both legs.  Whereas unilateral sciatica can be most unpleasant and painful, the indications for treatment are usually based on the severity and duration of the pain rather than any need for emergency treatment.  However, should you be describing bilateral sciatica, this represents a potential neurosurgical emergency and urgent referral is recommended.