Sciatica UK: Causes, Nerve Root Symptoms & Red Flags

A clinically reviewed UK guide to sciatica — what causes it, which nerve root is involved, red flag symptoms requiring emergency care, and how acute and chronic sciatica are managed.

What Is Sciatica?

Sciatica is not a diagnosis in itself — it is a nerve root symptom pattern that originates from a structural problem in the lower back — it is a description of a symptom pattern. The term refers to pain that radiates along the distribution of the sciatic nerve, the largest nerve in the human body. It typically presents as pain, tingling, numbness, or weakness that starts in the lower back or buttock and travels down through the leg, often reaching the calf or foot.

The underlying cause is radiculopathy — compression or chemical irritation of one or more lumbar or sacral nerve roots before they join to form the sciatic nerve. The pain experienced in the leg is referred pain from the nerve root, not from any pathology in the leg itself. This is why sciatica can cause severe foot or calf pain even when the problem is entirely in the spine.

The Sciatic Nerve: Anatomy

The sciatic nerve is formed by the union of the L4, L5, S1, S2, and S3 nerve roots as they exit the lumbar spine. It is the thickest and longest nerve in the body, running from the lower back through the buttock, down the back of the thigh, and branching into the common peroneal and tibial nerves behind the knee. It supplies sensation and motor function to most of the leg and foot.

The specific distribution of pain, numbness, or weakness in sciatica often indicates which nerve root is involved — a finding that helps guide diagnosis and management:

Causes of Sciatica

• Lumbar disc prolapse (herniated disc) — the most common cause, accounting for the majority of sciatica cases. The inner nucleus of a spinal disc protrudes through the outer annulus and compresses or chemically irritates an adjacent nerve root. The chemical irritation often causes more inflammation and pain than the mechanical compression alone.
• Lumbar spinal stenosis — narrowing of the spinal canal, usually from degenerative changes. More common in older adults; often causes bilateral leg symptoms that are worsened by walking and relieved by bending forward (neurogenic claudication).
• Spondylolisthesis — forward slippage of one vertebra over another, causing nerve root compression. Can be degenerative or secondary to stress fracture (spondylolysis).
• Piriformis syndrome — the sciatic nerve can be compressed by the piriformis muscle in the buttock. Relatively uncommon; pain is more buttock-centred; worsened by prolonged sitting.
• Sacroiliac joint dysfunction — can refer pain into the buttock and thigh mimicking sciatica, but without true neurological deficit.

Symptoms

• Radiating leg pain — the defining feature; pain follows a dermatomal distribution from the lower back or buttock into the leg; often worse below the knee than in the back itself
• Burning, shooting, or electric-shock quality — the neuropathic character distinguishes sciatica from simple muscle pain
• Numbness or tingling — in the leg or foot, in the distribution of the affected nerve root
• Muscle weakness — may affect foot dorsiflexion, plantarflexion, or knee extension depending on the root involved
• Worsening with sitting, coughing, or sneezing — all increase intradiscal pressure, exacerbating disc prolapse-related sciatica
• Some relief on lying flat — reduced spinal load; may also be relieved by the foetal position (flexed hips and knees)

Red Flags: When Sciatica Is an Emergency

Seek urgent GP assessment (same day) for:

• Sciatica associated with fever and back pain — possible spinal infection or epidural abscess
• Onset following significant trauma — possible fracture
• Progressive neurological deficit — worsening weakness or increasing numbness over days
• Sciatica in a person with known cancer or immune suppression

Acute vs Chronic Sciatica

Most acute sciatica from disc prolapse follows a predictable natural history — significant improvement or full resolution within 4–12 weeks in the majority of cases as the disc material resorbs and inflammation subsides. Remaining as active as possible, within the limits of pain, is associated with better outcomes than bed rest.

Sciatica persisting beyond 12 weeks is considered chronic. Psychosocial factors — fear of movement, catastrophising, poor recovery expectations — are among the strongest predictors of chronicity and are more powerful than imaging findings. Imaging that shows a prolapsed disc does not mean surgery is needed; most disc prolapses resolve without surgery.

Diagnosis

Sciatica is diagnosed clinically through history and neurological examination. The straight leg raise test (SLR) — pain reproduced at 30–70° of passive hip flexion — is the most sensitive clinical test for lumbar disc prolapse. Crossed SLR (contralateral leg raise causing ipsilateral leg pain) is less sensitive but highly specific.

Imaging is not required for acute sciatica without red flags. MRI is the investigation of choice if symptoms are not resolving at 6–8 weeks, if there are progressive neurological deficits, or if surgical management is being considered. Disc abnormalities on MRI are extremely common in people without symptoms and should be interpreted alongside the clinical picture.

Treatment