Sciatica on MRI & X-ray: Imaging Findings

The sciatic nerve is formed by the L4–S3 nerve roots. Clinically, the most commonly compressed roots are L4 (medial lower leg, knee extension weakness, diminished patellar reflex), L5 (dorsal foot and great toe, extensor hallucis longus weakness, no reliable reflex loss), and S1 (lateral foot and small toe, plantar flexion weakness, diminished Achilles reflex). Dermatomal mapping guides which level on MRI to prioritize. The femoral nerve (L2–L4) is not part of the sciatic nerve but can cause similar anterior thigh pain — a positive femoral stretch test (pain with hip extension while prone) implicates L2–L4 root compression, distinguishing it from true sciatica.

Sciatica is a symptom complex — radicular pain in a sciatic nerve distribution — rather than a standalone diagnosis. The most common underlying cause is a posterolateral lumbar disc herniation (contributing to 85–90% of cases), where the nucleus pulposus mechanically compresses and chemically irritates the nerve root. Other causes include foraminal stenosis from osteophytes, piriformis syndrome (extraspinal), sacroiliac joint pathology, lumbar facet synovial cysts, epidural hematoma, or neoplasm. MRI of the lumbar spine is the imaging standard to identify the structural cause and confirm the level. When MRI shows no compressive lesion, electromyography (EMG) and nerve conduction studies help localize pathology and exclude peripheral neuropathy or plexopathy.

Cauda equina syndrome is a surgical emergency arising from compression of the sacral nerve roots (S2–S5) below the L1 conus medullaris. Red flags include new bilateral leg weakness or sciatica, saddle anaesthesia (numbness of the perineum, inner thighs, and genitalia in an S3–S5 distribution), and dysfunction of the bladder (urinary retention with overflow incontinence being most sensitive), bowel (faecal incontinence), or sexual organs. Any patient presenting with sciatica plus bladder or bowel symptoms requires same-day emergency MRI and urgent surgical decompression — typically within 24–48 hours of symptom onset — to maximize neurological recovery.