Herniated Disc MRI Signs: Protrusion, Extrusion, and Red Flags

The Pfirrmann grading scale (I–V) classifies intervertebral disc degeneration on sagittal T2-weighted MRI based on signal intensity, disc height, and homogeneity of the nucleus pulposus. A grade I disc is bright and homogeneous; grade V is completely black with lost disc height. Herniation type is further described as protrusion, extrusion, or sequestration based on the relationship of displaced nucleus to the annular margin. Modic endplate changes (type I edema, type II fatty conversion, type III sclerosis) on the adjacent vertebral bodies indicate active inflammation or chronic degeneration and correlate with axial pain severity.

Each lumbar and cervical nerve root supplies a predictable dermatomal strip of skin and a myotomal group of muscles. A posterolateral L4–L5 herniation typically compresses the L5 root, causing numbness along the dorsum of the foot and weakness of great-toe extension (extensor hallucis longus). L5–S1 herniation compresses S1, producing lateral foot numbness and reduced ankle plantar flexion with a diminished Achilles reflex. Cervical C6 compression causes lateral forearm and thumb numbness with biceps weakness; C7 produces middle-finger numbness and triceps weakness. Accurate dermatomal mapping guides surgical planning and post-treatment assessment.

Cauda equina syndrome occurs when a massive central disc herniation compresses multiple sacral nerve roots simultaneously. Red flags include bilateral leg weakness, saddle anaesthesia (S3–S5 dermatomal numbness around the perineum and inner thighs), and dysfunction of bladder, bowel, or sexual function — often presenting as urinary retention with overflow incontinence. These findings mandate same-day emergency MRI and surgical decompression, typically within 24–48 hours of onset, to prevent permanent sphincteric and motor deficits. Any delay significantly worsens neurological prognosis.

A herniated disc more often produces dermatomal arm or leg pain from nerve root contact, while facet joint syndrome usually causes axial neck or back pain that worsens with extension and rotation. MRI can show both problems in the same spine, so the report should be reviewed with the symptom pattern, neurological exam, and clinician plan rather than treated as a stand-alone diagnosis.