What is a central disc protrusion?

Central Disc Protrusion: Definition and Clinical Significance

A central disc protrusion is a type of intervertebral disc herniation where disc material extends posteriorly into the spinal canal in the midline, potentially compressing the thecal sac and central neural structures rather than laterally affecting individual nerve roots. 1

Anatomical Characteristics

Central protrusions are distinguished by their location and direction of herniation:

• Central protrusions extend posteriorly along the midline of the spinal canal, as opposed to lateral or posterolateral protrusions that affect the neural foramina 1
• In thoracic disc disease, central protrusions represent the predominant pattern (55% of cases), while lateral protrusions are less common 1
• The herniated material typically consists of annulus fibrosus tissue with myxomatous degeneration, rather than pure nucleus pulposus, particularly in cases where disc material is removed as a free fragment 2

Pathophysiology

The development of central protrusions involves specific degenerative changes:

• Myxomatous degeneration of the annulus fibrosus increases with age and causes reversal of the usual bundle direction in the internal layer, leading to posterior convex bulging 2
• Loss of disc height or abnormal signal intensity on MRI is highly predictive of symptomatic tears extending into or beyond the outer annulus (associated with Stage 2 or 3 annular disruptions in 100% of protrusions) 3
• Acute disc herniations demonstrate significant inflammatory response and edema that contribute to greater mass effect and compression of neural structures, according to the American College of Radiology 4

Clinical Implications

Central protrusions have distinct clinical consequences compared to lateral herniations:

• Central protrusions at the lumbosacral level can compress the cauda equina or conus medullaris, potentially causing bilateral symptoms, bowel/bladder dysfunction, and saddle anesthesia 4, 5
• Acute sacral disc herniations with significant mass effect have higher risk of causing cauda equina syndrome requiring emergency intervention, as reported by the British Journal of Neurosurgery and American College of Radiology 4
• Mediators from the nucleus cause root inflammation and root tension signs, and these inflammatory changes can resolve even though the protrusion remains 6

Diagnostic Imaging

MRI is the preferred imaging modality for evaluating central disc protrusions:

• MRI without IV contrast is usually sufficient for initial evaluation, providing superior visualization of disc morphology, neural compression, and associated inflammatory changes 4
• Acute disc herniations show more pronounced thecal sac effacement on imaging compared to chronic herniations 4
• In thoracic spine, calcified disc material in the spinal canal (seen in 55% of cases) in association with linear calcification in the adjacent disc space is diagnostic of thoracic disc protrusion 1
• CT offers superior depiction of bony structures but is less sensitive for evaluation of nerve root compression compared to MRI 4

Clinical Correlation and Prognosis

Understanding the relationship between imaging and symptoms is essential:

• Centralization of symptoms (pain moving from periphery toward midline) usually implies that the annulus fibrosus is intact, with MRI revealing herniations or extrusions where the spinal canal is wide and liquid reserve is preserved 7
• Peripheralization of symptoms occurs with sequestration and extrusions when there is stenosis of the spinal canal 7
• The natural history shows improvement within 4 weeks with conservative management in most patients as the inflammatory process resolves 8
• Space in the vertebral canal is a major factor determining whether there is nerve root compromise, and symptoms can resolve even when the protrusion remains 6