What is a sequestered disc fragment of the lumbar spine?

What is a Sequestered Disc Fragment of the Lumbar Spine

A sequestered disc fragment is a piece of the nucleus pulposus (the gel-like center of the intervertebral disc) that has herniated through the annulus fibrosus (the tough outer ring) and completely separated from the parent disc, migrating as a free fragment into the epidural space. 1

Pathophysiological Mechanism

The sequestration process begins when the nucleus pulposus herniates through a defect in the annulus fibrosus, and the fragment then loses all continuity with its disc of origin, becoming a free-floating piece of disc material in the spinal canal 1, 2. This independent fragment can migrate in multiple directions—upward (cranial), downward (caudal), laterally, or even posteriorly behind the spinal cord 2, 3, 4.

Clinical Significance and Neurological Impact

Sequestered fragments cause neural compression through two distinct mechanisms:

• Mechanical compression: The free fragment acts as a space-occupying mass that directly compresses nerve roots or the cauda equina 1
• Chemical irritation: The disc material triggers an inflammatory response that irritates adjacent neural structures 1

This dual mechanism produces radiculopathy with characteristic symptoms including pain, sensory impairment, weakness, or diminished deep tendon reflexes in the affected nerve root distribution 1.

Critical Emergency: Cauda Equina Syndrome

The most dangerous complication is cauda equina syndrome (CES), which occurs when the sequestered fragment compresses multiple nerve roots from the lower cord segments. 1, 3 This presents with:

• Urinary retention or incontinence 1
• Bilateral lower extremity motor weakness 1
• Saddle anesthesia (numbness in the perineal region) 1
• Bowel dysfunction 1

CES requires emergency surgical decompression—delays in recognition and treatment can result in permanent neurological deficits including irreversible bladder/bowel dysfunction and lower extremity paralysis. 1, 3 In one series of posterior epidural sequestrations, 77.8% of patients presented with CES, emphasizing the high risk of this complication 3.

Diagnostic Challenges and Imaging Characteristics

Sequestered disc fragments frequently mimic spinal tumors on imaging, creating a significant diagnostic pitfall. 2, 5, 6, 4 The fragment undergoes independent degeneration processes after separating from the parent disc, which causes variable signal intensity on MRI that can resemble neoplastic lesions 2.

Key imaging features include:

• Variable MRI signal intensity due to independent degeneration of the sequestered fragment 2
• Peripheral rim enhancement on contrast-enhanced MRI, reflecting angiogenesis and granulation tissue formation around the isolated fragment 2
• Complete loss of continuity with the parent disc on imaging 2
• Migration patterns that can place fragments in unexpected locations, including intradural positions that cannot be identified without durotomy 2, 6

Multiple case reports document misdiagnosis as neurilemmoma, meningioma, or other spinal tumors based on imaging appearance alone 5, 6, 4.

Management Approach

MRI without contrast is the preferred initial imaging modality for evaluating suspected disc pathology, though MRI with and without contrast may be necessary when sequestered fragments mimic tumors or infection. 7

For surgical planning in sequestered disc cases:

• Posterior microsurgery with sequestrectomy and discectomy is the standard surgical approach, allowing complete excision of the free fragment 3
• Early surgical intervention is critical when CES is present—the mean duration from symptom onset to hospital admission in one series was only 7.4 days, yet outcomes varied significantly based on timing 3
• Reoperative discectomy alone (without fusion) provides satisfactory outcomes in 69-85% of patients with recurrent or sequestered disc herniations 7, 8, 9

Fusion is NOT routinely indicated for sequestered disc fragments unless there is associated lumbar instability, spondylolisthesis, or chronic mechanical low back pain. 7, 8, 9 The increase in morbidity, cost, and complications associated with fusion are not justified for isolated disc pathology 9.

Critical Clinical Pitfall

The most dangerous error is delayed recognition of cauda equina syndrome. 1 Any patient presenting with urinary retention, bilateral leg weakness, or saddle anesthesia in the context of acute back pain requires immediate MRI evaluation and urgent neurosurgical consultation 1, 3. Waiting for "routine" imaging or attempting prolonged conservative management in the presence of these red flags can result in irreversible neurological damage 3, 6.