What is an arachnoid web in the spine?

Arachnoid Web in the Spine

An arachnoid web is a rare spinal condition characterized by focal thickening of the arachnoid membrane that causes displacement and compression of the spinal cord, leading to progressive neurological symptoms and deficits. 1

Clinical Characteristics

• Location: Predominantly occurs in the thoracic spine, particularly on the dorsal (posterior) aspect of the spinal cord 2, 1
• Presentation: Patients typically present with:
  • Progressive myelopathic symptoms 2
  • Back pain (commonly thoracic) 3
  • Spastic gait and lower extremity weakness 4
  • Sensory deficits in the trunk or lower extremities 1
  • Bladder-bowel dysfunction in some cases 4

Pathophysiology

• Consists of abnormal formation of arachnoid mater into a dense web-like structure 3
• Creates a focal obstruction in the subarachnoid space 5
• May function as a one-way valve mechanism for cerebrospinal fluid (CSF), causing gradual cord compression 5
• Often associated with development of syringomyelia (focal intramedullary syrinx) in 83% of cases 1, 4

Diagnostic Imaging

• MRI: First-line imaging modality for evaluation 2

  • Characteristic "scalpel sign" - a focal dorsal indentation of the spinal cord resembling a scalpel blade 1, 6
  • May show associated syringomyelia adjacent to the web 4

• CT Myelography: Useful when MRI findings are inconclusive 2, 3

  • Can help differentiate arachnoid web from other conditions like ventral cord herniation 2

• Intraoperative Ultrasound: Valuable diagnostic tool during surgery 1, 5

  • Helps visualize CSF flow obstruction
  • Confirms web location before dural incision
  • Verifies restored CSF flow after resection

Differential Diagnosis

Arachnoid web must be distinguished from:

• Ventral cord herniation 2
• Arachnoid cysts 5
• Dorsal arachnoid webs 3
• Other causes of myelopathy (degenerative, neoplastic, infectious) 2

Management

Surgical Treatment

• Surgical intervention is the definitive treatment for symptomatic arachnoid webs 1, 6, 4
• Surgical approach typically involves:
  • Laminectomy or laminoplasty with intradural exposure 1
  • Fenestration and resection of the web 4
  • No spinal instrumentation/fusion typically required 4
• Intraoperative ultrasound is recommended to:
  • Localize the web before dural opening
  • Confirm adequate resection and restored CSF flow 1

Outcomes

• Surgical treatment generally results in favorable outcomes 1, 6
• Most significant improvement seen in:
  • Motor function and locomotion 4
  • Spasticity 6
• Variable improvement in:
  • Sensory deficits (may persist in some patients) 4
  • Bladder-bowel dysfunction 4
• Rapid neurological recovery may occur as early as postoperative day 1 6

Follow-up

• Regular neurological assessments are recommended after surgical intervention 2
• MRI follow-up to assess resolution of syringomyelia if present 1
• Long-term prognosis is generally favorable after surgical resection 1

Important Considerations

• Arachnoid web is often underdiagnosed due to subtle imaging findings 5
• Preoperative imaging may be inconclusive in some cases, requiring intraoperative confirmation 5
• Early diagnosis and treatment may prevent irreversible neurological damage 1, 4
• Should be considered in patients with progressive myelopathy and the characteristic "scalpel sign" on imaging 1, 6