Spinal Arachnoid Cyst Classification
Spinal arachnoid cysts are classified into two main categories: primary (idiopathic) and secondary (related to inflammation, trauma, surgery, or infection), with further anatomical subdivision based on location as intradural versus extradural. 1
Primary Classification System
The most clinically relevant classification divides spinal arachnoid cysts by etiology:
Primary (Idiopathic) Cysts
- No identifiable causative factor
- Account for approximately 83% of cases (109 of 130 patients in the largest series) 1
- Associated with syringomyelia in 46% of cases 1
- Significantly better surgical outcomes with 83% progression-free survival at 10 years 1
Secondary Cysts
- Result from inflammatory leptomeningeal reactions related to:
- Meningitis
- Subarachnoid hemorrhage
- Intrathecal injections
- Prior intradural surgery
- Trauma 1
- Account for approximately 17% of cases (21 of 130 patients) 1
- Associated with syringomyelia in 86% of cases 1
- Poor surgical outcomes with only 15% progression-free survival at 10 years due to extensive arachnoiditis 1
Anatomical Classification
By Dural Relationship:
Intradural Arachnoid Cysts
- Located within the dural sac
- Most common presentation
- Further subdivided into:
- Extramedullary (outside spinal cord)
- Intramedullary (within spinal cord) 2
Extradural Arachnoid Cysts
- Also termed epidural spinal arachnoid cysts
- Correspond to Nabors classification Types I and II meningeal cysts 3
- Arise from focal dural defects 4
- Less common than intradural variants
By Spinal Level:
- Thoracic spine: Most common location (approximately 80-90% of cases) 1, 3
- Thoracolumbar junction: Second most common 1
- Lumbar spine: Less frequent 1
- Cervical spine: Rare (only 1 case in 130 patients in one series) 1
Clinical Significance of Classification
The distinction between primary and secondary cysts is critical for surgical decision-making and prognostication:
- Primary cysts: Complete or partial resection with duraplasty yields profound improvements in the first postoperative year with excellent long-term outcomes 1
- Secondary cysts: Surgery provides only marginal clinical changes and temporary stabilization due to underlying arachnoiditis; recurrence rates exceed double that of primary cysts 5
Important caveat: Despite differences in etiology, progression-free survival is nearly identical for patients with or without associated syringomyelia, so syringomyelia presence should not alter surgical planning 1
The presence of nerve root involvement should also be documented in spinal arachnoid cyst classification 6, though this primarily affects surgical approach rather than prognosis.