Spinal arachnoid cyst is a benign cystic lesion containing CSF (cerebrospinal fluid)-density fluid, arising from duplication or defect of the arachnoid membrane. Most commonly intradural extramedullary in location (~70% of all spinal arachnoid cysts); less frequently extradural. The thoracic posterior epidural region is the most common location (~60-70%), followed by cervical and lumbar regions. Can be congenital or acquired (post-trauma, infection, surgery). Shows isointense signal to CSF on all MRI sequences — T1 hypointense, T2 hyperintense, complete suppression on FLAIR. NO enhancement — this feature is critical in differential diagnosis from tumoral lesions and infectious collections. No diffusion restriction — the most important differentiating finding from epidermoid cyst. Mostly asymptomatic and discovered incidentally. Large cysts may cause myelopathy, radiculopathy, or sphincter dysfunction through spinal cord compression. Surgical drainage or fenestration is performed for symptomatic cysts.
Age Range
10-70
Peak Age
40
Gender
Male predominant
Prevalence
Uncommon
Spinal arachnoid cysts form from congenital or acquired duplication of the arachnoid membrane. In congenital cysts, disruption occurs in normal arachnoid membrane formation during embryological development — anomalous adhesions between arachnoid trabeculae or membrane duplications form the cyst wall. The cyst lumen is filled with CSF — this fluid accumulates through one-way flow via small defects in the arachnoid membrane (ball-valve mechanism): when CSF pressure increases (cough, Valsalva), fluid enters the cyst but cannot exit when pressure normalizes as the defect closes → cyst progressively enlarges. In acquired cysts, post-trauma, infection (arachnoiditis), or post-surgical arachnoid adhesions compartmentalize CSF flow regionally → localized CSF collection forms. Cyst content is of normal CSF composition — protein, glucose, and cell count are identical to CSF. This water-like composition determines imaging characteristics: water density on CT (0-10 HU), isointense signal to CSF on all MRI sequences (T1 hypointense, T2 hyperintense), complete suppression on FLAIR (free water signal eliminated by inversion-recovery like CSF), and absence of diffusion restriction (free water diffusion). As the cyst enlarges, it compresses the spinal cord or nerve roots → neuronal damage.
The signature finding of spinal arachnoid cyst is the combination of three MRI criteria: (1) isointense signal to CSF on all sequences (T1 hypo, T2 hyper, complete FLAIR suppression), (2) absence of diffusion restriction on DWI (high ADC), (3) absence of enhancement. This triple combination distinguishes arachnoid cyst from other intradural cystic lesions (epidermoid, neurenteric cyst, cystic tumor) with high confidence.
Spinal arachnoid cyst shows completely isointense hyperintense signal to CSF on T2-weighted images — no signal difference from surrounding CSF. Cyst boundaries are determined by thin arachnoid membrane wall but this wall is usually not visible on T2; cyst presence is understood from compression/displacement of cord. In large cysts, cord is displaced anteriorly or laterally, thinning over the cord may be visible. No septa, debris, or solid component within cyst — homogeneous CSF signal. In intradural extramedullary cysts, cord-cyst interface is sharp and smooth; irregular interface should be evaluated for intradural tumor suspicion.
Report Sentence
Well-defined cystic lesion at ___ level, intradural extramedullary in location, showing CSF-isointense hyperintense signal on T2-weighted images, displacing the spinal cord anteriorly.
Spinal arachnoid cyst shows complete suppression on FLAIR — dark signal like CSF. This finding confirms cyst content is pure CSF and differentiates from lesions with proteinaceous or cellular content. Epidermoid cyst does not fully suppress on FLAIR (remains mildly hyperintense — due to keratin content); this feature is a secondary criterion complementing DWI in arachnoid-epidermoid differentiation. Neurenteric cyst may also not suppress on FLAIR (mucinous/proteinaceous content). Spinal FLAIR is technically more challenging than intracranial FLAIR — CSF flow artifacts may cause false signal increase; spatial saturation bands or cardiac gating should be used.
Report Sentence
The cystic lesion shows complete suppression on FLAIR like CSF, confirming pure CSF content; epidermoid cyst is excluded.
Spinal arachnoid cyst shows no diffusion restriction on DWI — ADC values are elevated at the same level as CSF (>3.0 × 10⁻³ mm²/s). This finding is the single most critical criterion sufficient alone to differentiate arachnoid from epidermoid cyst: epidermoid cyst shows marked diffusion restriction on DWI (hyperintense at high b-value, low ADC) because keratin lamellae and cholesterol crystals restrict water diffusion. Arachnoid cyst may appear hyperintense at low b-values (b=0) due to T2 shine-through but signal is lost at high b-values (b=800-1000) and shows high value on ADC map.
Report Sentence
No diffusion restriction in the cystic lesion on DWI with ADC values elevated at CSF-compatible levels; this finding excludes epidermoid cyst and supports the diagnosis of arachnoid cyst.
Spinal arachnoid cyst shows no enhancement on contrast-enhanced MRI — no gadolinium uptake in either cyst wall or cyst content. This feature definitively distinguishes arachnoid cyst from intradural extramedullary tumors (schwannoma, meningioma, myxopapillary ependymoma): tumoral lesions show marked solid enhancement. Also differentiated from infectious collections (abscess) by absence of enhancement — abscess shows rim enhancement. Rarely, thin arachnoid inflammation in the cyst wall may show minimal linear enhancement — this may be seen in chronic cysts and is not a malignancy marker.
Report Sentence
No enhancement in the cystic lesion on contrast-enhanced series, excluding intradural extramedullary tumor; consistent with arachnoid cyst.
Spinal arachnoid cyst appears as a cystic lesion at CSF density (0-10 HU) on CT. No enhancement. Large cysts may show spinal canal widening and scalloping of adjacent vertebral bodies (posterior wall erosion) — especially in long-standing extradural cysts. CT has a limited role compared to MRI in diagnosing spinal arachnoid cyst but is complementary in evaluating associated bony anomalies and degree of cyst-related bone erosion. CT myelography may be useful in demonstrating communication between cyst and subarachnoid space — though this invasive method is rarely used today.
Report Sentence
CSF-density cystic lesion at ___ level in the spinal canal on CT with no enhancement; scalloping of adjacent vertebral structures is present/absent.
Criteria
Located within dura but outside spinal cord. Comprises ~70% of all spinal arachnoid cysts. Thoracic posterior region most common.
Distinct Features
Cord displaced posteriorly or laterally. Formed by arachnoid membrane duplication. Can be congenital or acquired. Myelopathy develops when symptomatic.
Criteria
Located outside dura. Formed by arachnoid herniation through dural defect. Thoracic and thoracolumbar region common.
Distinct Features
Dural defect can be demonstrated by CT or MR myelography. Prominent scalloping and canal widening of vertebral bones may be present. Surgical treatment also requires dural defect repair.
Criteria
Cyst developing after trauma, spinal surgery, or arachnoiditis. Arachnoid adhesions compartmentalize CSF flow.
Distinct Features
History is critical (trauma, infection, surgery). Irregular cyst wall and accompanying arachnoiditis findings (dural thickening, enhancement). May show more irregular morphology than congenital cysts.
Distinguishing Feature
Epidural hematoma shows time-dependent T1/T2 signal changes due to hemoglobin degradation (T1 hyperintense in subacute, T2 hypointense in acute); arachnoid cyst shows stable CSF signal on all sequences. Hematoma has biconvex morphology in posterior epidural space; arachnoid cyst may be intradural or extradural. Clinical: hematoma acute presentation, arachnoid cyst usually chronic/incidental.
Distinguishing Feature
Tarlov cyst is perineurally located at sacral nerve root level with nerve root identified within cyst wall (axial MRI); arachnoid cyst shows thoracic posterior intradural extramedullary location without nerve root identification in wall. Both show CSF signal but location and nerve root-cyst relationship are differentiating.
Distinguishing Feature
Tethered cord is congenital anomaly characterized by low conus + thick/fatty filum terminale; arachnoid cyst is a CSF-density cystic lesion. T1 hyperintensity in tethered cord is from fat signal; arachnoid cyst is T1 hypointense (CSF signal). Different pathologies that may coexist.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
12-monthThe vast majority of spinal arachnoid cysts are asymptomatic and require no treatment; when detected incidentally, follow-up MRI at 6-12 months is recommended to assess size stability. Further follow-up may not be needed for stable asymptomatic cysts. Surgical treatment is planned for symptomatic cysts (myelopathy, radiculopathy, bladder dysfunction, progressive neurological deficit): cyst fenestration via laminectomy (creating opening in cyst wall for CSF drainage) or cyst-subarachnoid shunt placement. Fenestration is simple and effective; however, recurrence from cyst wall re-closure is 10-20%. Post-operative MRI is recommended to evaluate cyst size reduction and cord decompression.
Most spinal arachnoid cysts are asymptomatic and require no treatment. Surgical drainage or fenestration is performed for symptomatic cysts (myelopathy, radiculopathy, bladder dysfunction). Size stability on follow-up is important. Surgery is planned for cysts showing growth or developing new symptoms.