Embolization of CSF-venous fistula for syrinx resolution in acquired Chiari malformation: illustrative case

Abstract

BACKGROUND

Chiari malformation type I (CM-I) shares some overlapping radiological and clinical features of spontaneous intracranial hypotension (SIH). Recent work has recognized CSF-venous fistula (CVF) as an underdiagnosed cause of SIH. The authors present a case of presumed CM-I with tonsillar descent and syrinx that progressed despite posterior fossa decompression (PFD), with subsequent diagnosis of SIH due to CVF. Treatment of the CVF by transvenous embolization rapidly improved intracranial brain sag with early syrinx reduction.

OBSERVATIONS

Endovascular embolization of CVF produces durable improvements in brain sag and reduction in syrinx at 1 year, by alleviating the effects of the acquired Chiari and restoring CSF flow at the foramen magnum. Unrecognized SIH/spinal CSF leakage may account for some cases of failed syrinx improvement after PFD for CM-I.

LESSONS

Patients presenting with tonsillar herniation and syringomyelia should have brain and spine imaging to assess for evidence of SIH. The presence of syrinx, although more common in CM-I, may occur in SIH. Persistent or worsening syringomyelia after PFD should prompt reassessment for possible SIH, including invasive CSF leak testing. Endovascular embolization of the CVF effectively treats SIH by restoring CSF homeostasis and reduces syrinx formation from acquired tonsillar herniation.

https://thejns.org/doi/10.3171/CASE26202

Chiari malformation type I (CM-I) is a structural disorder of the posterior fossa, defined by descent of the cerebellar tonsils and brainstem, which may cause dysregulation of CSF circulation. Variations of posterior fossa decompression (PFD) can alleviate symptoms caused by inadequate posterior fossa volume.¹ CM-I frequently presents with syringomyelia, with a prevalence of 12%–60%.² Despite technical advances, up to 22% of patients treated with PFD experience persistence or progression of the syrinx.³

Spontaneous intracranial hypotension (SIH) due to a spinal CSF leak has emerged as an overlooked cause of cerebellar tonsillar descent, which can mimic Chiari malformation on neuroimaging.^4–6 Although low-lying cerebellar tonsils can be seen in both conditions, syrinx formation is thought to be an uncommon feature of SIH.⁷ Among the subtypes of spinal CSF leakage, CSF-venous fistula (CVF) is believed to be the least common and can present the greatest diagnostic challenge. In contrast to dural tears, with extradural CSF collection, CVFs are typically undetectable on noninvasive imaging. CVFs, which occur most frequently at the perineural venous plexus within the neural foramen, require decubitus dynamic CT or digital subtraction myelography for diagnosis.

Here, we describe a case of suspected CM-I with syrinx treated with PFD that developed progression of the syrinx and clinical deterioration. Subsequent investigation revealed a large spinal CVF that was successfully treated with transvenous embolization, resulting in rapid improvement of both conventional markers of SIH (Bern score) and significant improvement in syrinx.

Illustrative Case

A 59-year-old man presented with new headache after a fall from standing. The headache was exacerbated with coughing and Valsalva maneuvers. Over several months, he reported numbness and stiffness of his dominant right hand with decreased dexterity. Brain and cervical spine MRI showed tonsillar descent and a syrinx cavity extending from the cervicomedullary junction to C7 (Fig. 1A). He underwent suboccipital craniectomy, C1 laminectomy, expansile duroplasty, and tonsillar coagulation. Two days after discharge, he presented with nausea and right upper extremity weakness, requiring inpatient rehabilitation. His symptoms normalized to his preoperative neurological baseline at 3 months, and imaging showed initial improvement in syrinx size. At 8 months, he underwent a posterior fossa revision for progression in syringomyelia, in which detethering of the pericranial flap to the underlying tonsils and cervicomedullary junction was performed. Initial postoperative imaging showed improved retrotonsillar CSF space but persistent yet smaller syrinx. He continued to experience fluctuating headache, brain fog, and gait unsteadiness and developed bilateral hand fine motor weakness. Repeat MRI of the brain and cervical spine approximately 6 months after the revision surgery showed an enlarged syrinx cavity greater than the original presenting syrinx and progressive crowding of the foramen magnum (Fig. 1B).

FIG. 1.

. T2-weighted axial (C2–3 neural foramen; upper row) and sagittal (lower row) MR images. A: Preoperative images showing tonsillar descent and a cavitating syrinx from C2 to C7. B: Repeat images obtained several months after PFD showing progression of cavitation to involve the brainstem and the cord through T1. C: Images obtained 2 months after CVF embolization showing involution of the syrinx with more pronounced improvement at the upper cervical cord. The ventral and dorsal CSF space at the foramen magnum also increased posttreatment. D: Images obtained 1 year after CSF-venous embolization revealing near-complete resolution of the syrinx.

Review of total spine images showed no spinal longitudinal extradural fluid collection, excluding the diagnosis of a type 1 spinal CSF leak.^8,9 However, closer inspection of the brain images showed persistent crowding of the decompressed posterior fossa with notable brain sag features and a paucity of CSF volume at the foramen magnum. The Bern score was 8, due to the venous engorgement, suprasellar and prepontine effacement and decrease in the mamillopontine distance, and dural enhancement, indicating a high probability of spinal CSF leakage.¹⁰ Based on these findings, further evaluation with dynamic myelography was recommended.

Dynamic CT myelography was performed under local anesthesia with the patient in the right decubitus position with approximately 15° of pelvic-cranial tilt. Prior to contrast injection, 10 mL of normal saline was infused to augment CSF pressure,¹¹ and the patient was instructed to perform resisted inspiration during scanning.¹² First- and second-pass images revealed a large CVF at the right T8 neural foramen, with opacification of the azygous vein (Fig. 2A and B). Lower cervical meningeal diverticula were noted, but no extradural contrast or other CVFs were identified.

FIG. 2.

. A and B:Dynamic MIP reconstruction (A) and volumetric reconstruction (B) CT myelograms showing a large CVF at the right T8 neural foramen (arrows) with visible contrast accumulation in the azygous vein. C and D:Intraprocedural venograms showing the corresponding right T8 paraspinal vein. E and F:Postembolization flat-panel CT reconstructions showing embolic material filling the paraspinal vein, perineural venous plexus, and internal epidural venous plexus.

After multidisciplinary discussion, transvenous embolization of the CVF was recommended.^13,14 Under general anesthesia, a Benchmark 071 catheter (Penumbra) was advanced over the azygous vein from a transfemoral approach. Azygous venography enabled selection of the right T8 paraspinal vein. A Scepter balloon microcatheter (Balt) over a hybrid 0.008-inch wire (Asahi Intecc) was advanced to a position abutting the nerve root sleeve (Fig. 2C and D). With the balloon inflated, ethylene vinyl alcohol copolymer (Onyx 34, Medtronic) was injected to target filling of the perineural venous plexus with penetration to the internal vertebral venous plexus. Embolization was continued until reflux along the intercostal and paraspinal veins was observed. Postembolization flat-panel CT showed good deposition of embolic material in the location of the CVF (Fig. 3E and F). The patient recovered well from the procedure and was discharged on postoperative day 1.

FIG. 3.

. A: Preembolization T1-weighted midline sagittal MR image showing features of intracranial hypotension, with effacement of the suprachiasmatic cistern and prepontine interval, convexity of the pituitary gland, and decreased mamillopontine interval. B:Postembolization sagittal MR image showing improvement in these brain sag features with a marked increase in volume of the dorsal and ventral CSF space at the foramen magnum (arrows).

MRI of the brain at 2 months showed improvement in suprasellar effacement, mamillopontine interval, and prepontine cistern space, consistent with the previously described effects of treated SIH (Fig. 3A and B).¹⁵ Cervical spine imaging showed marked improvement in the syrinx and cross-sectional area of the cervical cord and restoration of the CSF space at the foramen magnum at 2 months (Fig. 1C) and continual syrinx reduction at 1 year (Fig. 1D). At the 1-year ­follow-up, the patient had recovery to full strength (5/5), resolution of both his daily headaches and brain fog with normal occupational therapy evaluations, and only mild residual right upper extremity numbness.

Informed Consent

The necessary informed consent was obtained in this study.

Discussion

This case highlights the challenges of contemporary diagnosis and management of disordered CSF flow at the craniocervical junction. Classically, the presence of tonsillar descent and syrinx suggests CM-I, and many patients with these findings show dramatic improvement with PFD.¹⁶ In a systemic review of the literature, the rate of persistent or recurrent syringomyelia after Chiari decompression surgery in adults averages approximately 6.7%.³ Persistent syrinx has often been attributed to inadequate decompression, but the present case demonstrates that intracranial hypotension from a remote CVF can produce an acquired brain sag and tonsillar ectopia. It is plausible that historic instances of refractory syrinx and tonsillar descent after PFD may reflect an undiagnosed, untreated CSF leak, as in this case.

Observations

The concept of an acquired tonsillar herniation with Chiari-like appearance in patients with CSF leakage in SIH was described by Atkinson et al. more than 2 decades ago.⁵ Schievink et al. identified a particular spinal CSF leak, CVF, formed by an aberrant connection between the nerve root sleeve and a paraspinal vein.^{9, 17} These CVFs represent an important subtype of leak that is typically undetectable on conventional spine imaging and routine myelography. Advanced, dynamic imaging with CT or fluoroscopy is necessary to document CVF, which may be transient and sensitive to fluctuations in CSF and venous pressure.^11,12 The brain imaging features of SIH are summated in the Bern score, which includes tonsillar descent but notably omits syrinx.¹⁰ A more recent study attempting to differentiate CM-I from SIH also did not recognize syrinx as a discriminating feature.⁴ Several cases of syrinx due to a documented spinal CSF leak have been reported, but the prevalence is likely quite low.^6,7 The presence of a syrinx can therefore be considered an atypical finding in SIH, but this diagnosis cannot be excluded.

The mechanism of syringogenesis in CSF leakage is not understood, and theories of syrinx in CM-I have been debated for decades.¹⁸ Various theories implicate disordered CSF pulsatility during the cardiac cycle,¹⁸ intramedullary-subarachnoid pressure dissociation, and venocapillary pressure dissociation.¹⁹ Common to these mechanisms is a mechanical disruption of physiological CSF flow at the foramen magnum. This case, which showed improved CSF space throughout the posterior fossa after CVF embolization, supports a shared mechanism of syrinx formation in CSF leakage (acquired tonsillar herniation) and CM-I. A chronic or intermittent drop in spinal CSF pressure may draw the posterior fossa structures caudally, resulting in functional disconnection of cranial and spinal CSF compartments. In this case, the persistent “pull” of the CVF resulted in obstruction of foramen magnum CSF flow, despite bony decompression of the posterior fossa. Distortion of spinal CSF compartment compliance, a feature of CVF,^11,20 may accentuate the craniospinal pressure gradient and intramedullary-subarachnoid gradient, further drawing interstitial fluid into the central canal.²¹ Functional separation of spinal and cranial epidural venous systems can be aggravated by respiratory and cardiac cycle–dependent venous pressure fluctuations.¹² This case represents the second report in the literature showing that obliteration of a CVF can treat syringomyelia.¹⁴

Lessons

While significant progress has been made in recent years, spinal CSF leakage remains a challenging and easily overlooked diagnosis in neurosurgery. This case highlights the striking clinical and radiographic overlap of symptomatic SIH due to CVF and classical CM-I. Cerebellar tonsillar descent is nonspecific, and other imaging features with greater discriminatory power (e.g., midbrain sagging and/or pachymeningeal enhancement) should be scrutinized in each case of suspected CM-I prior to surgery.⁴ The presence of syrinx is uncommon in SIH but likely arises owing to similar foramen magnum obstructive mechanisms as in CM-I; therefore, syrinx alone should not preclude the diagnosis of SIH. Cases of persistent syrinx or tonsillar descent despite adequate decompression warrant particular attention and may represent a population of unrecognized SIH. When SIH is suspected by brain imaging markers (high Bern score) or failed response to PFD, a CSF leak study should be initiated with whole-spine MRI and invasive myelography.⁸ Our findings support the durable effectiveness of embolization of CVF in treating acquired Chiari malformation and syringomyelia.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Caton, Barros, Yoon, Roonprapunt. Acquisition of data: Caton, Barros, Yoon, Ghatan, Roonprapunt. Analysis and interpretation of data: Caton, Barros, Bolognese, Ghatan, Roonprapunt. Drafting the article: Caton, Yoon, Roonprapunt. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Caton. Study supervision: Caton.

Supplemental Information

Previous Presentations

Portions of this work were presented at the American Society of Neuroradiology Annual Meeting 2025, Philadelphia, PA, May 17–21, 2025.

Correspondence

Michael T. Caton: Mount Sinai South Nassau Hospital, Oceanside, NY. michael.catonjr@mountsinai.org; travis.caton@gmail.com.