Azygos Venous Pressure and Respiratory Modulation of CSF-Venous Pressure Gradients in SIH and IIH patients.

BACKGROUND AND PURPOSE: The azygos system is a major drainage pathway of the thoracic spinal venous network and may participate in the regulation of spinal CSF resorption. CSF-pressure disorders such as spontaneous intracranial hypotension (SIH) related to a CSF-venous fistula (CSFVF) and idiopathic intracranial hypertension (IIH) could therefore be influenced by the hemodynamics and the pressure of this system. However, azygos pressure and its respiratory modulation remain poorly characterized.

MATERIALS AND

METHODS

This retrospective single-center study included 54 consecutive patients who underwent combined venous and CSF pressure measurements between September 2025 and February 2026. Forty-five patients were investigated for SIH/CSFVF and 9 for IIH. Pressures were recorded in the azygos system, intracranial venous sinuses, right atrium (CVP), and lumbar CSF compartment. Dynamic measurements were obtained during resisted inspiration, breath holding, and Valsalva. Values are reported as mean ± SD or median [IQR], as appropriate. Paired respiratory changes were assessed with Wilcoxon signed-rank tests.

RESULTS

Overall, median azygos pressure was 8.0 [6.0-9.8] mm Hg and was modestly but significantly higher than CVP: 6.5 [4.0-8.0] mm Hg; median paired difference, +2.0 [1.0-3.0] mm Hg; p < .001. Azygos pressure did not differ significantly between SIH/CSFVF and IIH groups: 8.0 [6.0-9.0] versus 9.0 [8.0-10.0] mm Hg; p = .34. Level-by-level measurements in 25 patients showed only limited cranial pressure decline, with median pressure of 8.0 [6.0-9.0] mm Hg in the caudal L2-T10 segment and 7.0 [5.0-8.0] mm Hg in the cranial T9-T5 segment; p = .007. Resisted inspiration produced the largest CSF-azygos gradient, increasing from 0.0 [-3.0 to 5.0] to 12.0 [11.0-15.0] mm Hg, primarily because azygos pressure decreased from 7.0 to -6.0 mm Hg while CSF pressure remained stable.

CONCLUSIONS

In the investigated population, median azygos pressure was 8mmHg, close to CVP, and stable along the spinal axis. Resisted inspiration markedly increased the CSF-azygos gradient by lowering azygos pressure, providing a hemodynamic explanation for improved CSF-venous fistula conspicuity during resisted-inspiration myelography.