CSF-venous fistulas Reconsidered: Pressure paradox and the Volume-Elastance Relationship.

BACKGROUND AND PURPOSE Spontaneous intracranial hypotension (SIH) is a debilitating condition most often caused by spontaneous cerebrospinal fluid (CSF) leaks, with CSF-venous fistulas (CVF) representing an increasing number of cases. Pathophysiological understandings of CVFs, particularly those concerning pressure dynamics between the CSF and venous systems, remain elusive. This study aimed to mathematically model CVFs using first principles physics and to explore pressure dynamics and their implications for treatment and complications.

MATERIALS AND METHODS CSF-venous pressure dynamics were modelled using physics first-principles. Adjustable parameters included initial CSF pressure, CSF production rate, fistula radius, and dural elastance. Dynamic equilibrium pressures and volumes were calculated iteratively, with results plotted against fistula radius, CSF production rate, and dural elastance.

RESULTS The model demonstrated that an increase in CSF daily production increased the dynamic equilibrium pressure. Greater dural elastance lowered CSF volume at equilibrium without changing equilibrium pressure. CSF pressure rapidly equilibrated to that of venous pressure for larger fistulas and physiological CSF pressure for smaller fistulas.

CONCLUSIONS Fistula radius, altered states of CSF production, and dural stiffening complicate the clinical presentation of CVFs. Current hypotheses do not adequately account for raised opening pressures and rebound intracranial hypertension. Instead, global CSF dysregulation, including increased production and impaired secondary outflow, may co-exist in patients with CVFs. Future management should evaluate CSF volume-pressure dynamics, rather than exclusively focusing on the fistula, to improve both diagnosis and anticipate complications.

KEY MESSAGES CSF-venous fistulas are an increasingly recognised cause of spontaneous intracranial hypotension. Current understandings of CSF-venous pressure dynamics remain elusive. This study modelled the behaviour of CSF pressures to inform a discussion on potential secondary pathophysiological factors. In doing so, it seeks to ensure clinicians consider patient-specific compensatory mechanisms when providing treatment, to anticipate complications and ensure efficacy.