Spontaneous Intracranial Hypotension Without CSF Leakage-Concept of a Pathological Cranial to Spinal Fluid Shift

Abstract

Objective: Spontaneous intracranial hypotension (SIH) is typically caused by CSF leakage from a spinal dural tear, a meningeal diverticulum, or a CSF venous fistula. However, some patients present with classic orthostatic symptoms and typical intracranial imaging findings without evidence of CSF leakage despite repeated diagnostic work-up. This article aims to elaborate a hypothesis that would explain a pathologically increased orthostatic shift of CSF from the cranial to the spinal compartment in the absence of a CSF leak. Medical Hypothesis: The symptoms of SIH are caused by a decrease in intracranial CSF volume, intracranial hypotension, and downward displacement of intracranial structures. A combination of pathologically increased spinal compliance, decreased intracranial CSF volume, low CSF outflow resistance, and decreased venous pressure might result in a pathological orthostatic cranial-to-spinal CSF shift. Thus, in rare cases, intracranial hypotension may occur in the absence of CSF leakage from the dural sac. Conclusion: We propose a pathophysiological concept for the subgroup of SIH patients with typical cranial imaging findings and no evidence of CSF leakage. In these patients, reducing the compliance or the volume of the spinal compartment seems to be the appropriate therapeutic strategy.

Keywords: CSF leak or fistula; CSF physiology and anatomy; compliance; orthostatic headaches; spontaneous intracranial hypotension.

Figure 1

Imaging of a patient with SIH symptoms without evidence of CSF leakage or CSF venous fistula. A 68-year-old male patient exhibiting classic SIH symptoms with typical cranial imaging findings, such as dural enhancement (A) and bilateral subdural hygroma (B) and no evidence of CSF leakage or CSF venous fistula despite repeated diagnostic work-up. Spinal MRI revealed a large number of prominent meningeal diverticula (C).

Figure 2

Pressure-volume curve of the craniospinal compartment. Pressure (P) is displayed on the x-axis and volume (V) on the y-axis. Compliance (C) is represented by the slope of the pressure–volume curve. Due to its exponential function, compliance varies with different pressure–volume status. A shift to the left on the pressure–volume curve leads to increased compliance (C1), whereas a shift to the right leads to decreased compliance (C2).

Figure 3

Hypothetical CSF volume shifts in different constellations of SIH and healthy individuals. Schematic illustration of craniospinal CSF distribution in the supine (left) and upright position (right; A–D). The red line indicates the threshold for development of orthostatic symptoms. In a healthy individual only a small proportion of the cranial CSF volume is shifted to the spinal compartment after a positional change (A). CSF leakage in patients with classical SIH leads to CSF hypovolemia and a shift to the left on the pressure–volume curve with increased compliance. Thus, a larger volume (absolute and proportional) is shifted to the spinal compartment (B). In a state of increased spinal compliance, a larger absolute volume is shifted to the spinal compartment (C,D). In patients with normal or low intracranial CSF volume, the proportion of CSF shifted is large and causes internal SIH (C). In patients with high intracranial CSF volume (e.g., because of brain atrophy) the proportionally smaller CSF shift is compensated and no symptoms occur (D).

Figure 4

Overview of factors contributing to internal SIH. Predisposing factors are (A) low CSF outflow resistance (Rout), low intracranial venous pressure (Pss) and (B) low intracranial CSF volume. The main pathophysiological factor is increased spinal compliance due to (C) meningeal diverticula, dural tears with prolapsing arachnoid around meningeal diverticula or (D) increased distensibility of the dura mater (I = normal distensibility, II = increased distensibility). A combination of these factors can lead to a pathological cranial-to-spinal CSF shift causing brain sagging (B) and orthostatic symptoms without CSF leakage.