Spinal Compliance Curves: Preliminary Experience with a New Tool for Evaluating Suspected CSF Venous Fistulas on CT Myelography in Patients with Spontaneous Intracranial Hypotension

DISCUSSION

We report a method of generating craniospinal compliance curves derived from CT myelography and characterize these curves in a preliminary analysis of patients with SIH and suspected CVF. Compliance measurements (C_op, overall CSC compliance) in patients with confirmed CVFs were statistically different from those in patients with SIH in whom a final etiology of CSF leak could not be identified. Using a hand-bolus injection as a simple and reproducible alternative to continuous infusion techniques, we show how patient-specific models of pressure-volume dynamics can be reconstructed and the PVI can be estimated.

Although these findings are preliminary, we propose several potential uses of CSC curves in patients with SIH who undergo CTM: First, a drop in CSF pressure observed on the CSC curve may help identify patients with dural leaks or fistulas. Second, there are probably pressure-dependent CVFs, that otherwise remain occult without adequate pressure augmentation (Fig 1). Finally, the documentation of CSF compliance may also define a patient-specific baseline, allowing quantification of treatment response, as we show with the epidural blood patch (Fig 3). For example, baseline patient-specific CSC parameters could be used in patients who undergo surgical fistula ligation as a means of confirming successful or unsuccessful CVF exclusion.

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FIG 3.

Craniospinal compliance curve in a 63-year-old woman suspected but not radiographically confirmed to have CVF. Estimated linear compliance for each CTM is shown as a dashed line, approximating the shape of the pressure-volume curve (A). The second CTM was performed after the blood patch showed an increase in opening pressure (B and C). The effect of the blood patch is diminished on delayed repeat CTM (D), in which the left side of the curve, including opening pressure, more closely matches the prepatch curve.

The theoretic basis for CSC curve analysis in patients with CSF leak warrants additional discussion. Abnormal profiles of PVI and the resistance to CSF outflow have been described in patients with intracranial pathology, including normal-pressure hydrocephalus, subarachnoid hemorrhage, and traumatic brain injury.^17⇓-19 However, contemporary models of CSF-space compliance treat the cranial and spinal spaces as communicating compartments of the same system.⁶ In these models, the CSC is analogized to an electrical circuit with systemic compliance determined by the additive effects of cranial and spinal compartments.^16,13,20 Abnormal dural compliance in the spinal compartment should, therefore, influence the entire craniospinal system. CVF, like all spinal CSF leaks, should, in theory, increase compliance, but the pressure-volume relationship is likely dictated by the dynamic equilibrium of CSF pressure and venous pressure. Our observation that in some patients, a sudden collapse of pressure occurred during saline infusion may indicate that we had reached a CSF-venous pressure equilibrium point. A further increase in CSF pressure could conceivably open the leak and provides one explanation for the inflection in compliance observed in this study (Fig 4). While unproven, this mechanism could provide a physiologic explanation for why CVFs are often occult in the supine position but manifest when the patient is upright or in the decubitus position or provocation maneuvers are performed. Our observation is also concordant with work from Amrhein et al,¹¹ which showed increased conspicuity of the CVF with inspiration, which augments the CSF-venous pressure gradient increasing venous return to the right atrium. It has been postulated that such maneuvers open the leak, which could be detected as a sudden increase in compliance of the craniospinal system.^5,9,21 A similar mechanism was postulated by Kumar et al,²² who reported indirect treatment of CVFs by reduction of the CSF-venous gradient using an inferior vena cava stent.

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FIG 4.

Schematic of the components of craniospinal compliance and hypothesized physiology of CVF (CSF = green, arterial blood = red, venous blood = blue). A normal CSC encompasses both intraventricular and subarachnoid CSF and is defined by cranial and spinal compartments (larger and smaller boxes, respectively) as well as the arteriovenous vascular bed (A). In dural tear CSF leak or CVF at low pressure, an equilibrium state (B) may exist in which the leak is occult by CTM. With special maneuvers (dynamic CTM, respiratory-phase variation, jugular pressure, and bolus-pressure augmentation), pressure gradients may open the leak, allowing detection on CTM (C).

The hand-injection bolus pressure augmentation method is easily reproducible and does not require additional equipment, unlike previously described methods using steady-state infusion.^20,23,24 The hand-bolus technique is more appropriate for patients with SIH than serial CSF withdrawal, which was described by Griffith et al²⁵ as a means of estimating compliance and PVI. This approach is based in mathematic and empiric observations showing that the PVI can be accurately measured using bolus (rather than steady/continuous) infusion.⁸ Although only a single bolus injection is needed to estimate PVI, multiple measurements, as used in our study, may be advantageous to identify zones in which the pressure-volume relationship changes more quickly due to autoregulatory mechanisms.²⁶ While dynamic CTM is the preferred technique at our institution, the bolus-pressure augmentation method could be performed with conventional or dynamic fluoroscopy. In this preliminary experience, we observed no periprocedural complications and only 2 instances (5.8%) of transient headache during bolus infusion. These findings suggest that this technique has a reasonable safety profile when pressures and symptoms are closely monitored.

The interpretation of CSC curves has several important technical and theoretic limitations. First, because we did not investigate CSC parameters in controls without SIH, it is difficult to estimate the diagnostic value of CSC curves in routine practice. A corollary limitation is that comparisons between confirmed and unconfirmed (suspected) CVFs are inherently limited and should be interpreted conservatively. Another important limitation is that CSC has been shown to vary with posture, largely due to shifts and redistribution of venous outflow through the jugular veins.²⁷ While our data suggest that a rapid drop in CSF pressure during pressure augmentation may reflect opening of a pressure-dependent CVF, this effect could plausibly be due to other physiologic effects such as respiratory phase variation and the cardiac cycle.¹¹ Definitive correlation could be established if the bolus-pressure augmentation technique were performed during digital subtraction myelography to allow real-time visualization of the CVF; we would expect that the leak would become visible or more conspicuous as CSF pressure is increased with serial intrathecal boluses, controlling for respiratory phase and cardiac cycle variations.

Another important limitation of our study is that the population included in this analysis comprised patients with suspected CVF who remained undiagnosed after conventional diagnostic and interventional studies (eg, MR myelography, conventional CTM). Accordingly, patients with other, non-CVF leaks are not represented, and the relatively low rate of confirmatory diagnosis (8/22, 36.4%) in this cohort reflects the diagnostic challenge of CVF relative to other forms of CSF leak.² Some of these patients may have leaks in locations that are more difficult to visualize (eg, sacrum) or simply CVFs that were not visible at that time. An additional important limitation of our study is that we did not assess control patients, and as a result, we did not define discriminatory threshold values of PVI and compliance in healthy patients. Nonetheless, the mean PVI values in our study (63.9 [SD, 7.5] mL) are higher than those in control patients reported in the literature, for example by Shapiro et al⁷ (PVI = 25.9 [SD, 3.7] mL) and by Wahlin et al¹³ (PVI mean = 9.8 [SD, 2.7 mL]), though methodologic differences may affect comparison.^, This finding is consistent with the proposed pathophysiology of CVF as a high-compliance state. A final limitation is that we did not use a standardized approach to positive-pressure augmentation because each operator selected the bolus number and volume before imaging.

This work is a preliminary exploration of a novel technique in a dynamic disease process; while these results are promising, investigation of CSC curves and correlation of steady-state and bolus-infusion methods should also be performed in the CSF leak population to confirm the validity of this approach. Future studies should also establish the boundaries of normal physiology in control patients.