Resisted Inspiration Improves Visualization of CSF-Venous Fistulas in Spontaneous Intracranial Hypotension

DISCUSSION

Our investigation found that in 5/8 (63%) cases, resisted inspiration resulted in superior visualization of the CVF compared with maximum suspended inspiration or the Valsalva maneuver. A recent investigation hypothesized that resisted inspiration might prove superior to maximum suspended inspiration on the basis of venous manometry data obtained during various phases of respiration, but it did not directly test the effect on patients with actual CVFs.¹⁰ Our investigation conducted in cases of patients with SIH supports the hypothesis posited by Mark et al¹⁰ that resisted inspiration can be useful in enhancing the visibility of a CVF.

Resisted inspiration did not result in improved visibility of CVFs in all cases, however. In 2 cases, the CVF was best visualized on a different phase of inspiration (maximum suspended inspiration and the Valsalva maneuver, respectively). Both of these CVFs were located at T3 on the left. While it is possible that this association is coincidental, venous drainage of the upper left thoracic spine differs from that in the remainder of the thoracic spine. Specifically, the second through fourth thoracic vertebral levels on the left usually drain into the superior intercostal vein, while the rest of the thoracic spine drains into the azygous system.¹³ It is possible that resisted inspiration may affect augmentation of venous drainage differently in the upper left thoracic spine on the basis of this anatomic difference, though more experience with this technique will be needed to determine if this observation is reproducible.

We also observed that in some cases, the pattern of venous drainage may change on the basis of the respiratory phase. Variations in intrathoracic pressure and the resultant changes in venous pressures during different phases of respiration may influence the route of venous drainage for a single fistula. This possibility could be consequential because some patterns of venous drainage may be more difficult to detect than others. For instance, drainage into the internal epidural venous plexus or basivertebral venous plexus could lie behind the overlying intrathecal contrast column and, therefore, be obscured on planar imaging such as digital subtraction myelography. Similarly, on CTM, drainage into the basivertebral venous plexus could potentially be less conspicuous than drainage into other routes because of the surrounding bone of the vertebral body. With greater experience, it may be possible to predict which phase of respiration is most likely to augment CVF visualization. It is also possible that multiple phases of respiration may be needed during an examination, depending on the vertebral level being imaged.

Some institutions use decubitus Trendelenburg patient positioning coupled with multiple phases of scanning within seconds after intrathecal contrast bolus injection, sometimes referred to as “dynamic CTM,” to visualize CVFs.^9,14 We did not use this technique for the CTMs in our study because we have found in our experience that maintaining a dense pool of contrast over the nerve roots is more important than temporal resolution when investigating possible CVFs with CTM, which can be accomplished without a dynamic contrast bolus. Moreover, a dynamic decubitus CTM technique would have made comparison of respiratory phases impossible because the migration of the contrast bolus would not provide adequate time for investigation of multiple respiratory phases. Although we make an effort to scan as quickly as possible after intrathecal contrast injection, our study does show that some delay in scanning does not preclude detection of CVFs as long as dense contrast can be maintained over the spinal nerve roots.

Our study has several limitations. First, this investigation included only CVFs seen or suspected during the initial imaging in maximum suspended inspiration. Although this study design has the potential to bias the results in favor of this particular inspiratory phase, we, nevertheless, found resisted inspiration to be superior to maximum suspended inspiration in most cases. However, because of this study design, the study cannot directly determine whether resisted inspiration will result in greater detection of fistulas in cases in which no fistula is seen at all on maximum suspended inspiration. Although it seems plausible that it might be helpful, the use of resisted inspiration as the sole respiratory phase used during an examination may be difficult to implement in practice in some cases, for several reasons: First, the length of time a patient can maintain inspiration against resistance may be insufficient to allow scanning of all vertebral levels of interest during CTM. When examinations are performed using DSM, resisted inspiration may be difficult to implement because the progressive expansion of the thorax may result in misregistration during image subtraction. Furthermore, resisted inspiration also depends on active cooperation and would not, therefore, be possible if examinations were performed with the patient under general anesthesia, which is used by some practitioners when performing DSM.

A second limitation of the study is that all variables that affect CVF visibility are not currently known; thus, factors unrelated to the respiratory phase (such as changes in contrast density in the thecal sac, differences in the cardiac cycle, or other unknown factors) may serve as confounders. Individual patients being scanned in the same session at very close time intervals may mitigate these effects, but this scenario does not completely exclude such influences. Finally, our study design relied on subjective assessments of CVF visibility by experienced, unblinded readers. Future investigations may consider whether more explicitly structured assessments by readers with varying levels of experience may better reflect the diagnostic performance of this technique across varied clinical practice environments.