Cerebrospinal Fluid Drainage in Endovascular Aortic Repair: Reply

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In Reply: We would like to thank Collange et al. for their kind words1 regarding our review2 and agree with their team about the need for randomized controlled trials for cerebrospinal fluid (CSF) drainage after high-risk thoracic endovascular aortic repair. We also agree with them regarding the importance of considering all the factors integral to spinal cord perfusion including organ-specific variation in critical closing (and by extension, opening) pressures, modifying factors related to vascular tone and external compression, and the critical influence of venous pressure.1 Paralysis after aortic aneurysm surgery was described by Doppman3 as “Russian roulette” due to the inability of the perioperative team to predict with certainty which patients would develop paralysis and which would not. He also stated that “the blood supply to the spinal cord is more variable anatomically and more complex hemodynamically than the blood supply to any other vital organ,”3 a point clearly made by Collange et al.1 The importance of venous pressure was well elucidated by Etz et al. in their a single-center retrospective review of 858 patients that found that the mean central venous pressures were significantly higher (P = 0.03) in patients who developed paraplegia 1 to 5 h postoperatively.4 In their discussion, they highlighted the fact that “outflow from the spinal canal depends directly on CSF and venous pressures and whether spinal cord edema is present.”4 Etz et al. further described the vertebral venous system as “a unique, large-capacity, valveless venous network in which flow is bidirectional and includes the vertebral venous plexuses, which course along the entire length of the spinal cord and anastomose with the intracranial veins in the suboccipital region”4 following the anatomic description by Tobinick and Vega.5 The close anatomic relationship of the spinal venous plexus to the spinal cord and its surrounding CSF within the semiclosed space of the vertebral canal imparts an important interrelationship between venous pressure and measured CSF pressure. That is, when the venous plexus surrounding the spinal cord is engorged, this is often (but not always) reflected in an increase in CSF pressure as well. Thus, CSF pressure can often reflect the venous pressure itself, partly negating the need for it to be considered as a completely separate variable in the standard equation of spinal cord perfusion pressure. However, in circumstances where CSF pressure is not being directly measured (which appears to be the recent trend in thoracic endovascular aortic repair), central venous pressure becomes a surrogate of CSF pressure itself. The future of eliminating postaortic surgery paralysis after stent grafting will likely depend on more than just a better understanding of postoperative changes to spinal cord blood flow (e.g., biomechanics, fluid dynamics, neurovascular physiology, collateral network blood flow). It will also require a more nuanced understanding of the complex molecular pathophysiology involving both ischemia–reperfusion injury and other processes (e.g., inflammatory white matter changes) that may differ from conventional open surgery techniques.6 There will be no easy solution to solving this devastating and clinical perplexing complication of thoracic endovascular aortic repair. We agree with French Nobel laureate, physicist, and chemist Marie Curie, who wisely stated about her own research struggles that “I was taught that the way of progress was neither swift nor easy.”7 Competing Interests Dr. Grocott serves as a consultant to Anesthesia Labs Inc. (San Francisco, California), for which he receives stock options. He also serves as a consultant for Cyban PTY Inc. (East Melbourne, Australia). Additionally, he has received payment as a scientific advisory board member from Edwards Lifesciences (Irvine, California) and as a speaker from Masimo Inc. (Irvine, California). Dr. Grocott also serves as a paid section editor for UpToDate (Waltham, Massachusetts). The other authors declare no competing interests.

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