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The Spine Blog

Friday, August 30, 2019

How well do we understand the pathophysiology of cauda equina syndrome?

Cauda equina syndrome (CES) is a condition that causes spine surgeons a high degree of angst that is far disproportionate to its prevalence. For one, the specter of CES is frequently raised by non-spine physicians for any patient who presents with some combination of urinary dysfunction and back pain. Given the huge numbers of patients with either urinary dysfunction or back pain, the overlapping section on the Venn Diagram for the two conditions is quite large. As such, the spine surgeon is doing his or her best to not miss the rare patient with true CES among the plethora of potential CES patients on whom they are consulted. Additionally, for the small number of patients who present with true CES including urinary incontinence, it is somewhat unclear if emergent surgery (frequently in the middle of the night) really changes the long-term outcome. Finally, and possibly most bothersome, is the threat of litigation surrounding CES regardless of what the surgeon does. Given the low prevalence of true CES, it is a very difficult topic to study, and the best data that exist are meta-analyses of Level IV case studies. These analyses have led to conflicting results about whether or not emergent surgery (i.e. middle of the night) leads to better outcomes than urgent surgery (i.e. the next morning). As a result, spine surgeons frequently operate on CES patients emergently despite any clear evidence that doing so justifies the increased risk and cost associated with going to the OR emergently. When clinical data do not yield sufficient evidence to guide treatment, in some cases basic science or animal data can shed some light on the underlying pathology that can help to guide treatment. To that end, Dr. Pronin and colleagues from the United Kingdom performed a meta-analysis of animal studies evaluating the effect of duration of compression and magnitude of pressure on the function of the cauda equina. They identified 17 studies on dogs and pigs looking at this topic, with most of the models using a balloon inflated in the spinal canal. Nerve function was evaluated using electrophysiological studies (i.e. nerve conduction velocity or motor or sensory evoked potentials). Using complex statistical modeling, they determined that pressure was a stronger predictor of neurological change and recovery than duration of compression. When pressure was below mean arterial blood pressure (MABP), electrophysiological changes tended to be minor and reversible. Pressures between MABP and systolic blood pressure (SBP) generally caused partial deficits that were partially reversible. As pressures exceeded SBP, deficits tended to be complete and irreversible. The meta-analysis suggested that deficits tended to develop after one hour of compression and longer duration of compression did not substantially change the severity of deficit or degree of recovery. The strongest predictor of function following decompression was the degree of pre-decompression deficit.

The authors have performed a complex analysis using very heterogeneous data from experiments using a relatively wide variety of methods. While this is likely the best that can be done with the available data in the literature, the results need to be interpreted carefully. For one, it is unclear if inflating a balloon in the spinal canal has a similar effect as a herniated disk, which also results in a major inflammatory response. Additionally, most of the studies did not maintain pressure on the cauda equina for more than 4 hours, and most CES cases do not undergo surgical decompression in under four hours even in the best-case scenario. The final electrophysiological evaluation was typically performed 90 minutes after the balloon was deflated, while nerves likely continue to recover for months after decompression for CES. Finally, it is unclear how well electrophysiological results following CES correlate with actual nerve function. Despite these limitations, the data do suggest that the die may be cast for many patients who present many hours (or days) after the onset of CES. There is no way for surgical decompression to occur within one hour of symptom onset, and this study suggests that the electrophysiological changes that are present for over an hour generally do not improve much with decompression. Additionally, in cases where the cauda equina pressure exceeded SBP and neurological deficit was complete, decompression did not lead to improvement. This study suggests there may be a small proportion of patients who present with incomplete deficits early in the course of CES who might benefit from an emergent operation. Neither these animal data nor the clinical data that currently exist allow the spine surgeon to determine who these patients are. Until we have a better understanding of which CES patients benefit from emergent decompression, spine surgeons will likely continue to decompress all CES patients emergently for medicolegal reasons if not for purely medical reasons.

Please read Dr. Pronin's study on this topic in the September 1 issue. Does this change how you approach CES? Let us know by leaving a comment on The Spine Blog.