Redundans Nervi Radix Cauda Equina: Pathophysiology and Clinical Significance of an Intriguing Radiologic Sign

Abstract

he scenario is very simple and familiar to most neuro-surgeons: The magnetic resonance imaging (MRI) scan ofa patient with lumbar canal stenosis shows the nerves ofthe cauda equina in a tortuous, serpiginous, and tangledappearance above the site of constriction of the lumbar canal. Iremember being told in my early residency days to interpret sucha radiologic finding with the same concern for neural tissuecompression (and its associated deleterious effects) of anabnormal fluid attenuated inversion recovery signal in a brain MRIscan. Previous studies have demonstrated that the lumbar canalconstriction that causes the appearance of redundant nerve rootsalso leads, at the histologic level, to disarrangement and reduc-tion of the number of nerve fibers, demyelination, endoneuralfibrosis, and Schwann cell proliferation in these roots (13). On thebasis of such secondary histologic changes, it is not surprisingthat in a subset of patients presenting with such a radiologic sign,the long-term functional outcomes (in terms of both pain andfunction) remain poor even after adequate decompression of thespinal canal.Although the finding of redundant nerve roots in the cauda equinawasfirstdescribedmorethan50yearsagoinmyelographystudies(15),relativelylittleisknownaboutthepathophysiologyandclinicalsignificance of this radiologic phenomenon. A literature search atPubMed/Medlinewiththekeywords“redundantnerveroots”and“cauda equina” yielded only 29 indexed articles, with 25 of themhaving being published more than 20 years ago. Most of the olderreports focused not on the association between redundant nerverootsandlumbarcanalstenosisbutratheronthespecificradiologiccharacteristicsthatmighthelpdifferentiatesuchfindingsfromthepresence of arteriovenous spinal malformations in spinal myelo-grams(9,12).Asignthatanetiologicrelationshipbetweenlumbarcanal stenosis and the presence of redundant nerve roots (nowclearly appreciated) was not understood in those early days is thefactthatsomeofthesepreviousreportsevenproposedopeningoftheduramatertodisentanglethesupposedly“knottedandcurled”elongated nerve roots, followed by a questionable duraplasty torelieve the presumed “increased pressure” under which suchroots might be (9).Even today, the reality is that any statement regarding thepathophysiology of the appearance of redundant nerve roots inthe setting of lumbar canal stenosis remains purely speculative.One hypothesis is that the underlying etiology leading to such anabnormality would be essentially mechanical. Because previouspostmortem anatomic studies in patients with severe lumbarcanal stenosis have demonstrated that all redundant nerve rootspassed through the maximum point of constriction in the spinalcanal, it has been suggested that such squeezed roots would besignificantly stretched during concomitant leg and trunk exten-sion, ultimately leading to their elongation (13).Another possible etiology for such a finding would be a vascularabnormality in the cauda equina (related to either a compromisedarterial supply or an impaired venous drainage) induced by theconstrictive forces at the stenotic sites. Vascular changes have