Clinical Significance of Redundant Nerve Roots of the Cauda Equina

Abstract

Redundant nerve roots (RNRs) of the cauda equina are a phenomenon characterized by the presence of enlarged, tortuous nerve roots in the lumbar subarachnoid space. First described by Verbiest in 1954 (6), RNRs previously were believed to be a rare congenital anomaly. The association of RNRs with lumbar spinal stenosis is now well recognized, although they have also been associated with arachnoid cysts (1) and plexiform neurofibromas (3). Although the etiology of this entity remains unclear, it is hypothesized that a focal constriction or tethering of the nerve roots leads to restriction of the normal movement of the spinal nerves, their stretching during flexion and extension of the spine, and subsequent elongation and redundancy. This hypothesis is supported by the fact that most observed RNRs occur above the level of constriction or stenosis. Lumbar spinal canal stenosis is a common condition. At the present time, 0.5 million Americans, most >60 years old, experience symptoms attributable to lumbar spinal stenosis. All spinal surgeons see and treat patients with lumbar spinal stenosis regularly, and it remains one of the most prevalent conditions for spinal surgeons. The pathophysiology of lumbar spinal stenosis is multifactorial, but facet hypertrophy, disk space height loss, and subsequent ligamentum flavum buckling are critical components. The clinical syndrome that accompanies this anatomic narrowing is likely due to direct local trauma to the nerve roots, ischemia, focal demyelination, and inhibition of axonal transport. As the disease progresses, stretched and redundant nerve roots can be observed grossly. Recognizing RNRs on preoperative imaging is important for the surgeon clinically because these nerve roots may have a higher tendency to herniate out through an inadvertent dural tear incurred during the operation. With improved and more widespread imaging, it is now appreciated that RNRs are much more common than previously believed. Suzuki et al. (4) found that >40% of patients with severe constriction in the lumbar region demonstrated RNRs. The diagnosis of RNRs has traditionally been made by myelography, and although this remains the “gold standard,” increasing evidence points to the efficacy of magnetic resonance imaging (MRI) for diagnosis. During lumbar spine MRI, the knees and hips are typically flexed, and this may lead to an increase in falsenegative results associated with this modality. Nevertheless, the noninvasive nature of MRI has increasingly made it the mainstay of current imaging techniques. Studies investigating the clinical significance of RNRs are scarce. One of the few available clinical studies on RNRs demonstrated that they were associated with advanced age and a longer duration of symptoms related to canal stenosis, particularly neurogenic claudication (5). More recent work by a Korean group replicated the finding that RNRs were observed in older patients but failed to show a statistically significant difference between patients with RNRs and patients without RNRs in terms of surgical outcomes, although there was a tendency toward improved outcomes in patients without RNRs (2). Very little is known about the time course of the resolution of RNRs, if any, and implications on disease prognosis. In the present issue of WORLD NEUROSURGERY ,Y okoyama et al. report their results of the clinical significance of postoperative changes in patients with RNRs following decompressive laminectomy for lumbar spinal canal stenosis. This is a retrospective study from a single center, in which 33 patients with RNRs as demonstrated by MRI were treated with decompressive