Nerve Root Anomalies with Application to the Lumbar Plexus

Anatomy

Spinal nerves originate in the spinal column as a set of ventral and dorsal rootlets. Emerging from the spinal cord, these rootlets converge to form the ventral and dorsal roots, respectively. Unlike cervical and thoracic regions, lumbosacral nerve roots must descend distal to the conus medullaris within the dural sac to reach their appropriate root sleeves. At their respective pedicle, nerve roots diverge inferolaterally, traversing the inferior surface of the corresponding pedicle before exiting through the intervertebral foramen. As nerve roots diverge, they remained bound by a meningeal sheath. ¹ Continuing laterally, the subarachnoid extension is terminated near or on the dorsal root ganglion (DRG). Distally, the dorsal and ventral roots join to become the very short spinal nerve, before diverging again into dorsal and ventral rami.

18.2 Congenital Nerve Root Anomalies

18.2.1 Conjoined Nerve Roots

Conjoined nerve roots (CNRs) are defined as two adjacent nerve roots that, at some point during their course from the thecal sac, share a common dural sheath (▶ Fig. 18.1, ▶ Fig. 18.2, ▶ Fig. 18.3, ▶ Fig. 18.4, ▶ Fig. 18.5). Remaining conjoined for a variable course, the two roots separate, following independent courses either through the same or separate foramina. This anomaly tends to display no disturbance of any adjacent segments. CNRs are the most commonly reported anomaly in the literature, generally involving the L5 and S1 nerve roots. ²^, ³^, ⁴^, ⁵^, ⁶^, ⁷^, ⁸ These anomalies tend to appear unilaterally, although there are reports of bilateral CNRs. ⁹

Fig. 18.1 Cannon et al classification: (I) conjoined nerve root, (II) extradural anastomosis, (III) caudal origin (transverse root)

Fig. 18.2 Postacchini et al classification: (I) cranial origin, (II) caudal origin (transverse root), (III) closely adjacent nerve roots, (IV) conjoined nerve root, (V) extradural anastomosis

Fig. 18.3 Neidre and MacNab classification: (IA) conjoined nerve root, (IB) closely adjacent nerve roots, (IIA) two nerve roots exit through common foramen leaving one foramen unoccupied, (IIB) nerve roots in all foramina, but one foramen contains two separate roots, (III) extradural anastomosis.

Fig. 18.4 Kadish and Simmons classification: (I) intradural anastomosis, (IIA) cranial origin, (IIB) caudal origin, (IIC) closely adjacent nerve roots, (IID) conjoined nerve roots, (III) extradural anastomosis, (IV) extradural division of nerve root.

Fig. 18.5 Chotigavanich and Sawangnatra classification: (I) intradural anastomosis, (II) extradural anastomosis, (III) extradural division of nerve root, (IV) intradural anastomosis and extradural division of nerve root, (V) intradural and extradural division of nerve root, (VI) closely adjacent nerve roots

The L5–S1 level has unique characteristics, interesting considering its role as the locale of so many CNRs. The L5 nerve root has the greatest amount of space in the spinal canal and the narrowest lateral recess and intervertebral foramen, and S1 and L5 dorsal roots have the largest overall diameters. ² Taken together, along with knowledge of the root sleeves attachments to surrounding structures within the lateral recess and neural foramen, L5 spinal nerves have a significantly susceptibility to entrapment and compression. ²^, ¹⁰^, ¹¹

18.2.2 Closely Adjacent Nerve Roots

A similar variant of the CNR anomaly occurs when two nerve roots arise through closely adjacent openings in the dural sac. In many cases, these closely adjacent nerves are tightly adherent to one another, each contained within an individual dural sheath requiring careful dissection to separate.

18.2.3 Caudal/Cranial Nerve Root Origin

Nerve roots are sometimes found arising from the dura in an abnormally cranial or caudal location, resulting in an exceedingly oblique or transverse course from the dural sac, respectively. In these cases, there is no abnormality associated with the nerve or its dural sheath. It is simply the root’s abnormal origin of emergence from the dural sac which gives rise to its atypical course.

18.2.4 Anastomosed Nerve Roots

Nerve interconnections (anastomoses) have been found to occur both intradurally and extradurally. The anomaly consists of a nerve fiber connection between two adjacent nerve roots. Cadaveric studies have revealed a greater number of anastomoses between sacral roots. ¹²

18.3 Embryology

Knowledge of the embryologic fault underlying these anomalies is imprecise. It has been suggested that the emergence of nerve roots from an abnormally caudal location, as well as the presence of conjoined and/or closely adjacent nerve roots, likely results from defective migration of nerve roots during embryonic development ¹³^, ¹⁴ Emergence of nerve roots from an abnormally cranial location, as well as bilateral anomalies, has been suggested to result from defective emergence of affected roots from the spinal cord. ¹⁴ Finally, abnormal connections between two roots have been suggested to result from the abnormal exit of a root from the dural pouch during fetal development. ¹⁵^, ¹⁶

18.4 Prevalence

Nerve root anomalies (NRAs) have been detected in patients between the ages of roughly 15 and 73. ⁵^, ¹⁰^, ¹⁷^, ¹⁸ The mean patient age is approximately 38 years. ⁵^, ¹⁸ NRAs most frequently involve the L5 and S1 nerve roots, routinely accounting for 50 to 70% of reported NRAs. ³^, ⁴^, ⁶^, ⁷^, ⁸^, ¹⁴^, ¹⁹^, ²⁰^, ²¹^, ²² Various case series have reported either L4 or S2 to be the next most commonly involved nerve root, present in approximately one-third of cases. ³^, ⁷^, ²³^, ²⁴ NRAs are apparently equally common on left and right sides. ³^, ⁴^, ⁷^, ¹⁰^, ¹³^, ¹⁴^, ²⁴

Despite broad reporting in the literature, these anomalies remain underdiagnosed, best appreciated by the significant differences in reported prevalence rates.

18.4.1 Surgical Prevalence

Prevalence rates of intraoperatively discovered CNR anomalies range from 0.32 to 5.8%, ⁶^, ²³ averaging around 2.38% (▶ Table 18.1). Most cases of intraoperative discovery involve patients undergoing surgery for diagnosed intervertebral disc herniations. ¹⁸^, ²³^, ²⁵^, ²⁶ These cases demonstrate the ease with which anomalies are overlooked in preoperative imaging. Concern has been raised over increasingly minimal invasive surgical techniques diminishing a surgeon’s likelihood of intraoperative identification of anomalous roots, thus increasing prevalence of surgical failure. ¹⁶^, ²⁷ However, among patients undergoing microendoscopic discectomy, Morishita et al reported an intraoperative discovery rate for CNR anomalies of 3.6%, a relatively large figure among the surgical literature. ²⁸

A surgical case series by Pamir et al, detailing intraoperatively discovered anomalies among patients undergoing lumbar disc herniation surgery, reported a prevalence rate of 3%. ⁵ Of interest, over the 3-year period of this study 2.5% of all patients required reexploration. Among this group, secondary exploration revealed anomalies in 20% of cases. ⁵ One-third of Cannon et al’s patients underwent secondary surgery before their anomalies were discovered and half of Neidre and MacNab’s patients only had their anomalies discovered at repeat exploration for failed disc surgery. ⁴^, ¹³

18.4.2 Radiologic Prevalence

Reported prevalence rates of NRAs from radiologic studies also display significant variability. Importantly, these prevalence rates must be teased apart for appropriate comparison (▶ Table 18.2). For instance, magnetic resonance imaging (MRI) studies have reported prevalence rates ranging from 0.25 to 6.7%. ¹⁵^, ²⁹ The 0.25% prevalence rate reported by Artico et al specifically described the prevalence of CNR anomalies. ¹⁵ In contrast, the 6.67% prevalence rate reported by Aota et al included four types of extradural anomalies. ²⁹ Of the 300 patients examined, 7 patients had CNR anomalies (2.33% prevalence), 7 patients had closely adjacent nerve roots (2.33% prevalence), 5 patients had two separate nerve roots in a single foramen (1.67% prevalence), and 1 patient had anastomosed nerve roots (0.33% prevalence). These distinctions are frequently unnoted in the literature, leading to improper prevalence comparisons. One such example is a cited 17.3% prevalence of anomalous nerve roots, originally reported by Haijiao et al. ²⁷ Teased apart, there is merely a 0.53% prevalence of CNR anomalies, a 1.6% prevalence of nerve roots of abnormally cranial or caudal origin, and a 15.1% prevalence of furcal nerve roots.