|Year : 2018 | Volume
| Issue : 2 | Page : 60-62
Rami communicans fibers in discogenic low back pain: The controversies
Gautam Das, Chinmoy Roy
Department of Pain Management, Daradia: The Pain Clinic, Kolkata, India
|Date of Web Publication||31-Aug-2018|
Dr. Gautam Das
Department of Pain Management, Daradia: The Pain Clinic, Kolkata
Source of Support: None, Conflict of Interest: None
Intervertebral disc innervation and mechanism of discogenic low back pain is a subject of debate. There is no controversy regarding innervation of anterior and lateral annuli. Nociception from this part is carried by sympathetic pathway. Sinuvertebral nerves innervate posterior annulus, but there are controversies about nociceptive pathway from posterior annulus. It can be mixed somatic and sympathetic or can be completely sympathetic pathway through rami communicans fibers. Local anesthetic block of rami communicans fibers with or without steroid gives very good results in discogenic low back pain. But radio frequency ablation of rami communicans fibers do not give consistent result. This article has tried to find the literature about disc innervation, role of rami communicans nerves in transmission of discogenic back pain, and role of rami communicans nerve block in management of discogenic back pain.
Keywords: Discogenic back pain, rami communicans block, rami communicans, sinuvertebral nerve
|How to cite this article:|
Das G, Roy C. Rami communicans fibers in discogenic low back pain: The controversies. Indian J Pain 2018;32:60-2
Innervation of intervertebral disc and mechanism of discogenic low back pain is a subject of debate since 1950s. Initial publications suggest that sinuvertebral nerves are formed with fibers coming from both gray ramus communicans and ventral rami of spinal nerve root. This conjoint nerve reenters through intervertebral foramen along with segmental blood vessels and innervate posterior annulus. Nociception is carried by spinal somatic segmental nerve, whereas sympathetic nerves are efferent sympathetic vasomotor fibers that do not carry nociception.,, Some authors are of the opinion that network of sinuvertebral nerves can spread over three segments and that may explain poor localization of discogenic back pain.,
In early 1980s, microdissection and histological studies by Bogduk et al. and Bogduk had thrown new lights on this subject. The anterior and the lateral aspects of disc are innervated by sympathetic nerves through network formation. Nerves from anterior and lateral annuli either join ramus communicans or sympathetic trunk. These nerves receive afferent nociceptive signals though small sympathetic fibers from outer annulus. Thus, afferent nociceptive signals from anterior and lateral part of annulus are transferred via sympathetic pathway. All other studies support this view and there are almost no controversies about innervation and signal transmission from anterior and lateral annuli.
But, differences exist in published literatures regarding nociceptive signal transmission through posterior network of nerves along posterior longitudinal ligament, which innervate the posterior annulus and posterior cortex of vertebral body. These nerves are sinuvertebral nerves, which are also called recurrent meningeal nerves or in some literature they are described as branches of recurrent meningeal nerve. Differences of opinion exist regarding the origin of recurrent meningeal nerves, which may have anatomical variations. According to the available literature, recurrent meningeal nerves (or sinuvertebral nerves) may have three possible origins. They may originate at neural foramen or just outside neural foramen, either from (1) spinal nerve root or its (2) ventral or dorsal rami, or from (3) gray rami communicans. [Figure 1] shows these three possible origins of these sinuvertebral nerves. Recurrent meningeal nerve (or sinuvertebral nerves) enters the spinal canal and gives rise to an extensive network of fine nerves. So, there are two possibilities: It can be a somatic nerve because it originates from spinal nerve root or its ventral/dorsal rami, or it can be made up of afferent sympathetic fibers because it joins gray rami communicans.
|Figure 1: Schematic diagram of cross section of lumber spine at the level of disc. Sinuvertebral nerves fibers may have three possible origins: (1) spinal nerve root, (2) its ventral division, or (3) gray ramus communicans fibers|
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Now, question arises how nociceptive signals from posterior annulus are transferred to brain? There are two views. According to few literatures pain signal is carried by somatic nerve, but some others believe that it is carried by sympathetic nerves.
Bogduk et al. and Bogduk first indicated about possible dual innervation of the disc and described anatomy of rami communicans fibers in detail. They pointed out that pain from posterior annulus is carried via sinuvertebral nerves, having both somatic segmental component and sympathetic component, which may be carried via sympathetic fibers through rami communicans.
Studies by Kojima et al., also support dual somatic and sympathetic innervation of posterior annulus. They pointed out that there are two networks of nerves along posterior longitudinal ligament. One is superficial, and the other is deep. Deep network is segmental through somatic nerve, and this can be blocked by dorsal root ganglion of same segment. Superficial network is nonsegmental that spreads over a few segments and possibly sends signal via sympathetic pathway.
But Groen et al., and Raoul et al. have shown that sinuvertebral nerves do not join somatic segmental nerves but join gray ramus communicans and ascend via the sympathetic trunk. This pathway is supported by the research by Suseki et al., in which the study on rat by immunohistochemistry revealed that sinuvertebral nerves do not join spinal nerves at same segments, but they run via gray rami communicans fibers and then ascend via sympathetic fibers and reenter spinal dorsal horn at L2/L1 level via white rami communicans.
This was further supported by the study carried out by Das et al., in which they did rami communicans block prior to endoscopic discectomy and claimed that surgical pain from annulus was relieved completely. This ramus block was done unilaterally at one level only, that is along the vertebral body, which is immediately above the concerned disc. This indicates that a majority of the sympathetic fibers ascend and join the ramus communicans above, and not the lower one. This study draws our attention to another interesting aspect that ipsilateral ramus communicans nerve block gave complete relief of surgical pain arising from cutting of the same sided annulus. Earlier it was hypothesized that nerves from either side innervate both sides of annulus. This can be explain that in chronic pain nerves can grow inside and around the disc,, and thus may grow on either side and take sensations on both sides in chronic pain. But in acute disc prolapse of short history, these sympathetic nerves may not cross midline and therefore complete pain relief may be possible even with unilateral rami communicans block.
Clinical features provide an indirect evidence that sinuvertebral nerves carry sympathetic fibers. It is well accepted that area of discogenic low back pain do not correspond perfectly with the level of disc degeneration. Mostly these patients present with poorly localized low back pain without any typical clinical sign. This can be explained by the fact that sinuvertebral nerves do not send nociceptive signals through same level spinal somatic nerve root. They ascend with sympathetic nerves that are nondermatomal and nonsegmental, and therefore, pain becomes poorly localized.,
Rami communicans fiber block thus thought to be an important intervention in relieving discogenic back pain. Simopoulos et al. published partial relief of discogenic back pain by radio frequency lesioning of L2 ramus communicans. Oh and Shim, in randomized controlled trial, had shown efficacy of radio frequency ablation of ramus communicans in discogenic low back pain and suggested this treatment when other options had failed. Local anesthetic and steroid injection at rami communicans also gave good results in vertebral compression fracture because vertebral body also have similar innervation like disc., Rami communicans block was also successful in diagnosis of internal disc disruption.
Several other studies showed poor results of rami communicans block. Radio frequency ablation of rami communicans fibers in a randomized sham-controlled, double-blind, multicenter clinical trial by Tilburg has shown poor outcome even after successful nerve block with local anesthetic. Study by Jérôme Rigaud et al. has shown poor results after laparoscopic excision of rami communicans fibers, even though they included only those patients who have shown good pain relief with diagnostic rami communicans block.
Thus, inconsistent results after rami communicans block are common and may have the following explanations. Bogduk et al. explained that there were several rami communicans fibers at each level and not just one. There are at least three anatomical variations of rami communicans fibers, and also, there are several ways by which sympathetic nerves may communicate with rami communicans fibers. He also demonstrated that in one variation, rami communicans fibers may pass through groove along attachment of psoas muscle at concave lateral side of vertebral body. There are few other studies to support that ramus communicans fiber in each level is not just one, thus rami communicans term may be more appropriate even we are discussing it at a single level. Conventional radio frequency produces precise cylindrical lesion and thus this procedure may not be able to ablate all the fibers at each level and may give variable outcome.
There may be another explanation of failure of radio frequency procedure, even after successful local anesthetic block. By its anatomical location, rami communicans block is always accompanied by psoas block. Thus, diagnostic local anesthetic block may give false-positive result if the pain was because of psoas myofascial pain rather than discogenic low back pain.
In conclusion, several anatomical studies by microdissection, histological and immunohistochemical studies, and clinical studies confirm that nociception from anterior, posterior, and lateral annuli of lower lumber discs is transferred via gray rami communicans fibers, which ascend and enter dorsal horn via white rami communicans at L2/L1 level. Thus, local anesthetic block of rami communicans fibers gives good pain relief both in discogenic low back pain and in surgical discectomy. But radio frequency ablation gives inconsistent results in discogenic low back pain, which may be due to poor localization and because it is impossible to ablate all rami communicans fibers at single level by conventional radio frequency. We may need a wider or bigger size of radio frequency lesion that may include all rami communicans fibers and failure may be minimized. This may be possible with cooled or bipolar radio frequency. Further studies are needed with rami communicans fibers ablation by cooled and bipolar radio frequency.
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