The vertebral column is richly innervated. The dorsal and ventral nerve plexus is derived from branches of the sympathetic trunk, sinuverte-bral nerves, and the rami communicantes, as well as the perivascular nerve plexus of the segmental arteries.4 The double-layered peridural membrane forms the outer margin of the epidural space and lines the entire bony spinal canal. The epidural space is a circumferential compartment surrounding the thecal sac, but a median raphe may compartmentalize the dorsal epidural space. Also, because there is variable communication between the dorsolateral compartments and the ventral compartment of the epidural space, asymmetric filling of the epidural space is not uncommon upon injection of contrast media.5 However, the compartmentalization is usually incomplete, and the epidural space generally forms a contiguous compartment around the thecal sac from the skull base to the sacrum.
Low back and sciatic pain are likely a combination of mechanical compression and inflammatory changes resulting from degenerative disc disease. Histological studies demonstrate the presence of inflammatory cells and increased protein in the cerebrospinal fluid (CSF) of patients with degenerative spine disease.6 Recently, Rutkowski et al. demonstrated central nervous system neuroimmune activation and neuroinflammation following lumbar nerve root injury.7 The pharma cological basis for response to epidural steroid injections is based on mitigation of the inflammatory changes that cause pain symptoms.8,9 In a clinical study by Winnie et al., 80% of patients showed improved work status at 6 months following epidural injections.10 Benzon concluded that pain relief is produced by the "interruption of sustained neural activity that produced and perpetuated the paraspinal muscle spasm."8 It was formerly believed that peridural adhesions could be relieved by the volume effect of the injectate.11 Although adhesional lysis is practiced by some proceduralists, it is unlikely to be the mechanism responsible for improvement following an epidural steroid injection.
Although numerous studies have argued the efficacy of epidural steroid injections,10,12-21 many of these trials are flawed in design. Unfortunately, double-blind controlled and randomized studies are difficult to perform in the clinical area. Despite this, there are a number of investigations that provide convincing evidence that epidural steroid injections are effective. Coomes and coworkers showed that epidural injections with anesthetic agents are more effective than bed rest for the treatment of low back pain.16 Burn and Langdon showed improvement in two thirds of the patients at 6 months (complete resolution of symptoms or significantly decreased pain).14 These investigators stratified patients based on age and duration of symptoms and found the best responses when symptoms were less than one year in duration and patient age was greater than 40 years. Heyse-Moore reported 120 consecutive patients who received epidural steroid injections with local anesthetic and found an overall success rate of 62% in their series.19 In this study as well, the best results were demonstrated in patients with a relatively short pain history: of patients who had had symptoms for 6 months or less, 81% improved; only 45% of more chronic sciatic pain sufferers showed improvement.19 Berman and coworkers also demonstrated better results in patients with subacute radicular pain (<3 months duration) versus those with more chronic pain symptoms.12 Another variable that appears to affect outcome is spinal stenosis. Rivest and coworkers reported their findings in 212 patients and discovered that patients with herniated discs responded better to epidural steroid injections than those with lumbar spinal stenosis.20 In addition, a number of reports have argued the efficacy of diagnostic nerve blocks for pain relief and for diagnostic benefits.22-27
Yates compared the results of epidural injections of local anesthetic with and without the use of steroids.21 In this prospective double-blind study, patients with sciatica who received both the steroid and local anesthetics showed significant improvement at 1-month and 1-year follow-up, compared with those who received placebo.15 A double-blinded randomized control study by Ridley demonstrated short-term benefits in 39 outpatient sciatica pain sufferers. Patients receiving the injections showed significantly diminished rest and walking pain at 1 to 2 weeks following injection, compared with those who received placebo injections.28 Dilke and colleagues published a double-blind study comparing patients who received methylprednisolone versus others given a placebo injection. The treated patients showed decreased pain symptoms and fewer missed work days than the placebo group.17 When pain is addressed soon after the onset, many patients are able to return to work and remain active, thus preventing atrophy of stabilizing musculature and other undesirable sequelae of inactivity. Facilitating the return to work also minimizes the deleterious occupational and economic effects of missed workdays. For many, these short-term benefits justify the procedure. In our experience, a 6-month mean follow-up interval showed a 76% improvement in patients receiving epidural steroid injections.29
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