Discography is a diagnostic test in which radiographic contrast is injected into the nucleus pulposus of the intervertebral disc. Although originally developed for the study of disc herniation, discography is now used most commonly to identify symptomatic disc degeneration. There are two components of discography: (1) the anatomic appearance of contrast spread within the disc (using plain radiographs and/or computed tomography [CT]) and (2) the presence or absence of typical pain during contrast injection within the disc (pain provocation).
Discography was first described by Lindblom in 1948. He used the technique to demonstrate disc ruptures and to determine if patients’ pain symptoms originated from the abnormal disc. Although the procedure was initially performed more than 60 years ago, discography still remains controversial largely because of validity concerns. In 1968, Holt (cited later) reported that discography was falsely positive in 37% of an asymptomatic population. However, much subsequent work supports the validity of provocative discography in identifying symptomatic disc abnormalities.
Identifying the specific pathology responsible for spinal pain is often difficult. This is particularly true given the high incidence of anatomic abnormalities in asymptomatic individuals and the presence of normal anatomy in some painful individuals, at least as demonstrated on conventional imaging studies.1,2 The primary purpose of diagnostic injections for chronic spinal pain is to identify which anatomic structure of the spine is causing pain and what the pathologic disorder is that affects it. Discography remains the only test available that attempts to correlate pain response from the patient during provocation with abnormal discs identified on imaging studies.
Whether or not it is important to make an anatomic diagnosis in patients with spinal pain is a matter of some debate.3,4 While some would argue that in the majority of patients, attempts at making an anatomic diagnosis are contraindicated, others feel that, at a minimum, making a diagnosis will help patients to heal by providing them with a clear understanding of their problem.5 The most important reason to make an anatomic diagnosis, however, is if there are treatments that can be directed toward specific pathology, leading to good outcomes. Many patients with spinal pain can be treated with interventional pain management procedures. The success of these procedures may depend on an accurate anatomic diagnosis; however, typically, little harm will come to the patient if the procedure fails. Traditionally, the indications for surgery have been felt to be neurologic loss. Increasingly, however, surgery is being performed for pain without neurologic loss, essentially becoming a pain management procedure. Although surgery may help some patients with chronic pain, the tissue injury that necessarily accompanies surgery may potentially lead to devastating consequences. If surgery is being considered for patients with chronic pain, an accurate diagnosis is essential. In this chapter, we focus on the role of diagnostic injections in presurgical decision making.
There are two types of spinal pain: radicular pain and axial pain.6 Radicular pain results from mechanical compression or chemical irritation of a nerve root, or both. Establishing an anatomic diagnosis for patients with radicular pain is important, as surgical treatments have excellent outcomes in well-selected patients. The source of radicular pain, typically either a herniated nucleus pulposus or spinal stenosis, can be definitively diagnosed at surgery; therefore, there is a gold standard that can be used to assess the validity of diagnostic studies. Consequently, the ability of both clinical findings and imaging studies to diagnose the site of pathology is well defined. A diagnostic injection may be indicated when imaging studies suggest that more than one nerve root may be responsible for a patient’s symptoms. In that circumstance, a selective epidural injection may be useful.
In contrast to radicular pain, the relationship between spinal pathology and axial pain is uncertain. There are a number of anatomic structures that are potential sources of pain, including myofascial tissues, synovial joints, and the intervertebral discs. Although discogenic pain is felt by many to be an indication for surgical fusion, outcome studies have demonstrated variable results.7–16 Because there is no gold standard for diagnosing the tissue source of axial pain, it is not possible to rigorously validate diagnostic studies.17 Discography is frequently performed on patients with axial pain, but its use remains controversial.18,19
Complicating the diagnosis and treatment of spinal pain is the influence of psychosocial factors on pain. Pain is a complex phenomenon, with components secondary to both tissue injury and the emotional reaction to tissue injury. Although there is considerable controversy regarding the relative importance of psychosocial and biologic factors in causing spinal pain,20–22 there is evidence to suggest that the level of psychological distress can affect the results from diagnostic injections23 as well as the outcomes from treatment.24 It is important to recognize the potential importance of psychosocial factors in diagnosing and treating patients with spinal pain.
If surgery is being considered for a patient with spinal pain—and the source of pain is unclear despite a clinical evaluation, imaging studies, and, potentially, electrodiagnostics—a diagnostic injection may be indicated. Either discography or a selective epidural injection may be considered, depending on whether the patient has radicular pain or axial pain.
The lumbar intervertebral disc can be injected with contrast, local anesthetic, or other substances.
In addition to provoking pain that can be compared with the patient’s clinical symptoms, injecting contrast into the disc may demonstrate pathology that is not otherwise revealed on conventional imaging studies. Practitioners did not stipulate how strongly discs should be stimulated when tested, nor did they stipulate how severe the evoked pain should be in order for the test to be considered positive. Several studies have confirmed the accuracy of lumbar discography as a radiologic test in demonstrating both disc herniations and disc degeneration.2,26–31 With the advent of MRI scanning, and, in particular, the use of gadolinium enhancement in evaluating postoperative patients, the utility of lumbar discography purely as a radiologic study has diminished. However, clinical studies have demonstrated that discography is more sensitive than MRI in detecting disc degeneration, particularly when postdiscography CT scanning is added.2,28,31–33
The purpose of discography is to determine whether the intervertebral disc is a source of clinical symptoms. Although interpretation of the radiologic images obtained at the time of discography is important, in contemporary practice, discography is primarily a provocative clinical test, rather than a radiologic imaging procedure.
The functional anesthetic discography test involves the assessment of the effect of an injection of local anesthetic into one or more discs of the lumbar spine during the generation of a patient’s typical pain by a usually provocative position or activity. The injection is accomplished via a catheter that is inserted into the discs in question under fluoroscopic guidance. Without a clear “gold standard” to which the results of testing for discogenic pain can be compared, it is difficult to know how to critically examine the results of the test. Studies have shown negative results using the functional anesthetic discography in patients with positive provocative discography results.
Although observing the effect of a local anesthetic injection on pain and function (analgesic discography) can potentially provide useful information, the clinical utility of this has not been well defined.
The primary indication for provocative discography is to determine whether a patient with chronic spinal pain, who has failed aggressive efforts at conservative care, can be helped with spinal fusion.
A variety of pathologic processes affect the intervertebral disc, potentially causing noxious stimulation of nerve endings. A precision injection of contrast dye into the disc nucleus also stimulates nerve endings. The stimulus applied with discography has two components: a chemical stimulus resulting from contact between contrast dye and sensitized tissues and a mechanical stimulus resulting from a fluid-distending stress. The underlying premise of discography is that this applied stimulus replicates the clinical noxious stimulus responsible for the patient’s symptoms and that reproduction of the patient’s clinical symptoms during the injection confirms the disc as the source of pain.
As with any diagnostic test, it is important to know the false-positive and false-negative rates associated with provocative discography, which are used to calculate sensitivity and specificity. Defining the false-positive and false-negative rates requires comparing the test results against a gold standard.17,34,35 A gold standard is a method for definitively establishing a diagnosis and is typically obtained from biopsy, surgery, or long-term follow-up.17 Unfortunately, in contrast to radicular pain, there is no absolute method to determine the tissue origin of lumbar axial pain and, therefore, no way to determine the sensitivity and specificity of discography.
For the provocative discography construct to be valid, the injection must selectively affect the nerve endings in the annulus of the disc being studied, which is the presumed site of the clinical noxious stimulus. A number of authors have suggested alternate sources for the pain of discography, other than stimulation of annular nerve endings.36 Postulated pain mechanisms include increased pressure at the end plates or within the vertebral body,37 increased substance P and VIP in the dorsal root ganglion,38 or transmission of mechanical stimulation to the facet joints. Despite these hypotheses, there is good evidence that the provocative response resulting from discography is related to stimulation of nerve endings in the outer annulus, rather than other factors.39–41 Although relatively unusual, painful end-plate disruptions can also occur.42
The complex nature of anatomic structures can also lead to inaccurate results from discography. Anatomic structures are typically composed of several different types of tissues. A pathologic process can potentially affect just one component of a structure, which may not be the same component targeted by a precision injection. For example, discogenic pain is commonly felt to be a result of annular fissures originating in the nucleus and extending to the outer annulus, which is where the majority of the disc nerve endings reside. During discography, contrast is injected into the nucleus. If there are not fissures, the contrast will be confined to the nucleus. However, histologic studies have demonstrated that there can be middle or outer annular abnormalities that are not contiguous with the nucleus.28 In such cases, an injection into the nucleus could lead to a false-negative result.
Another potential source of inaccurate results from discography is the change in central nervous system (CNS) nociceptive processing that occurs with chronic pain. The neuroanatomic pathways mediating acute pain behave as a hard-wired system, with a pure stimulus-response relationship.43 However, these neuroanatomic pathways are plastic, and they change with the development of chronic pain. With chronic pain, central sensitization occurs and dorsal horn cell activity no longer depends on peripheral tissue injury.43,44 A pure stimulus-response relationship no longer exists. Both previously innocuous stimuli to the dorsal horn and stimuli from outside the original receptive field cause pain. As a result, the interpretation of diagnostic injections based on an acute pain paradigm may be inaccurate.44 In the presence of chronic pain, it is possible that an anesthetic injection of an injured nerve or structure may not produce complete pain relief, anesthetizing an adjacent normal nerve or structure may relieve pain, and provoking a normal structure or nerve may reproduce a patient’s clinical pain.
Finally, psychological factors are important sources of false-positive results with discography. There are two components to pain. The first is the nociceptive process initiated by tissue injury, and the second is the psychological and emotional reaction to nociception. The pain response that is amplified in patients with issues of chronic pain, social stressors (such as secondary gain or litigation claims), or psychological distress disorder increases the false-positive results injection. A patient with chronic pain should always be seen in the context of these interacting factors. Measuring the response to a diagnostic injection, and in particular to provocative discography, always relies, to some extent, on patients’ self-reports of pain. Therefore, psychological factors can clearly affect the measurement of the response to a diagnostic injection. As a result, when assessing patients’ responses to diagnostic injections, the relative contribution of nociception and psychological factors should be considered and the reliability of patients’ self-reports of pain estimated.
Clearly, there are a number of potential sources of both false-positive and false-negative responses with discography. In an effort to study the potential for false-positive results, several studies have investigated the ability of discography to provoke back pain in asymptomatic subjects.
Holt, in 1968, reported a 36% rate of positive discography in asymptomatic subjects, leading him to discredit the use of the test.45 However, there were several methodologic flaws with this study. The most notable flaws were that all of the subjects were prisoners, a highly irritating contrast medium was used, and, most importantly, Holt did not include a positive pain response as a criterion for a positive injection (i.e., the criteria for a positive result were based primarily on radiologic images).
Holt’s findings were subsequently refuted in a well-designed study by Walsh and colleagues, who demonstrated a 0% rate of positive discography in asymptomatic volunteers.46 Walsh and colleagues studied ten asymptomatic subjects and seven patients with chronic low back pain. The criteria for a positive result differed between the two groups. For both groups, a positive result required a 3-out-of-5 pain intensity (using a pain thermometer), two types of pain behavior (as assessed by videotape review), and structural degeneration. For the patients with chronic low back pain, a positive result also required that the provoked pain be similar to their usual pain. Obviously, it was not possible to evaluate the similarity of pain in asymptomatic subjects because they had no pain prior to the injection. Among the asymptomatic subjects, five of ten had at least one structurally abnormal disc; however, none satisfied the criteria for a positive test. Thus, the false-positive rate in these asymptomatic volunteers was 0%. Among the chronic low back pain patients, all seven had at least one structurally abnormal disc, and six of seven patients had at least one disc that satisfied the criteria for a positive result. Overall, 13 discs were structurally abnormal, with seven being positive and six negative. Of note, two of the seven had at least one disc that was structurally abnormal and was associated with intense, but atypical, provoked pain, as well as pain behaviors. In each case, the test result was considered to be negative, given that the provoked pain was different from the patient’s typical pain.
The Walsh study was important for several reasons. First, using strict criteria for a positive test, including postinjection review of videotaped responses, there was excellent interrater reliability. Diagnostic tests that rely on an observer’s interpretation are not clinically useful unless there is good interobserver reliability (i.e., the same test applied to the same patient should always produce the same result).47 Thus, Walsh and colleagues’ study established reproducible criteria for a positive result from discography. Second, by demonstrating a 0% false-positive rate in asymptomatic subjects, Walsh and colleagues effectively refuted Holt’s assertion that the false-positive rate of discography was so high as to make it useless. Third, Walsh and colleagues demonstrated that patients suffering from chronic low back pain were capable of developing different types of pain in response to provocation discography. According to their criteria, only provoked pain that was similar to the patient’s typical symptoms constituted a positive test. Atypical provoked pain, even if intense and accompanied by pain behaviors, constituted a negative test.
The asymptomatic subjects studied by Walsh and colleagues were all healthy volunteers, with an average age of 23. Caragee and colleagues recently expanded on the Walsh study of asymptomatic subjects by studying a cohort of subjects who did not have low back pain, but whose clinical characteristics more closely matched those of patients with low back pain who typically present for discography.23 Thirty subjects with no history of low back pain were recruited: Ten had previous cervical surgery with good results, ten had the same surgery but had persistent chronic pain, and ten had primary somatization disorders. Lumbar discography was performed and interpreted according to the Walsh protocol. Four somatization patients dropped out before beginning the study and two stopped the study after only one or two discs were injected, and, therefore, were not included in the study analysis.
Among the subjects with good results from previous cervical surgery, seven of ten had at least one disc that had an outer annular rupture (10 of 30 discs, total), while only one of ten had a positive result. The patient with a positive test had a high Zung depression score. Of the subjects with chronic pain, five of ten patients had at least one disc that had an outer annular rupture (11 of 32 discs, total), with four of ten having at least one positive disc. Of the 11 discs with significant structural abnormalities, seven were positive and four were negative. Among the subjects with somatization disorder, three-quarters had at least one disc that had an outer annular rupture (6 of 13 discs, total), with three-quarters having at least one positive disc. Of the six discs with significant structural abnormalities, two were positive and four were negative. Based on these data, Caragee and colleagues concluded that in individuals with normal psychometrics and without chronic pain, the rate of false-positives is very low if strict criteria are applied, and that the false-positive rate increases with increased annular disruption.
The study by Caragee and colleagues is important for a number of reasons. Firstly, it confirms the finding by Walsh and colleagues that in subjects without a history of low back pain, and without psychosocial risk factors, provocation of a significant pain response with discography is unusual, with an incidence of 0% in the Walsh study and 10% in the Caragee study. It also confirms the Walsh finding that although discs in this population are often structurally abnormal (combining the studies, 12 of 20 subjects had at least one structurally abnormal disc), they are no more likely to be positive than a structurally normal disc. More importantly, the Caragee study reveals that in subjects without a history of low back pain, but with a history of chronic pain or a somatization disorder, provocation of a significant pain response with discography is common, with an incidence of 40% in the chronic pain group and 75% in the somatization disorder group. Furthermore, the more disrupted the annulus, the greater is the chance of a positive response.
Caragee and colleagues’ study is a powerful reminder of the importance of psychosocial factors in modulating pain, while also demonstrating the potential of false-positive responses with discography. However, in assessing the importance of this information, it is necessary to reconsider the premise of discography.
The premise of discography is that reproduction of a patients’ clinical symptoms during the injection identifies the disc as the source of pain. The rationale for its use is that the results can help discriminate among the various structures that may be responsible for axial pain. Therefore, to establish its validity, the criteria for a true positive disc must be determined in the relevant population, which is back pain sufferers. The Walsh data on patients with chronic low back pain demonstrated that it is common for patients undergoing discography to have intense pain that is very different in location and character from their clinical symptoms. In the Walsh study, the criteria for a positive test in the chronic low back pain population required that provoked pain be similar to the patient’s clinical symptoms. Unfortunately, without a gold standard for axial pain, the validity of incorporating measures of familiarity of pain into the criteria for a positive test cannot be precisely defined.17
Caragee and colleagues clearly demonstrated the potential for false-positive responses with discography. However, given the premise of discography and the fact that patients with chronic low back pain frequently have intense but atypical pain during discography, it is difficult to know the significance of any pain response in an asymptomatic subject.
Although the sensitivity and specificity of provocative discography cannot be precisely defined, it is important to remember that the ultimate criterion for a diagnostic test is whether the patient is better off as a result. If a test can predict the response to treatment, and is reliable and reproducible, then it may be clinically useful,17,34,35 even without a defined sensitivity and specificity.
In individuals with disc degeneration and annular defects, discography may elicit low back pain (LBP) with injection whether the patient is symptomatic with serious LBP or not. The ability of an individual to differentiate the true site of LBP by the quality of sensation with disc injection (concordancy) of pain produced by the injected disc also may not be reliable. In fact, individuals may not have the neural discrimination to differentiate pain originating from different sites in the low back and pelvis.
In contrast to the surgical treatment of radiculopathy, the surgical treatment of axial pain is controversial, as studies have demonstrated a wide disparity in outcomes.7–15,48 This disparity has been attributed to a number of factors, including type of fusion (interbody versus intertransverse, instrumented versus noninstrumented), approach (anterior versus posterior versus 360°), surgeon variability, and methodologic differences. The results from discography have been an important part of the preoperative evaluation in most of these studies, but, typically, the criteria for a positive test have not been strictly defined. The validity of the criteria used to define a positive discogram is another variable that could potentially affect surgical outcome.
In an effort to develop criteria for discography that can be used to predict surgical outcome, Derby and colleagues reported on a cohort of patients who underwent provocative discography under pressure monitoring.39 Although the pathophysiology of lumbar discogenic pain is still uncertain, there is presumptive evidence that it results from both mechanical stimulation of nociceptors in the annulus as well as by chemical irritation by enzymes and breakdown products involved in the degradative process.39
Physiologic loading of the disc creates horizontal and vertical stresses within the nucleus, annulus, and end plates of the disc that are directly related to the weight of the body above the segment and any added moment stresses resulting from body position. The relationship between intradiscal pressure and body position has been quantified by several investigators.19,49 Derby and colleagues hypothesized that some discs were more sensitive to chemical stimuli than mechanical stimuli. Pain at discography that occurred at low pressures, below the typical weighted values, would result from chemical stimulation of the outer annulus by contact with contrast dye. Pain occurring at higher pressures would result from mechanical stimulation of the annulus by the fluid-distending stress of discography.
In order to establish the criteria for chemical and mechanical stimulation, Derby and colleagues used data from a preliminary study on disc pressure measurements at the time of discography.39 In a preliminary study, they combined provocative discography with measurement of intradiscal pressure, comparing results from discography performed in the lying position with results from discography performed in the sitting position. The criterion for a positive result was 6-out-of-10 concordant pain. In normal discs, the average opening pressure, representing the intrinsic pressure of the disc, was 27 pounds per square inch (psi) in the side-lying position and 85 psi in the sitting position. As the degree of degeneration increased, the opening pressure decreased in both positions. However, the threefold difference between the opening pressure in the sitting position versus the lying position was maintained between equally degenerated discs. In the majority of discs, concordant pain provocation occurred when contrast first reached the outer annulus, with the maximal pain response usually occurring at pressures only 10 to 30 psi above the opening pressure. From these findings, Derby and colleagues concluded that in degenerated discs with annular disruption, pain provocation during discography is usually caused by low-pressure stimulation of an irritable outer annulus by a chemical stimulus.
Based on this information, the authors created a protocol for grading the sensitivity of the disc annulus that could be used to predict surgical outcome. Four categories of discs were defined. In chemical discs, pain is provoked at minimal pressure; 15 psi above opening pressure was chosen as the threshold for a chemical disc because this is well below the mechanical load resulting from sitting. In mechanical discs, pain is provoked at pressures between standing and lying; that is, between 15 and 50 psi above opening pressure. In indeterminate discs, pain occurs between 51 and 90 psi above opening pressure, and, in normal discs, there is no pain.
This classification system was applied to a consecutive series of patients referred for lumbar discography prior to potential fusion surgery. Following discography, patients were returned to the care of their referring surgeons, who independently decided whether surgery was indicated, and if so, whether it should be an intertransverse or interbody fusion. The disc classification was not reported to the surgeon.
The subjects were contacted at two follow-up intervals, at a mean time of 16 and 32 months, with the overall outcome classified as favorable or unfavorable depending on the results from three different outcome tools. Looking at all surgical cases combined, there was no significant difference in outcome between patients undergoing interbody versus intertransverse fusion, with both groups having approximately a 50% favorable outcome.
However, among patients classified as having a chemically sensitive disc, there was a highly significant difference in outcome between patients undergoing interbody versus intertransverse fusion. Within that group, 89% of the interbody fusion patients had a favorable outcome, while only 20% of the intertransverse fusion patients had a favorable outcome. Patients with chemically sensitive discs who did not have surgery of any kind had an 88% unfavorable outcome. There was no significant difference in patient demographics, including the percentage of patients with workers’ compensation claims, between the patients with favorable outcomes and unfavorable outcomes. Other than workers’ compensation status, psychosocial risk factors were not assessed.
Until now, the clinical significance of degenerative disc disease in a patient with axial pain was uncertain because treatments directed specifically at the disc (i.e., fusion) led to variable outcomes. Based on the data of Derby and colleagues, it now appears that there is a subset of patients with degenerative disc disease who have chemically sensitive discs and who have outcomes with surgery that rival those of patients undergoing partial disc excision for herniated nucleus pulposus. The surgery performed must be an interbody fusion, presumably because the disc is completely excised, therefore removing the source of the noxious stimulus. If these results stand up to long-term follow-up—and are replicated by other investigators—then the use of pressure-controlled discography as a diagnostic test to predict patients who will benefit from surgical fusion will be validated.
In addition to potentially having the ability to predict outcome, adding pressure monitoring to provocative discography improves interobserver reliability and, therefore, reproducibility. Assessing the response to discography requires measuring pain before and after the injection. There are three components to pain: its intensity, location, and character. If the location and character of the pain provoked at discography is similar to or exactly the same as the patient’s clinical symptoms, it satisfies the criteria for concordant pain. The intensity of pain is measured both by the patient’s self-report (e.g., using a numerical rating) and by observed pain behaviors. However, the intensity of provoked pain is dependent on the intensity of the stimulus. In simple terms, the harder one pushes on the syringe, the more likely the disc is to hurt. By measuring intradiscal pressures, the intensity of the stimulus can be quantified, allowing more reliable comparisons between patients and discographers. Although it is possible to estimate injection pressures manually, using a controlled inflation syringe with digital pressure readout provides a precise value.
There are two approaches to the lumbar disc: posterior and lateral.50 The posterior approach necessitates a dural puncture and, therefore, should be avoided. (Disc puncture is typically performed with a 22- or 25-gauge needle.) There is some evidence that using an introducer needle can reduce the risk of infection, although this is not a universal practice.51 Although rarely encountered, a variety of complications are possible with discography, including neural injury, bleeding, and intradural leakage of injected substances.36,52,53 There have been case reports of disc herniations resulting from discography.54,55 Canine studies have had conflicting results on the potential for disc injury during discography; however, the weight of evidence in humans suggests that this is not a significant problem.55–58
The most significant risk associated with discography is infection. The rate of discitis reported in the literature is as high as 1.3% per disc, and serious morbidity has resulted.51,59,60 However, practice audits at centers performing a large volume of discography have demonstrated infection rates as low as 0 out of 10,000 (R. Derby, personal communication). There is experimental evidence that prophylactic antibiotics, both intravenous and intradiscal, can prevent discitis.61–64 As a result, many practitioners routinely administer prophylactic antibiotics, particularly to high-risk patients such as diabetics.
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