Although still controversial, discography (diagnostic intervertebral disc injection) is both an imaging study and a provocative physiologic study for determining whether an intervertebral disc is in fact a pain generator in a given patient (Table 68-7). Inserting a spinal needle into the center of the intervertebral disc and injecting contrast dye provides both physiologic information on whether a degenerative disc is painful and on anatomic features of the intervertebral disc. There is currently no other method to establish reliably whether a disc is a patient’s pain generator.
Clinical Presentation of Discogenic Pain
Patients with lumbar discogenic pain typically have low back pain but can also have pain referred to the buttock, hip, groin, thigh, or distal lower limb. (175). Discogenic pain is typically worse with lumbar flexion and unsupported sitting, as intradiscal pressures have been found to be higher in these positions (176). Physical examination should reveal a normal neurologic examination if the discogenic pathology does not affect the nerve roots.
MRI and CT of the lumbar spine can be useful initial assessment tools because they are noninvasive tests that allow for visualization of multiple discs. Although they have high sensitivity (Figs. 68-18 and 68-19) for detecting anatomic disc abnormalities, surgically proven internal disc disruption has been reported in cases of normal-appearing MRIs (177). MRI carries a high rate of false-positive findings, as shown in studies of asymptomatic patients (178). In addition, they cannot provide the physiologic information about whether an abnormal- appearing disc is actually a pain generator. Although highintensity signal zones (HIZ) in the posterior annulus on MRI have been linked to discogenic pain (Fig. 68-19), the HIZ can also occur in asymptomatic patients (179–182). With no pathognomonic features of discogenic pain and a high falsepositive rate of anatomic findings on noninvasive diagnostic tests, the diagnosis of symptomatic lumbar disc diseases requires a physiologic study for better clinical correlation of a patient’s pain with CT or MRI abnormalities. Provocation discography is a physiologic test for discogenic pain.
Anatomy and Pathophysiology of Discogenic Pain
The intervertebral disc is a well-innervated structure with A-delta and C-pain fibers (183) containing nociceptive substances such as substance P, calcitonin gene-related product, VIPs in the annulus fibrosis (184–186). Nerve growth factor has been found in both the annulus fibrosis and nucleus pulposus, which may increase pain sensitization (187). In healthy intervertebral discs, only the outer one third of the annulus fibrosis is innervated. Study of intraoperative samples from degenerative intervertebral discs of patients with chronic back pain demonstrated evidence of inward growth of nerve fibers along the radial fissures into the inner annulus (185,186). The presence of the neural structures and nociceptive fibers is believed to be the anatomic basis of chronic low back pain due to degenerative disc diseases. Discogenic pain may occur in internal disc disruption, which is a condition characterized by a degraded nucleus pulposus with radial fissures extending into the peripheral annulus fibrosis (Fig. 68-19) (188,189). The outer margin of the annulus is intact. The nucleus pulposus, reaching the innervated outer annulus through the annular fissures, invokes an intensive local inflammatory process. These inflammatory substances irritate and sensitize the nociceptive fibers in the outer annulus. The threshold for nociceptive mechanical stimulation is lowered in these chemically sensitized nociceptors. Chronic discogenic pain may result from mechanical stimulation of sensitized nociceptors with normal lumbar disc loading. In fact, intraoperative mechanical stimulation of the posterior annulus in the presumed painful segment induced low back pain in one study (190). Degenerative disc disease is believed to account for some 40% of patients with chronic low back pain of unclear origin (191).
FIGURE 68-18. Lateral plain film nucleography demonstrating a posterior annular fissure in the L4-5 and L-S1 discs, respectively, through which contrast leaked into the ventral epidural space. Compare this image with that in Figure 68-23. The nucleography of the L3-4 discs was normal. The patient had concordant back pain at the L4-5 and L5-1 discs but no pain at the L3-4 discs.
FIGURE 68-19. T2-weighted sagittal images of lumbar spine. MRI demonstrating degenerative disc disease, especially at L5-S1.
Indications for discography appear in the “Position Statement on Discography” from 1988 and 1996 by the Executive
Committee of the North American Spine Society (192):
- Patients with unremitting spinal pain of greater than 4 months and unresponsive to all appropriate methods of conservative therapy
- Patients in whom other investigations have failed to explain the source of pain
- Chronic back pain patients who are contemplating intradiscal or surgical procedures such as spinal fusion
Discography is generally performed in a radiologic suite or an operating room. The patient is placed in either a prone or oblique side-lying position. A pillow is placed under the patient’s abdomen to reverse lumbar lordosis. The lumbosacral area is prepared and draped in a sterile fashion. The patient’s vital signs should be monitored and oxygen saturation recorded with a pulse oximeter. A peripheral intravenous line should be established for light conscious sedation.
An AP fluoroscopic view is then used to identify the appropriate disc level. The fluoroscope is tilted in either a cephalad or caudad direction to best visualize the target disc space on radiograph. The C-arm is then rotated ipsilaterally to place the superior articular process of the subjacent vertebral body in a position that bisects the vertebral body above. An appropriate size spinal needle, typically a 25-gauge, 3.5-in. spinal needle is used to infiltrate the skin and subcutaneous tissue down to the superior articular process with 1% lidocaine. Caution should be exercised not to inject local anesthetic overzealously in the superior articular process area in order to avoid potential spread to the epidural or nerve root areas, thus compromising the patient’s ability to perceive pain during the subsequent provocative discography. To reduce the chance of discitis, a two-needle technique with 18- or 20-gauge, 3.5-in. introducers and 22- or 25-gauge, 6-in. inner needles, is recommended.
The introducer needle is inserted and directed to the outer edge of the superior articular process in the AP view and just to the anterior border of the superior articular process in the lateral view. The inner needle is then inserted through the introducer and slowly advanced to the center of the nucleus pulposus using alternating AP and lateral views. If the patient complains of radicular pain or paresthesias in a nerve root distribution during needle advancement, the needle should be withdrawn and redirected. If a pressure-controlled system is used for the injection, the needle is connected to an injection system with a manometer that is filled with nonionic and water-soluble contrast. The injectionist then injects contrast slowly, monitoring the pressure reading and the patient’s reports of pain simultaneously. The opening pressure is the pressure reading at the first appearance of dye in the disc on fluoroscopy. The endpoint of the injection occurs when the patient reports concordant pain (defined as reproduction of pain in the same location and intensity), or when 2 mL of total volume is injected into the disc or the pressure reading reaches 90 lb per square inch (psi). To help minimize the chance of discitis, 5 to 10 mg of cefazolin can be injected into each disc before needle removal. One level above and below the disc with concordant pain should be also studied so as to serve as a control (Figs. 68-20 to 68-22).
Information obtained from discography includes the volume of contrast injected, the patient’s pain response (no pain, dissimilar or discordant pain, similar pain, and exact pain provocation or concordant pain), degree of resistance to injection, morphology of the nucleogram, and postdiscogram CT morphology of the disc (193).
FIGURE 68-20. Sagittal T2-weighted lumbar spine MRI demonstrating degenerative disc disease at L5-S1 and possible degenerative disc disease at L4-5 and L3-4.
FIGURE 68-21. Lateral view of lumbar discograms reveals a posterior annular fissure at L4-5 and L5-S1 discs. The discography reproduced the patient’s clinical symptoms (concordant) at L4-5 and L5-S1 discs but no pain at L3-4 discs.
FIGURE 68-22. AP view of lumbar discograms demonstrated left side posterior annular fissures.
Postinjection CT scanning provides an axial view of the injected discs (Fig. 68-23). Patterns of radial and concentric annular fissures are more clearly defined in this plane. Postdiscography CT scanning should be performed within 2 hours of the discogram to prevent diffusion of dye out of the nucleus. The Dallas Discogram Description, in addition to recording the pain and contrast volume injected, describes morphologic degrees of annular degeneration and disruption (194). Annular degeneration and disruption are graded by the percentage of the contrast injected that fills the annulus and annular fissures toward the outer annulus as revealed by the contrast (Table 68-8) (195). A study by Derby et al. found that there was a significant correlation between the extent of annular disruption on CT and the rate of symptomatic disc on discography, with significant differences between grade 3 and 5 versus grade 0 and 2 (196).
FIGURE 68-23. Lumbar postdiscography CT image demonstrating the track of contrast that leaked through a radial fissure into a circumferential outer annular fissure.
A positive discogram requires a concordant pain response, an abnormal nucleogram, and a normal control level. In contrast, multilevel painful discs without a normal control disc on injection are unexplainable and cannot be regarded as positive.
Validity of Discography
In a prospective and controlled study in 1990, by applying modern manometry and postdiscography CT technology, refined needle placement techniques, less irritating contrast dye, stressing concordant pain, or discordant pain in addition to disc morphology (195), Walsh concluded that using stringent criteria, the false-positive rate of lumbar discography in asymptomatic individuals is 0% and the true-positive rate in symptomatic patients is 89% (189). However, Walsh’s study is limited by its small sample size (i.e., only ten controls and seven patients).
In a retrospective study, Derby et al. reported that a patient with a chemically sensitive disc, defined as one having a concordant pain response provoked with intradisc pressure of less than 15 psi as measured by a manometer, has a better outcome with interbody fusion versus intertransverse fusion surgery (197). This better surgical outcome presumably resulted from the removal of the mechanical load or stimulation to the chemically sensitive disc through removal of the painful intervertebral discs and stabilization with interbody fusion (197). Nevertheless, these investigators pointed out that discography should never stand alone as a diagnostic tool and sole factor for a clinical decision. The presence of a chemically sensitive disc does not rule out other coexisting sources of pain, nor does it exclude patients with excessive pain magnification. Discography should be used in conjunction with other diagnostic tests, such as MRI and CT scans, as well as the patient’s history, physical examination findings, response to a comprehensive therapeutic program, and psychometric profile information. Therefore, careful candidate selection is the key to maximizing clinically valuable data from discography. A recent study by Carragee et al. in 2000, although controversial, revealed a high false-positive rate with lumbar discography in a mixed population of 26 asymptomatic patients (182). A significant positive pain response and pain-related behavior with discography were found in 10% of the pain-free group and in 83% of the somatization disorder group. Discs with annular disruption were more likely to be painful on injection. The study concluded that if strict criteria are applied, the rate of false-positive discography may be low in subjects with normal psychometric profiles. Schwarzer et al. in 1995 (183) reported that a discogram is likely to provide highly specific information on the painful lumbar disc when the following characteristics are present: (a) unremitting low back pain persisting after 6 months of conservative care; (b) no prominent psychological dysfunction; (c) injection of all degenerated discs and one normal disc by MRI; or (d) combination of the results of appropriately and carefully performed provocative and imaging tests.
Complications of Discography
Potential complications of discography include discitis (0.05% to 4%), nerve root injury, subarachnoid puncture, chemical meningitis, bleeding, and allergic reaction (192). Use of sterile technique, a two-needle technique, as well as intradiscal antibiotics, can reduce the risk for infection (198).
In addition to providing imaging of disc morphology, discography is the only provocative physiologic test that can provide information on whether a degenerative disc is the source of pain. In appropriately selected patients, discography is a safe, reproducible, objective diagnostic tool when testing includes volume, pressure, fluoroscopic abnormalities, and pain provocation. Discography results help guide surgical management of chronic back pain. A negative discogram or a multilevel positive discogram without a painless control level can help to exclude a patient as a surgical candidate. Conversely, a singleor double-level positive discogram, along with other clinical data, can help to guide operative planning with respect to fusion or other surgical procedures. This potentially optimizes surgical outcomes. Whether diagnostic discography will actually change surgical outcome depends on many factors, such as a patient’s psychosocial factors, job situation, and the surgical technique employed. The true value of diagnostic discography will likely remain controversial until a prospective, randomized, double-blind, and controlled clinical trial on the outcomes of surgery based on discography is carried out.