Applied Radiology

Dr. J. Kevin McGraw is the Co-Director of the Center for Vascular and Interventional Radiology, St. Vincent Mercy Medical Center, Toledo, OH, and Riverside Methodist Hospital, Columbus, OH. Dr. Jeffrey S. Silber is an Interventional Radiologist at the Center for Vascular and Interventional Radiology, St. Vincent Mercy Medical Center, Toledo, OH, and Doylestown Hospital, Doylestown, PA.

Chronic lumbar pain is a common cause of disability, with an estimated 5% of the American population suffering from chronic, disabling low back pain. Acute low back pain often responds to physical therapy and activity modification. Approximately 90% of acute low back pain sufferers will have a resolution of their pain within 6 to12 weeks, but epidemiologic studies have shown that more than 60% of patients will suffer from recurrent symptoms. It has been reported that the prevalence of discogenic pain among patients with chronic low back pain approaches 40%. ¹ Treatment options for discogenic pain include conservative measures such as physical therapy and steroid injection; and more aggressive strategies include surgery with discectomy and spinal fusion. In recent years, a new, minimally invasive, fluoroscopically guided treatment option has been developed called intradiscal electrothermal therapy (IDET).

Pathophysiology

A combination of complex interactions underlies the basis for chronic discogenic pain. The disc is an innervated structure that is capable of pain generation. Pain fibers or nociceptors are present in the outer posterolateral portion of the lumbar disc. Nociceptor afferent transmission is relayed through the dorsal root ganglion (DRG). Ingrowth of granulation tissue and small unmyelinated nerve fibers has been shown to occur in the degenerated disc.

The natural history of the degenerating disc includes loss of nuclear hydrostatic pressure, which leads to buckling of the annular lamellae. This phenomenon leads to increased mobility and shear stress to the annular wall. The process may continue, resulting in either radial or concentric fissuring of the annulus. The progressive degeneration of the disc, manifested by any of the above morphologic changes, has been shown to alter disc mechanics. Mechanoreceptors in the disc wall can discharge in the presence of increased disc mobilization. ¹

Chronic discogenic back pain can develop with any combination of annular fissures, delamination, or microfractures of collagen fibrils, leading to mechanical distortion of the annular lamellae and subsequent sensitization of nociceptors. This disrupted disc can cause referred pain into the buttocks and legs due to dorsal root ganglion stimulation or from direct chemical irritation of the nerve roots. The combination of mechanical and neural properties creates an interplay that leads to chronic discogenic pain.

There are three general types of disc pathologies: the "leg pain" disc, caused by the classic disc herniation with nuclear migration and sciatica accompanied by true dural tension; the "back pain" disc, caused by the internally disrupted disc (discogenic pain) with annular pathology, resulting in back pain, variable amounts of leg and buttock pain without frank radiculopathy, and a mixed pattern, such as with small contained disc herniations and central herniations. In addition, the postoperative disc can be a source of pain that mimics internal disc derangement.

To treat discogenic pain successfully, the mechanical and neural factors must be addressed. The IDET procedure was developed to thermocoagulate annular tissue and to thermally modulate collagen. Granulation tissue, which may be both vascularized and innervated, can likewise be cauterized. Small, contained disc herniations may also be addressed by reduction in nuclear volume.

IDET: Mechanism of action

Heat energy has been used medically for many years. Tissue cutting, tumor ablation, thermal coagulation, and physical therapy are some of the areas in which heat has been applied in medical care. Recently, the use of heat therapy has expanded to control contraction or shrinkage of collagenous tissue. The intervertebral disc is a relatively avascular structure, which allows heat to be held in the tissues with little fluctuation in temperature during treatment. Adjacent structures, such as the spinal cord and nerve roots, are protected from thermal injury by the vascular circulation outside the disc and by cerebrospinal fluid, which quickly dissipates any heat conducted beyond the tissue.

Effect on neural tissue

As previously mentioned, the disc is an innervated structure. Bogduk ² illustrated the sources of lumbar disc innervation and Coppes et al ³ found nociceptive properties in nerves of the outer annular wall. Fremont et al ⁴ also discovered nerve fibers as deep as the inner third of the annulus fibrosis and into the nucleus pulposus in several disc samples.

Thermal destruction of nerve tissue is well documented and widely used in the brain and elsewhere in the body. Brodkey et al ⁵ established that irreversible nerve blocks occur at 45šC in the brain, and Cosman and Cosman ⁶ used radiofrequency energy to produce 45šC isotherms for neural tissue destruction. With the IDET procedure, the catheter used is actually a thermal resistive coil. Heat produced by the catheter is conducted to the annular wall. Temperatures produced by the IDET catheter in the outer third of the annulus (approximately 46š to 48šC) are felt to be sufficient for the destruction of pain fibers.

Effect on collagen

The intervertebral disc is composed primarily of type 1 and type 2 collagen, both of which have similar molecular structures. The strength of the collagen fibers is due to the triple-helix molecule, which is cross-linked with hydrogen bonds. Collagen will shrink when heated, secondary to the disruption of the heat-sensitive hydrogen bonds. The optimal temperature for collagen contraction is reported to be 65šC. Hydrogen bonds start to break at 60šC. It is unclear whether there is a significant shrinkage effect over 75šC. Thermal contraction of collagen depends not only on degree of heating but also on duration of heating. Lower temperatures over a longer period of time result in shrinkage comparable with that achieved with a higher temperature over a shorter period of time. Under these parameters, IDET can shrink the fibers of the disc annulus and nucleus, relieving pressure of a disrupted disc by decreasing the volume of the nucleus pulposus. Contraction, and therefore tightening, of the annular fibers may also enhance the structural integrity of the degenerated or damaged disc and could stabilize fissures. ⁷

Patient selection

The IDET procedure is best suited for patients with mild to moderate degenerative disc disease, absent radicular symptoms, and a positive discogram. The process of patient selection involves identification and exclusion of patients who would be unlikely to benefit from the IDET procedure. Contraindications include large disc herniations, nerve root irritation secondary to mass effect, central stenosis, instability, and disc height loss beyond 50%. This information is obtained during a clinic visit, at which time imaging studies, including MRI if available, are reviewed with the patient. Findings on MRI usually include disc desiccation and, possibly, the presence of a high-intensity zone (HIZ) that has been shown to correlate with an annular tear in 89% of cases (figure 1). If there are no contraindications, a discogram is performed. If concordant pain (reproducing the patient's typical symptoms) is reproduced at discography, the patient is an IDET candidate. We further counsel the patient and perform the IDET at least 1 week following the discogram. IDET cannot be performed at the time of discography since the liquid contrast would cool the IDET catheter and prevent even distribution of the thermal energy into the posterior annular wall. If the discogram is not clearly concordant, we do not offer IDET. Generally, about one-third of patients referred to our clinic eventually undergo IDET.

Many factors have been suggested to correlate with high clinical success rates following IDET. These factors include a young age, the presence of an annular tear, a nonsmoker, and a truly motivated patient. For example, an ideal patient would be a young, motivated nonsmoker with a single-level annular tear and a concordant discogram. This type of patient, unfortunately, does not make up the majority of our clinic referrals. Many patients are "borderline" IDET candidates, but most are clear regarding candidacy. In difficult cases, the use of electromyelography to clarify the source of the pain may be helpful, although it is rarely necessary.

Procedure

After obtaining informed consent, the patient is placed in the prone position. Intravenous conscious sedation is finely titrated, since patient feedback during the procedure is critical. The skin is prepped and draped for potential bilateral posterolateral approaches. A 17-gauge needle is directed into the center of the disc under fluoroscopic control in a path adjacent to the articular facets in order to avoid nerve roots (figure 2). The stylet is then removed and the Spinecath--a steerable, flexible probe with a distal thermal resistive coil portion--is advanced using frontal and lateral fluoroscopy. The catheter will pass through the nucleus and curl along the interface of the nucleus and annulus with little resistance (figure 3). If resistance is met or the path is unexpected, the catheter is partly withdrawn and redirected by turning the hub. Ideal placement is along the entire posterior annulus from the 3 o'clock to the
9 o'clock position. In approximately one-third of the cases, only half of the posterior annulus can be covered by this approach with the catheter being caught in annular fissures. This can sometimes be avoided by reviewing the computed tomography (CT) of the disc and choosing the more favorable side for approach. If this cannot be avoided, the procedure is completed by the contralateral approach. Before heating, one must confirm that the catheter is contained within the disc space and that the heat delivery portion of the catheter (indicated by two radiopaque markers) is beyond the needle tip (figure 4). The heating protocol, which includes slowly increasing the temperature of the catheter from 65šC to 90šC, is then begun. The protocol includes 16 minutes of heating and requires frequent patient feedback. Patients usually feel nothing until the temperature reaches 75šC, at which time reproduction of their usual symptoms is common, similar to discography. If radicular symptoms are present with pain extending below the knee, the unit is immediately turned off and the catheter is repositioned. Once the protocol is completed, antibiotics are administered intradiscally via the 17-gauge needle (5 mg cefazolin in 1 cc) and the needle is removed. The patient is transferred to the recovery area, maintained in the supine position for 1 hour, and slowly mobilized before discharge. During this recovery time, the nursing staff reviews basic postprocedural precautions, including avoidance of heavy lifting for at least 2 weeks. Additionally, use of anti-inflammatory medications is avoided over the next 6 weeks. The patient is seen at 2, 6, and 12 weeks for follow-up.

Results

Since 1998, 11 studies have reported clinical outcomes of IDET. Many of these studies have been published in the anesthesia literature, although some are in the physiatry and orthopedic literature. Only one study has been published in the radiology literature. Our data reflects 30 patients and 41 disc spaces with follow-up ranging from 2 and 10 months. A total of 21 (72%) of 30 patients reported significant improvement in their back pain with a visual analog scale (VAS) score decreasing by 4 or more points. Nine (28%) of 30 patients had either no improvement or a decrease of between 1 and 3 points on the VAS scale. ⁸

The most publicized of these studies is by Saal et al, a group of physiatrists from Stanford. These investigators reported a series of 1116 consecutive patients presenting with chronic low back pain of >3 months in duration. Of this group, 1025 patients were improved with aggressive nonoperative care (physical therapy, medication, etc.) and were discharged to self-care. The 91 patients who did not respond to nonoperative care were offered surgical fusion. Of these 91 patients, 62 underwent IDET as an alternative therapy and 29 remained in a control group to assess the impact of natural history on symptom resolution. These groups were not assigned randomly. Data was collected at 6, 12, and 24 months on 58 of the 62 IDET-treated patients. At 6-month follow-up, there was a significant improvement in patients' pain with a decrease on the VAS score of 3.71 ± 1.95. At 12 and 24 months, the mean decrease in VAS score was 3.52 ± 2.30 and 3.41 ± 1.96, respectively. Based on these data, the authors concluded that a statistically significant improvement in pain was obtained in patients with chronic discogenic low back pain treated thermally with the Spinecath. ⁹

Combining all data available in the literature, comprising a total of 448 patients, the mean decrease in pain following IDET is 2.7 points on the VAS scale. Some clinical studies have also assessed functional activity following IDET. The physical function scale (PF) of SF-36 ranges from 0 to 100. Low scores indicate that the patient is limited due to health conditions in performing physical activities, including bathing and dressing. High scores indicate that the patient can perform all types of physical activities without limitations. The bodily pain scale (BP) of SF-36 ranges from 0 to 100. Low scores indicate that the patient has very severe and limiting pain, whereas high scores indicate that there are no limitations due to pain. The efficacy of IDET was assessed by examining changes in these scores between pre-treatment and follow-up, with positive changes indicating improvements in quality of life. Five of the above-mentioned 11 studies cited the PF and BP scores. A total of 404 patients were involved in these five studies. An overall mean improvement in pain of 24.1 (BP) is noted. It is generally considered that an improvement of 7 or higher is clinically significant. The same 404 patients exhibited a 25.2 increase in their PF scores. This suggests that there is an overall improvement in level of pain and physical function. ^10-13 Although our study did not involve the BP scale or the PF scale, these results are consistent with what we have noted in our practice.

Complications

In our study of 30 patients and 41 levels, there was 1 minor complication of lower extremity radicular type pain that improved after 6 weeks. This was treated with conservative therapy that included a tapered dose of steroids. ⁸

Surprisingly, not all of the above-mentioned studies addressed complications. This may be secondary to the notion that some lower extremity discomfort is "accepted" for a short period following treatment. Saal et al, in their study of 62 patients followed to 24 months, reported no adverse events or complications. ⁹ Wetzel and Anderson ¹⁴ reported a "low complication rate" in his series of 75 patients, but did not report an actual percentage. The remainder of the 11 studies report no additional complications. This discrepancy may be secondary to the perception that transient lower extremity pain is an accepted side effect and not a complication.

There has been one report of a major complication of cauda equina syndrome. The details of this case include poor catheter position beyond the expected posterior location of the annulus, operator inexperience, and persistent thermal energy applied despite the fact that the patient complained of severe lower extremity and pelvic pain. It is felt that this potential complication could be avoided easily with good radiographic technique and incorporating patient feedback under light conscious sedation. ¹⁵

Approximately 25,000 IDET procedures have been performed in the United States, and there have been no reports of discitis. However, this is a potential complication given the similarity of this procedure to lumbar discography. One potential explanation is the coaxial nature of the catheter system, such that the catheter that is applying the heat to the posterior annulus does not contact the skin, which would be the source of infection.

Conclusion

Intradiscal electrothermal therapy for chronic discogenic back pain is a relatively safe and effective treatment for patients with chronic discogenic back pain. Interventional radiologists possess the necessary technical and clinical skills to perform the procedure safely and effectively. Radiologists who perform discography should consider incorporating IDET into their daily practice. AR