Discography

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CHAPTER 25 Discography

Richard Derby, Sang-Heon Lee, Byung-Jo Kim

INTRODUCTION

Discography was developed in the late 1940s for diagnosing lumbar intervertebral disc herniation.^1,² In contemporary practice, discography refers to provocation discography in which the most important component is the evaluation of pain reproduction caused by pressurizing the disc with contrast medium. Discography is conceptually an extension of clinical examination, tantamount to palpating for tenderness.³ A precision injection of contrast dye into the disc nucleus stimulates nerve endings⁴ (via two mechanisms: chemical stimulus from contact between contrast dye and sensitized tissues, and a mechanical stimulus resulting from fluid-distending stress).

Discography is a potential solution to the diagnostic dilemma concerning which patients to treat surgically and at what segmental level. In 1995, the North American Spine Society stated that discography, especially followed by CT scanning, may be the only study capable of providing a diagnosis or permitting precise description of the internal anatomy of a disc and the integrity of disc substructures.⁵ Although new diagnostic imaging tools have been developed and are widely used, discography is still practiced. Discography remains particularly useful in problematic cases unresolved by MRI or myelography and in patients for whom surgery is contemplated.⁶

In this chapter, the technical considerations and complications of discography are discussed.

INDICATIONS AND CONTRAINDICATIONS

According to the position statement on discography by the North American Spine Society:⁵

Discography is indicated in the evaluation of patients with unremitting spinal pain, with or without extremity pain, of greater than 4 months’ duration, when the pain has been unresponsive to all appropriate methods of conservative therapy. Before discography, the patients should have undergone investigation with other modalities which have failed to explain the source of pain; such modalities should include, but not be limited to, either computed tomography (CT) scanning, magnetic resonance imaging (MRI) scanning and/or myelography.

The single purpose of discography is to obtain information. Disc morphology is not diagnostic; it does not indicate whether a given disc is responsible for a patient’s pain. However, it is reassuring if a disc proven to be symptomatic on other grounds also happens to be morphologically abnormal. The prime indication for discography is to establish a diagnosis of discogenic pain when therapy is to be directed at that disc. A parallel application is to identify asymptomatic discs. When a single disc is found to be symptomatic in the presence of adjacent asymptomatic discs, focused surgical therapy can be entertained. Patients with symptomatic or abnormal discs at multiple levels constitute a greater surgical challenge.

Congenital anomalies of the vertebrae or nerve roots and postoperative spinal abnormalities constitute relative contraindications to discography. Such conditions require greater care and dexterity to negotiate the abnormal anatomy to access the target disc and avoid injury to surrounding structures.

In the case of cervical discography, spinal cord compression constitutes an absolute contraindication to discography. In such cases neurologic features are key, and information about discogenic pain is immaterial to management. Moreover, the uncertainty of the relationship between the prolapsed disc and the spinal cord and resistance of the disc to passage of a needle only invites morbidity.

PREPROCEDURAL EVALUATION AND PATIENT PREPARATION

Preprocedural evaluation

A thorough patient evaluation is performed before discography. The evaluation should include the general medical status and a detailed history, including previous medical conditions and allergies. Symptom onset, pain nature and distribution, pain duration and frequency, injury type, previous medication and surgery, and posture or movements that aggravate or diminish pain should also be recorded. In most cases of lumbar discography and all cases of thoracic and cervical discography an MRI or CT scan should be reviewed prior to discography. Further, since false-positive rates increase with somatization, psychometric testing should be included.

Prior to the procedure, the clinician should explain the nature of the procedure, its risks and complications, and what to expect. It is important that patients recognize and report any reproduction of accustomed pain and distinguish this pain from other pain. Subjects should be instructed in the use of pain scales for pain intensity responses. Unless the patient is able to describe the nature and intensity of provoked pain, the procedure has no value. In this regard, it is advantageous to have a trained observer independently monitor patient pain responses while an operator concentrates on technical aspects of the procedure.

Patient preparation

Since the disc is avascular, there is an increased risk of disc space infection and most discographers use prophylactic antibiotics.³ Animal studies have shown that intradiscal and intravenous antibiotics prevent discitis, and prophylactic intravenous antibiotics 20 minutes prior to the procedure are recommended.^7,⁸ In addition, many discographers add 2–6 mg/mL of a cephalosporin antibiotic to the nonionic contrast solution.⁸ All procedures should be performed under sterile conditions, including sterile scrub and double gloves.

At best, discography is uncomfortable; at worst, it can be very painful. For this reason, it is recommended that patients be sedated with the intravenous injection of either midazolam (2–5 mg) or intermittent does of 10–30 mg of propofol. Patient response should be monitored and the dosage titrated to establish a level of sedation permitting the patient to be conversant and responsive after needle placements, yet tolerant of procedural discomfort. The main disadvantage is that midazolam and propofol frequently cause procedural amnesia, and the patient report of experienced pain when asked at a later date is unreliable. In addition, patients taking significant narcotics for pain control should be given a preoperative dose of narcotic (e.g. 50–100 μg fentanyl) to reduce pain intolerance and withdrawal hypersensitivity. Although respiratory depression is uncommon using this protocol, subjects are monitored with pulse oximetry and a blood pressure cuff. Supplemental oxygen is administered by nasal cannula.

TECHNIQUE OF LUMBAR DISCOGRAPHY

Patient position

The patient lies in a prone oblique position on a fluoroscopy table. Elevating the target side approximately 15° allows the fluoroscopy tube to remain in a more AP projection and reduce radiation scatter. If required, a folded towel or soft wedge may be placed under the patient’s flank to prevent side bending of the lumbar spine. On the side selected for puncture, a wide area of the skin of the back is prepped and draped from the costal margin to the mid-buttock and from the midline to the flank. The puncture side should be opposite the patient’s dominant pain to eliminate confusion between pain reproduced during contrast injection and the pain of penetrating the outer anulus fibrosus.

Disc puncture

Prior to injection, a fluoroscopic examination of the spine is performed to confirm segmentation and determine the appropriate level for needle placement. Using AP view, the beam should be parallel to the inferior vertebral endplate. After selecting the target disc using AP view, the fluoroscopic beam is axially rotated until the facet joint space is located midway between the anterior and posterior vertebral margins. In this view, the insertion point is 1 mm lateral to the lateral margin of the superior articular process (Fig. 25.1).

Fig. 25.1 Oblique view of lumbar spine (*, needle insertion point; P, pedicle; FJ, facet joint).

The insertion point is marked on the skin. Since the distance between the opposite superior articular processes increases at lower levels, the usual distance from the midline increases from about 3–4 cm at the T12–L1 level to 6–7 cm at the L5–S1 level. Because of the iliac crest and increased interfacetal distance, at the L5–S1 level the fluoroscopy tube is rotated only far enough to bring the facet joint space approximately 25% of the distance between the anterior and posterior vertebral margins.

Prior to needle placement, the skin, subcutaneous tissues, and deep muscular tissues along the needle trajectory are infiltrated with local anesthetic (1% lidocaine). Techniques vary depending upon the number of needles used. The single-needle technique has the advantage of a single needle insertion and perhaps shorter procedure duration, but the risk of disc infection may be higher.

To avoid potential neural injury, the needle should be directed into the safe triangle. The borders of the safe triangle include the nerve root for the superior tangential border, the vertebral endplate of the target disc for inferior border, and the lateral margin of the superior articular process for the medial side line (Fig. 25.2). To minimize nerve trauma, one should use a needle with a short, noncutting bevel or blunt-pointed tip with a side port.

Fig. 25.2 Safe triangle for needle insertion (NR, nerve root outlined by contrast media; SAP, superior articular process; EP, endplate).

Puncture of T12–L1 through L4–L5 intervertebral discs

In the double-needle technique, a styletted 25-gauge, 6-inch needle is placed into each disc through a 20-gauge 3.5-inch introducer needle under fluoroscopic guidance. To protect the discographer’s hand from radiation exposure, forceps may be used to grasp the introducing needle. The introducer needle is advanced parallel to the fluoroscopic beam using an oblique fluoroscope view (Fig. 25.3).

Fig. 25.3 The introducing needle should proceed parallel to the fluoroscopic beam using an oblique fluoroscope view.

A slight ‘hockey-stick’ bend at the end of the introducer needle can improve navigation. If bony obstruction is encountered, the physician should confirm whether the needle has contacted the superior articular process or the vertebral body. If necessary, the needle may be slightly withdrawn and its trajectory modified. The introducer needle can be either advanced just over the lateral edge of the superior articular process or advanced to the margin of the disc. At the L5–S1 level, advancement may proceed just over the lateral edge of the superior articular process. When the introducer or discogram needle contacts the disc margin, the ideal position in the AP projection is on a line drawn between the midpoints of the pedicles above and below (Fig. 25.4A).

Fig. 25.4 The ideal position of needle when the needle contacts the disc is on the line between midpoint of pedicles at AP (A) and posterior vertebral margin at lateral view (B) (P, pedicle).

In no case should one advance the introducer or discogram needle medial to the inner pedicle margins before contacting the intervertebral disc. In the lateral view the needle should contact the disc between the posterior vertebral margins (Fig. 25.4B).

After confirming introducer needle position with a lateral view, a 25-gauge, 6-inch needle is advanced into the center of the disc through the introducing needle while monitoring by lateral view. A slight ‘hockey-stick’ bend on the end of the discogram needle facilitates navigation. When the needle contacts the disc, position should be checked using AP and lateral views. Contact with the anulus fibrosus is characterized by the perception of firm but resilient resistance, and frequently the patient experiences a momentary, sharp, or sudden aching sensation in the back or the buttock. The needle is then advanced to the center of the disc. Needle position must be monitored and checked by both AP and lateral imaging.

L5–S1 intervertebral disc

Due to increased facetal width and the presence of the iliac crest diacrest, puncture of the lumbosacral disc is more challenging. Instead of a direct lateral approach, once the introducer needle is advanced to the anterior border of the superior articular process of S1, a slight curve or ‘hockey-stick’ bend is needed to advance the discogram needle in a medial and slightly posterior direction around the SAP to contact the disc just anterior to the vertebral margin as viewed in the lateral fluoroscopy projection. Less experienced operators may find an 18/23-gauge needle combination easier to direct than the 20/25-gauge combination used for upper levels. Longer needle combinations may be required in muscular or obese patients. The fluoroscopy tube is rotated until about 1–2 cm of the L5–S1 disc is visualized between the superior articular process of S1 and the sacral ala (Fig. 25.2).

Using the oblique fluoroscopic view, the guide needle is introduced toward the bony notch between the sacral ala and superior articular process of S1 until the needle tip lies immediately adjacent to the anterolateral aspect of the superior articular process of the sacrum (Fig. 25.5A). The needle tip should not be handled directly, but should be wrapped in sterile gauze. The distal 2–3 cm of the needle should be bent in a direction opposite the bevel. The degree of curve is determined by the operator on the basis of how much deflection is required in the patient at hand for the needle to approach the target disc center. In a lateral fluoroscopic projection, the 25-gauge discogram needle is passed through the guide needle while the guide needle is held firmly in position. The inner needle is advanced until the tip emerges from the guide needle. The inner needle is advanced under direct fluoroscopic vision. As it emerges, the guide needle is retracted slightly (Fig. 25.5A). This unsheathes the procedure needle, which should be turned so that the curve or bend bows the introducer needle in a medial and posterior direction through the safe triangle. Once the needle encounters the anulus fibrosus, its position is checked and confirmed in both AP and lateral fluoroscopy views. In the lateral view the needle should contact the disc 2–3 mm anterior to the vertebral margin (Fig. 25.5B) and in the AP view the needle should ideally be on a line bisecting the midpoint of the L5 and S1 pedicles.

Fig. 25.5 (A) Oblique view of L5–S1 discography. The inner needle is advanced slightly under direct fluoroscopic vision, and the guide needle is simultaneously retracted slightly. (B) Lateral view. The inner needle should contact the disc 2–3 mm anterior to the vertebral margin.

If the inner needle fails to curve medially it will not pass toward the center of the disc, and may strike the ventral ramus. As a result, the needle course must be monitored. If the needle fails to track medially and posteriorly, the needle should be removed and its curvature accentuated. Should the procedure needle meet with bony obstruction, the fluoroscope should be turned to determine whether the superior articular process or the vertebral body has been encountered. If the vertebral body has been encountered, the course of the needle can be corrected by withdrawing it slightly and rotating the needle appropriately. If the needle is blocked by the superior articular process, the inner needle is retracted into the guide needle and the pair are advanced to the lateral edge of the S1 superior articular process. The inner needle may then be directed toward the disc as described above. The ideal final needle position is in the disc center; however, there is leeway. In severely degenerated discs the needle position is not as critical, since contrast medium will spread throughout the disc. Ideally, the needle should be within 4–5 mm of the center on AP and lateral fluoroscopy.

Provocation using pressure manometry

Provocation

Once the needle tip has been properly placed in the center of the nucleus pulposus, nonionic contrast medium mixed with antibiotic is injected into each disc at slow velocity using a controlled injection syringe with digital pressure readout. The total volume injected should probably be limited to 3.5 mL. Although a few severely degenerated discs will accept more volume, the incidence of false-positive pain responses may increase. If one cannot achieve 50 psi above opening pressure at 3.5 mL due to the degree of disc degeneration or a leak through the endplates or anulus, a dynamic pressure of 50 psi above opening can usually be achieved by more rapid injection. In general, however, the media should be injected as slowly as possible. Higher injection speeds may cause rapid pressure elevations leading to increased pressure differences between the nucleus pulposus and manometer and between the dynamic and static pressures. At 0.05 mL/sec (one revolution of the Merit monometer) the difference between static and dynamic pressures is minimal. The intrinsic disc pressure is the pressure required to start the flow of contrast medium into the nucleus (opening pressure) resulting from osmotic forces within the disc resisted by anulus tension and the tension of the anterior and posterior longitudinal ligaments. Typical opening pressures are 5–25 psi, depending on the degree of nuclear degeneration. An opening pressure >30 psi usually indicates that the needle tip is within the inner anulus. In this case, the needle tip should be repositioned.

The disc is slowly pressurized by injecting 0.5 mL increments through a syringe attached to a pressure measuring device. At each 0.5 mL increment, the injection pressure, location of contrast medium, and any pain response are recorded. At a slow injection speed, the dynamic pressure more closely reflects the real intradiscal pressure than the static pressure taken without pressure applied to the injecting syringe. Injection continues until one of the following end points is reached: subject pain = 6/10, intradiscal pressure >50 psi above opening in a disc with a grade 3 annular tear or 80–100 psi in a normal-appearing nucleogram, or a total of 3.5 mL of contrast medium has been injected.

Imaging

The appearance of the normal nucleus following the injection of contrast medium is unmistakable: the contrast medium assumes a lobular pattern or a bilobed ‘hamburger’ pattern (Fig. 25.6A). A variety of patterns may occur in abnormal discs.⁹ Contrast medium may extend into radial fissures of various lengths but remain contained within the disc (Fig. 25.6B), or it may escape into the epidural spaces through a torn anulus (Fig. 25.6A). In some cases (Fig. 25.6C), the contrast medium may escape through a defect in the vertebral endplate.³ However, none of these patterns alone is indicative of whether the disc is painful; that can be ascertained only by the patient’s subjective response to disc injection.

Fig. 25.6 The appearance of the nucleus following the injection of contrast media. (A) The contrast medium assumes either a lobular pattern or a bilobed (‘hamburger’) pattern (black arrowhead) as a normal nucleogram. Contrast medium escaped into the epidural space through a radial fissure at L5–S1 level (white arrow). (B) Contrast medium extended into radial fissures (arrow) but remain contained within the disc. (C) The contrast medium escaped through a defect in the vertebral end plate (white arrowheads).

Immediately after discography, CT–discography may be performed to define fissures extending to the outer third of the anulus and extending circumferentially within the anulus fibrosus. Radial annular tears can be found by discography, but only the postdiscogram CT axial view clearly shows the location and size of fissures within the anulus fibrosus.³ Sachs et al.¹⁰ developed the Dallas discogram scale, in which annular disruption is graded on a 4-point scale. Grade 0 describes contrast medium contained wholly within a regular nucleus pulposus. Grades 1 to 3 describe the extension of contrast medium along radial fissures into the inner third, middle third, and outer third of the anulus fibrosus, respectively. Aprill and Bogduk¹¹ proposed a modified Dallas description scale which includes grade 4, distinguished from grade 3 by the spread of contrast medium circumferentially within the substance of the anulus fibrosus and subtending a >30° arc at the disc center.