Provocative Discography: Section 2

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/emotional reaction to nociception. 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 a patients self report 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, a number of 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. There were several methodological flaws with Holt’s study, however. The most notable flaws were that all of his subjects were prisoners, he used a highly irritating contrast medium, and most importantly, he did not include a positive pain response as a criterion for a positive injection (i.e. the criteria for a positive result was based primarily on radiologic images).

Holt, EP. The question of lumbar discography. JBJS 1968, 50-A:720-26.

Holt’s findings were subsequently refuted in a well-designed study by Walsh, who demonstrated a 0% rate of positive discography in asymptomatic volunteers.

Walsh, et al. Lumbar Discography in Normal Subjects. JBJS 1990, 72A:7;1081-1088.

Walsh studied 10 asymptomatic subjects and 7 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 3/5 pain intensity (using a pain thermometer), 2 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, as they had no pain prior to the injection.

Among the asymptomatic subjects, 5/10 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 6/7 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 patients typical pain.

Post injection pain flare.

Walsh’s study was important for several reasons. First, using strict criteria for a positive test, including post-injection review of videotaped responses, there was excellent inter-rater reliability. Diagnostic tests that rely on an observer’s interpretation are not clinically useful unless there is good inter-observer reliability; i.e., the same test applied to the same patient should always produce the same result (1). Thus, Walsh’s study established reproducible criteria for a positive result from discography.

(1) Sackett, et al. Clinical epidemiology. A basic science for clinical medicine. Second edition. Little Brown and Company, 1991.

Second, by demonstrating a 0% false positive rate in asymptomatic subjects, Walsh effectively refuted Holt’s assertion that the false positive rate of discography was so high as to make it useless.

Third, Walsh demonstrated that patients suffering from chronic low back pain were capable of developing different types of pain in response to provocation discography. According to Walsh’s criteria, only provoked pain that was similar to the patients typical symptoms constituted a positive test. Atypical provoked pain, even if intense and accompanied by pain behaviors, constituted a negative test.