Pressure Controlled Discography

In an effort to develop criteria for discography that can be used to predict surgical outcome, Derby has reported on a cohort of patients who underwent provocative discography under pressure monitoring.

Derby, et al. The ability of pressure controlled discography to predict surgical and non-surgical outcome. Spine, in press.

While the pathophysiology of 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.

Derby, et al. The ability of pressure controlled discography to predict surgical and non-surgical outcome. Spine, in press.

Physiologic loading of the disc creates horizontal and vertical stresses within the nucleus, annulus, and endplates of the disc, directly related to the weight of the body above the segment and any added moment stresses due to body position. The relationship between intradiscal pressure and body position has been quantified by several investigators.

Nachemson. Disc pressure measurements. Spine 6:1; pg. 93. Unloaded disc= 20 psi, standing=50 psi, sitting=90 psi, lifting=200 psi.

Quinnel, et al. Observations of Pressures within Normal Discs in the Lumbar Spine. Spine 1983 8:2;166-9.

Derby hypothesized that some discs were more sensitive to chemical stimuli than mechanical stimuli. Pain at discography which 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 used data from a preliminary study on disc pressure measurements at the time of discography.

Derby. Lumbar discometry. ISIS newsletter.

In his preliminary study Derby 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 criteria for a positive result was 6/10 concordant pain. He found that in normal discs the average opening pressure, representing the intrinsic pressure of the disc, was 27 lb/in 2 in the side lying position and 85 lb/in 2 In the sitting position. As the degree of degeneration increased the opening pressure decreased in both positions. However, the three fold 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 lbs/in 2 above the opening pressure.

From these findings Derby 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, Derby 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. Chemical discs have pain at minimal pressure. 15 psi above opening pressure was chosen as the threshold for a chemical disc, as this is well below the mechanical load resulting from sitting. Mechanical discs have pain provoked at pressures between standing and lying, that is between 15 and 50 psi above opening pressure. Indeterminate discs have pain between 51 and 90 psi above opening pressure and normal discs have no pain.

This classification system was applied to a consecutive series of patients referred for lumbar discography prior to potential fusion surgery. Following discography, the patient was returned to the care of their referring surgeon 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 vs 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 vs 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 worker’s compensation claims, between the patients with favorable outcomes and unfavorable outcomes. Other than workers compensation status, psychosocial risk factors were not assessed.

Until now, however, the clinical significance of degenerative disc disease in a patient with axial pain was uncertain, as treatments directed specifically at the disc (i.e. fusion) led to variable outcomes. Based on Derby’s data, 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 HNP. 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- it’s intensity, location, and character.

If the location and character of pain of the pain provoked at discography is similar to or exactly the same as the patients clinical symptoms it satisfies the criteria for concordant pain.

The intensity of pain is measured both by the patients self report, for example, 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. While it is possible to estimate injection pressures manually, using a controlled inflation syringe with digital pressure readout provides a precise value.