Spinal Probing & Discography

Background

Treatment of back pain is often unsuccessful due to suboptimal identification of the pain source. Discography is a direct means of localizing pain related to disc distortion. Spinal probing is a minimally invasive method of evaluating extra-discal pain sources. This study examines the validity of discography and spinal probing as a diagnostic tool.

Methods

Between January 1994 and September 1995, prospective data were collected on 230 patients undergoing discography and spinal probing. Diagnostic validity was assessed by comparing the diagnostic findings with those of conventional endoscopy and by evaluating the clinical outcome of endoscopic laser foraminoplasty, performed at the spinal level indicated by discography and spinal probing, at 3 months.

Results

Spinal probing and discography localized symptoms to a single spinal level in all 230 patients, who proceeded to endoscopic laser foraminoplasty at the indicated level. Spinal probing and discography findings correlated with those of endoscopy in 99% of patients. Overall, 90% of patients achieved ‘Excellent’, ‘Good’ or ‘Improved’ outcomes at 3 months. Based on correlation with endoscopy findings, discography and spinal probing yielded a sensitivity of 100%, a specificity of 0% and a positive predictive value of 98.7%. Based on positive clinical outcome at 3 months, discography and spinal probing yielded a sensitivity of 99.5%, a specificity of 10.5% and a positive predictive value of 92.4%.

Conclusions

Discography and spinal probing offers a highly sensitive system for identifying the index level in patients with back pain and warrants further study in a large, randomized, controlled clinical trial.

Introduction

Back pain is a common condition in the developed world but treatment is often unsuccessful due to inaccurate identification of the pain source.1-8 Localization of back pain conventionally relies upon clinical examination, X-ray findings, CT-myelography, CAT and MRI scan results and, in certain centres, discography. With the exception of discography, these techniques are indirect methods of assessment that are not able to demonstrate any correlation between a given abnormality and the patient’s predominant presenting symptoms.

Discography is a patient-interactive means of evaluation that aims to reproduce the stimulus responsible for the patient’s symptoms in order to locate the source of the pain. The biochemical stimulus of injecting contrast medium into the disc can evoke pain within the nucleus, annulus or endplates of the disc itself9-13 while the mechanical stimulus caused by fluid distension of the disc can evoke pain in discal and extra-discal structures such as the dorsal root ganglion, the vertebral body or the facet joints.14-16

The diagnostic role of discography is controversial. It is used in the determination of index levels in patients requiring spinal fusion.6, 17-22 and several studies have also confirmed its accuracy as a radiological test to demonstrate both disc herniation and disc degeneration.23-29 However, discography is limited to evaluation of pain sources related to disc distortion.

Endoscopic aware-state evaluation of the foraminal zone provides a new but unrecognised means of diagnosis, which may prove to be a new gold standard in spinal diagnosis. It is however, intrusive, particularly in patients with multilevel disc disease, who would require endoscopy at multiple levels for optimal diagnostic accuracy. Spinal probing is a minimalist, versatile alternative to endoscopic aware-state evaluation. It involves fluoroscopic palpation of extra-discal tissues, including those not excited by discography, through a 2mm incision. Combined with discography, spinal probing offers a minimally invasive method of evaluating multilevel discal and extra-discal pain sources for maximum diagnostic accuracy.

This prospective study assesses the validity of discography and spinal probing as a diagnostic tool in patients suffering from multilevel disc disease by comparing spinal probing and discography findings with those of endoscopy findings and monitoring clinical outcome in the early post-operative period.

Methods

Between January 1994 and September 1995, prospective data were collected on all patients undergoing spinal probing and discography at a single center in the UK.

Patients presenting with 2-level disc disease demonstrated by MRI scanning and at least a 1-year history of back, buttock or leg pain resistant to ≥3 months of muscle balance physiotherapy were eligible to participate in the study. Patients were excluded if they had evidence of:

• • Facet joint cysts

• • Cauda equina syndrome

• • Painless motor deficits

• Tumors.

During an initial assessment, participants described their predominant presenting symptoms, indicated symptom distribution on a pain manikin and quantified back, buttock or leg pain using the Visual Analog (Pain) Scale (VAP).

Eligible participants were consented for a staged procedure consisting initially of spinal probing and discography on two spinal segments that clinically reproduced the site of back pain or peripheral radiation or were shown to evidence clinically relevant pathology on radiological or imaging investigations.

This technique was performed under aware-state analgesia with the patient in the prone position on a humpback radiolucent table extension. Under X-ray guidance, a spinal probing cannula was inserted into the spinal foramen via a posterolateral approach facilitated by the use of a specially designed X-ray alignment jig. The distribution of evoked sensations and the degree to which they reproduced the patient’s predominant presenting symptoms was recorded on a data sheet by a trained observer during probing of the paravertebral musculature, the lateral facet joint surface, the anterior facet joint margin, the anterior facet joint–annular interval and the annular wall. Radio-opaque dye (Omnipaque® 240 [Nycomed Ltd, Romsey, Hampshire, England]) was then injected into the intervertebral disc to evaluate its integrity. The pattern of dye distribution, acceptance volume and leakage were recorded, together with pain reproduction during discography pressurization.

Evoked sensations that reproduced the patient’s predominant presenting symptoms were classed as ‘concordant’ symptoms. Patients in whom spinal probing and discography demonstrated concordant or overlapping symptoms arising at more than one spinal level were referred for further investigation and were excluded from the study cohort. If spinal probing and discography were able to localize symptoms to a single spinal level, patients underwent immediate endoscopic laser foraminoplasty30 at the level evidencing the most common source of symptom reproduction.

Modification of symptoms was assessed using the Visual Analog (Pain) Scale (VAP) and Oswestry Disability Index (ODI). Scores were recorded before and after surgery. The outcome was measured by observing the percentage change in scores. An index of ³90% was deemed an ‘Excellent’, ³50% deemed ‘Good’, ³20% deemed ‘Improved or satisfactory’ and the remainder deemed ‘Poor’. Modification of symptoms was also assessed using a pain diary relating back, buttock and leg pain levels to activity levels. Patients completed the diary 3 times a day for 6 weeks.

Patients were discharged the day of, or morning following, surgery. A muscle balance physiotherapy regime was re-commenced on the first day following surgery, amplified with neural mobilization drills and continued on a monitored self-help basis for 3 months. Patients were reviewed at 6 and 12 weeks unless clinical symptoms required closer supervision.

Clinical outcomes were evaluated 3 months post-operatively using the VAP and ODI scores described previously. This evaluation point was considered more likely to reflect the appropriateness of the targeting rather than later evaluation that would reflect the ability of the technique to sustain outcome.

A postal review was performed 2 years following surgery to ascertain whether the patient had required additional surgical intervention.

Results

Baseline characteristics

A cohort of 277 patients was recruited into the study, of whom 47 were excluded due to >2-level disc disease. A summary of the baseline characteristics of the remaining 230 patients is shown in Table 1. All patients had multilevel chronic lumbar spondylosis with back or referred pain and multilevel degenerative disc disease on MRI scan. Preoperative diagnoses included compressive radiculopathy, lateral recess stenosis, perineural scarring and advanced disc settlement. A total of 67 patients had undergone prior surgery (discectomy, decompression or spinal fusion), including 13 patients who had undergone two or more surgical procedures.

Table 1 Summary of baseline patient demographics

Spinal Probing and Discography Results

All eligible patients underwent spinal probing and discography at two adjacent spinal levels. Concordant symptoms were localized to a single spinal level in all 230 patients (Table 2). At the adjacent level, 88 patients experienced discordant symptoms (i.e. the procedure elicited pain that did not replicate the predominant presenting symptom) while 142 patients were asymptomatic. All discs examined showed evidence of degenerative disc disease.

In all, 96 patients demonstrated concordant symptoms during both spinal probing and discography, 115 during spinal probing alone and 19 during discography alone (Table 2).

Table 2 Outcome of spinal probing and discography

Endoscopic Laser Foraminoplasty Results

All 230 patients proceeded to endoscopic laser foraminoplasty at the spinal level that demonstrated concordant symptoms during spinal probing and discography. Operated levels ranged from T12 to S1. Endoscopy demonstrated concordant symptoms that correlated with spinal probing and discography findings in 227 of the 230 patients (99%). Of the remaining three patients, two had demonstrated concordant symptoms during spinal probing alone and the third during discography alone. Pathology was treated in all three patients.

The clinical outcome of endoscopic laser foraminoplasty was assessed 3 months post-operatively. Overall, 159 of 230 patients (69.1%) achieved ‘Excellent’ or ‘Good’ outcomes (Figure 1). Of the 115 patients who demonstrated concordant symptoms during spinal probing alone, 98 (85.2%) achieved ‘Excellent’ or ‘Good’ outcomes. Conversely, only 11 of 19 patients (57.9%) who demonstrated concordant symptoms during discography alone achieved ‘Excellent’ or ‘Good’ outcomes (p=0.001 vs spinal probing alone by Fisher’s Exact test).

Figure 1 Outcome of endoscopic laser foraminoplasty (to be added)

Revision Surgery

At 2 year review, 6 patients (2.6%) had required revision surgery at the same spinal level and 9 patients (3.9%) had required revision surgery at an adjacent level.

Sensitivity and Specificity of Spinal Probing and Discography

The sensitivity and specificity of spinal probing and discography as a tool for diagnosing the spinal level responsible for disc-related symptoms was calculated in two ways: firstly, by comparing the findings of spinal probing and discography with those of endoscopy (the current diagnostic benchmark); and secondly by comparing the clinical outcome of surgery in patients with endoscopically confirmed concordant or discordant symptoms on spinal probing and discography. For the purposes of the calculation, surgical outcomes of ‘Excellent’, ‘Good’ and ‘Improved’ were taken to indicate that spinal probing and discography correctly identified the pathology requiring intervention. Unknown outcomes were not included in the calculations.

The presence of pathology as assessed by a concordant response provoked during endoscopy reveals an apparent sensitivity of 100%, a specificity of 0% and a positive predictive value of 98.7% (Table 3). The presence of pathology as assessed by a positive clinical outcome reveals an apparent sensitivity of 99.5%, a specificity of 10.5% and a positive predictive value of 92.4% (Table 4).

Table 3 Sensitivity and specificity based on endoscopy findings

Table 4 Sensitivity and specificity based on outcome measures

Discussion

Discography has been the subject of much controversy14, 19, 20 and is generally reserved for identification of target levels for spinal fusion.6, 14, 22 In this study, discography alone elicited the predominant presenting symptom in only 50% of cases, but this may be because 38% of study subjects were suffering with non-compressive radiculopathy. Such patients are found at endoscopy to have highly irritated nerves that are sensitive to light touch and may suffer significant symptoms in the absence of positive discographic provocation tests.

Spinal probing alone identified the source of the predominant presenting symptom in an additional 42% of subjects. However, used alone, spinal probing would have been unable to identify the index level in 8% of cases. These patients had central protrusions that were too medial for spinal probing to access but were amenable to discography. This is confirmed by a significant difference in the outcome of endoscopic laser foraminoplasty in patients whose pain source was identified by discography alone compared to those whose pain source was identified by discography and spinal probing, which reflects the presence of more medial pathology that is less likely to produce a positive probing result and is also less adequately addressed by endoscopic laser foraminoplasty.

When combined, spinal probing and discography were able to localize the source of the predominant presenting symptoms in 100% of cases, 98.6% of which were confirmed by endoscopy. Failure to confirm the remaining 1.4% of pain sources may be attributable to gentle tissue handling. These results suggest that discography is a valuable supplement to spinal probing to maximize diagnostic sensitivity.

In this study, the sensitivity of discography and spinal probing at identifying the spinal level responsible for the predominant signs and symptoms of back pain was calculated at over 99% when assessed by either immediate endoscopy or by early post-operative clinical outcome. The fact that this was not a placebo effect was confirmed by the low revision rate at the 2-year review. This high sensitivity may be due in part to appropriate selection of patients to undergo the investigation and to the exclusion of patients with concordant symptoms at multiple levels for further investigation – but both of these factors should also apply to clinical practise.

It has been demonstrated that MRI cannot be relied upon to accurately select the appropriate spinal level for intervention.29, 31, 32 At over 99%, the sensitivity of discography and spinal probing is much higher than the reported sensitivity of MRI-determined high intensity zones, at 27%.33 However, unlike MRI, discography and spinal probing is an invasive technique, however minimally.

The most significant risk associated with discography is infection,34-36 which can be reduced by the use of an introducer sleeve and separate internal needle and by prophylactic antibiotic cover.37-39 The results of the present study were included in a large database started in 1991, including over 8000 disographies spanning a decade. The incidence of infection in this database is 0.08%.

In conclusion, discography and spinal probing offers a highly sensitive system for identifying the index level in patients with either compressive radiculopathy or non-compressive radiculopathy. It directs the surgeon to the causal level by concordant feedback with encouraging accuracy where the response from the concordant level is clearly distinguishable from adjacent levels. Addressing the target level is attended by a low revision rate at 2 years. Encouraging early post-operative clinical results attest to the validity and efficacy of this method of investigation and warrant further study in a randomized, controlled clinical trial.