Interpretation of a discogram includes both a morphologic and a functional evaluation. The functional evalua tion is more important because MRI is well suited for characterization of morphologic findings. The tenet of discography is that injection into the discs and subsequent increased intradiscal pressure will elicit a concordant pain response (one that mimics the patient's typical pain) if that disc is a pain generator. A scale of subjective pain severity from 0 (no pain) to 10 (maximal pain) can be determined during the procedure by asking the patient to relate what his or her level of pain is during each injection. The patient is also asked whether the pain mimics his or her typical pain (i.e., "concordant"). To evaluate the patient's pain response more objectively, multiple vertebral levels around the suspected pain generator are injected during the procedure; the patient is not told which level is being injected, or when the injection is starting. The authors coach the patient prior to the procedure regarding reporting of pain response and monitor for spontaneous pain elicited during the examination. It is important to establish a "reference level," or relatively pain-free level with injection. For discography to be considered positive, there should be at least one reference level, which is defined by the absence of pain or lack of concordant symptoms on injection. An unquestionably positive discogram consists of a single concordantly symptomatic disc with control discs above and below that level (if it is not the lumbosacral junction). Optimal benefit results when one or two levels demonstrate a highly concordant pain response, with a relatively pain-free adjacent reference level(s). If all levels are painful, a limited fusion may not result in patient satisfaction, and these results can suggest that continued medical management might be the best course, rather than surgery.
There is an interest in characterizing results based on pressure-controlled manometric discography (Table 1).
The information obtained includes opening pressure, pressure at onset of pain, and maximum pressure. The opening pressure is noted when contrast is first visualized in the disc space. The correlation of opening pressure and pressure at pain onset is important, potentially influencing surgical technique. The integrity of the disc can be evaluated by assessing the amount of pressure it can hold. An incompetent disc will fail to maintain pressure because of leakage of contrast. A typical nonpainful disc should be able to hold a pressure of at least 90 mmH2O. Manometric discography may help stratify patients into categories that are more likely to improve from interbody fusions (32). However, there are issues regarding whether intradiscal injection, which produces a tensile load, is comparable pathophysiologically to the compressive load that is exerted by virtue of our bipedal existence.
The Modified Dallas Discogram Scale (33) is the standard used for describing the radiographic and CT discographic appearance of annular disruptions of the lumbar, thoracic, and cervical discs. A normal disc is considered grade 0 and has the appearance of a cotton ball in younger patients and a hamburger bun in older patients (10) (Fig. 6). A radial tear confined to the inner third of the annulus is considered a grade 1. On fluoro-scopy it has the appearance of a small tail extending from the central nucleus but not reaching the disc margin. A grade 2 tear extends to the middle one third of the annulus, while a grade 3 tear extends to involve the outer annular fibers (Fig. 7). A grade 4 tear extends to the outer annulus and covers >30° of the disc circumference. Diffuse, severely degenerated discs often fall into this category. A grade 5 radial tear extends through all layers of the outer annulus and extends into the ventral epidural space (33) (Fig. 7).
Care must be taken to avoid an annular injection, as it can lead to a false-positive pain response. Annular injections can be avoided by placing the needle in the middle third of the disc in both the anteroposterior and lateral projections. An annular injection appears as a collection of contrast within the annulus along the periphery of the disc (Fig. 8).
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