Relation of foraminal (lateral) stenosis to radicular pain.

In their article in this issue of AJNR, Nowicki and coauthors (1) take as their point of departure the embarrassing fact that we are quite poor at relating the sensory and motor symptoms of spinal stenosis to the radiologic picture of the spinal canal and its foramina. They quote figures of 30% false negatives (ie, pain with no obvious disease), and 20% false positives (ie, asymptomatic disease). We suspect that these numbers are overly optimistic for most clinical settings. The situation is no better for pain associated with intervertebral disk herniation. Why are we so inaccurate at diagnosing back pain in the radiology suite? Are there more informative ways to image the spine? Clinical-radiopathologic correlation is generally carried out with the patient recumbent and at rest. In daily life, however, people are constantly in motion. It is well known that in patients with lumbar spinal stenosis, postural factors and load bearing can elicit and increase pain. With this in mind, the authors carried out a detailed combined radiologic and histologic study of spinal roots and their foramina under conditions of precisely controlled spinal loads. Observations were made of isolated cadaveric “motion segments,” tissue units made up of two adjacent spinal segments and the intervening disk, joint capsule, and nerves. Calibrated, physiologically relevant loading forces were applied, and effects on the spatial relation between the nerve roots and the disk/ligament were monitored. The authors refer to nerve root compression seen only under load deformation as “dynamic stenosis,” and point out that such compression is hidden (occult) under normal imaging conditions. The bottom line of their study is that in terms of radiologic diagnosis, things might be even worse that we thought! If we were to monitor dynamic stenosis routinely, the percentage of cases calling for a diagnosis of functionally significant lateral stenosis would be far higher than at present, based on classic recumbent resting images. For example, in spines with “normal” aging signs, such occult stenosis occurs in about half of all individual foramina sampled from T-12 to S-1. Moreover, it is universal (100%) in persons with advanced disk degeneration. Had these data been collected from a population of patients with severe back pain, we might have expected this sky-high incidence of radiologically defined disease. But they weren’t. Spines of patients with diagnosed or suspected spinal disease, or with significant spinal radicular symptoms, were specifically excluded from the investigation. These were mostly healthy folks. A major weakness of the study is that premorbid symptoms were not specifically provided. Nonetheless, the message is clear that the observed high incidence of stenosis with movement is characteristic of healthy, largely asymptomatic persons. Imagine that we could routinely image the spines of our patients under conditions of physiologic loading comparable to those used by Nowicki et al in their cadavers. Factoring in occult stenosis along with classically evident stenosis, there would no doubt be fewer instances of false-negative diagnoses. The cost, however, would be a greatly inflated number of false positives. More than ever before we are left with twin dilemmas: (a) Why does spinal nerve compression cause radicular symptoms? and (b) Why does it so often not cause symptoms? To gain some insight into these questions it is important to consider a few fundamental and, on the face of it, trivial, facts. First, stenosis causes pain only to the extent that it causes