Why MRI Cannot Directly Show Nerve Impairment
MRI is one of the most powerful diagnostic tools in modern medicine, yet it has a fundamental limitation that clinicians often overlook: it shows anatomy, not physiology. When a radiologist reads a lumbar MRI, they see the shape, size, and signal of discs, vertebrae, and nerve roots — but they cannot see whether a nerve is actually conducting impulses normally, whether it is ischemic, inflamed, or firing ectopically. In other words, MRI tells you what structures look like, not how they function.
This distinction is clinically critical. A nerve root can be visibly compressed on MRI and cause zero symptoms. Conversely, a patient can suffer severe, debilitating L5 or S1 radiculopathy with a completely “normal” or near-normal MRI. Both situations are common in everyday clinical practice. Miranda et al. (2021) provide direct numerical evidence for this: in a cohort of 101 patients whose lumbar MRI confirmed disc herniation or bulging, the Slump Test was positive in only 55.3% of herniations and 85.7% of bulging cases, while Lasègue reached only 18.1% and 28.6% respectively. Nearly half of all patients with an MRI-confirmed hernia showed no positive response to either root-tension test — suggesting that a large proportion of those structural lesions were not functioning as actively symptomatic, tension-sensitive root compressions at the time of clinical examination.
What MRI Misses
Several common causes of radicular pain are invisible or poorly visible on a standard supine MRI. Foraminal ligaments that constrict nerve roots — these thin fibrous bands are rarely captured on imaging. Spondylolisthesis-related root stretch — the instability and dynamic nerve tension only becomes apparent in weight-bearing or standing position, not in the supine position used for MRI. Arachnoiditis — adhesions of nerve roots inside the dural sac can sensitize roots and cause radicular symptoms with no clear “compression” seen. Wide dural sac syndrome and epidural adhesions — fibrovascular bands attaching the sciatic nerve to surrounding structures (documented endoscopically in piriformis syndrome cases) cause nerve traction that imaging simply does not capture. Foraminal stenosis — when predominantly foraminal rather than central, it is frequently underestimated or missed on routine sequences.
Additionally, MRI does not directly demonstrate the key pathophysiological driver of radicular pain: root ischemia from increased mechanical tension. A stretched nerve, even without compression, can become ischemic and symptomatic — a mechanism invisible to imaging.
The Numbers That Matter
One meta-analysis cited in the Berthelot review found that MRI sensitivity for detecting true nerve root impingement in patients with clinically obvious radicular pain was only approximately 25%, even though its specificity was high (around 92%). This means that in three out of four clinically real radiculopathies, MRI may not show the culprit. EMG fares no better: in one series of 108 patients with clinically disc-related sciatica confirmed by CT, EMG was abnormal in only 42% of cases.
Miranda’s data add another dimension to this picture. Their study used MRI as the gold standard and measured how often clinical tests agreed — yet even within that already MRI-confirmed group, the Slump Test reproduced sciatic pain in fewer than 6 out of every 10 herniation patients. Miranda et al. themselves note that both tests are vulnerable to false positives in patients with shortened hamstrings, and that test results vary between examiners and with each patient’s pain threshold. This means the overlap between “structurally abnormal on MRI” and “clinically active root compression” is substantially smaller than most clinicians assume.
The Practical Conclusion
These numbers explain why Berthelot and colleagues argue so forcefully that neurodynamic clinical tests cannot be evaluated against MRI as a “gold standard” — the two tools measure completely different things. Dismissing the Lasègue test as “low sensitivity” because it is sometimes positive when MRI is clean, or negative when imaging looks alarming, reflects a fundamental logical error. Miranda et al. conclude their paper with exactly this spirit: the Slump Test, superior to Lasègue in every category they tested, should be more consistently present in clinical practice precisely because it engages the nervous system directly, not its shadow on a scan.
For a manual therapist, this means: trust a well-performed, positive neurodynamic test cluster (SLR + Bragard + Slump + Bowstring) even when imaging appears unremarkable. The patient’s nervous system is telling you something the scanner cannot.
Sources:
Berthelot JM et al. Inadequacies of the Lasègue test, and how the Slump and Bowstring tests are useful for the diagnosis of sciatica. Joint Bone Spine. 2020. https://pubmed.ncbi.nlm.nih.gov/32561431/
Miranda IH, Raymundo JLP, Klein KM. Sensitivity of Laségue Sign and Slump Test in Hernia and Disc Bulging Diagnoses Compared with Magnetic Resonance Imaging. Rev Bras Ortop. 2021. DOI: 10.1055/s-0040-1722590
