A bulging disc is the most common explanation given for sciatica, and it is often the right one. The problem
is that it is also the explanation most likely to be accepted without being tested, because a lumbar MRI almost
always supplies something that could plausibly be blamed.
Here is the number that should govern how much weight a scan carries. In a systematic review of imaging in
3,110 asymptomatic people — individuals with no back pain at all — disc bulges were found in
30% of 20-year-olds, rising to 84% of 80-year-olds. Disc degeneration ran from 37% to 96% across the same age
span, disc protrusion from 29% to 43%, and annular fissures from 19% to 29%.1 The authors concluded that
many imaging-based degenerative features are likely part of normal aging and unassociated with pain, and must be
interpreted in the context of the patient’s clinical condition.1
Which means a report describing a bulge at L4–5 has told you that you have a common finding. It has not
told you that this finding is generating your leg pain.
The test that answers what the scan cannot
A selective nerve root block is a
diagnostic instrument first. A small volume of local anesthetic is delivered to a single nerve root under
fluoroscopic guidance, with contrast injected first to document exactly where the fluid sits. If the leg pain
that root carries goes quiet, that root is implicated. If nothing changes, it is not.
This is the same logic as a piriformis injection, applied one structure over. In both cases the medication is
incidental to the question; the question is whether silencing a specific candidate changes your pain.
Why this is not an epidural
The distinction is easy to lose and it matters. An epidural
steroid injection is designed to spread across a region, which is what makes it a reasonable treatment and a
poor diagnostic. A selective nerve root block is designed to stay put. When a block spreads, it stops being
selective, and the result it produces is no longer trustworthy.
How accurate is it, honestly
Moderately accurate, and worth stating in real numbers rather than reassurance. A prospective, controlled,
single-blinded study performed 105 fluoroscopically guided blocks in 47 patients who had pure radiculopathy from
a single confirmed level — 47 at the symptomatic level and 58 at an adjacent asymptomatic control level.
Using a 70% pain-relief threshold determined by ROC analysis, the block showed a sensitivity of 57%, specificity
of 86%, accuracy of 73%, positive predictive value of 77% and negative predictive value of 71%.2
The authors’ own summary is that the accuracy of diagnostic lumbar selective nerve root blocks is only
moderate.2 That is a more useful thing to know than a marketing claim, and it shapes how the result should
be used: as strong evidence weighed alongside examination, not as a single deciding vote.
What causes the wrong answers — and why contrast fixes most of it
The same study identified the mechanisms behind its false results, all of them identifiable on the spot
radiographs:2
- False negatives — insufficient infiltration, insufficient passage of the injectate, and
intraepineural injection. In plain terms: the anesthetic never properly reached the root, so a real generator
was missed. - False positives — overflow of injectate from the asymptomatic level being tested into
the epidural space or into the symptomatic level. The wrong root got the credit because the fluid did not stay
where it was put.
Every one of those failures is a spread problem, and every one is visible in the contrast images. This is why a
block performed without documented contrast is not simply less precise — it produces a result that cannot be
graded, because there is no way to tell a true negative from an inadequate injection.
When the disc is not the answer
If a well-performed, contrast-documented block at the level your MRI implicated does not change your leg pain,
that is information worth acting on. It argues against the structure everyone had been treating and it redirects
attention to the candidates that produce identical symptoms without a nerve root being compressed:
- The deep gluteal space — piriformis
syndrome, gemelli-obturator internus syndrome, ischiofemoral impingement, proximal hamstring syndrome. - The hip joint itself, which refers into the buttock
more often than its textbook description suggests. - The sacroiliac joint.
- The cluneal nerves over the posterior iliac
crest.
Each of those can be tested the same way, which is the point of building the workup this way in the first
place. Sciatica names a route down the leg. It does not name what started it.
The sequence that avoids wasted years
The failure pattern is recognizable: MRI shows a bulge, treatment is aimed at the bulge, treatment does not
work, treatment is repeated with more force. Each step is defensible in isolation and the sequence never revisits
its own first assumption.
The alternative is not more imaging. It is testing the candidate the imaging nominated, documenting the test
well enough to trust its answer, and being willing to accept a negative result as progress.
Frequently asked questions
Does a bulging disc on MRI mean it is causing my sciatica?
Not on its own. In a systematic review of 3,110 people with no symptoms at all, disc bulges were present in 30% of 20-year-olds and 84% of 80-year-olds, and disc degeneration in 37% and 96% respectively. Those are pain-free people. A bulge on your scan is a candidate for your pain, not a verdict — and the older you are, the weaker the inference. Learn more: herniated disc.
What is a selective nerve root block and how is it different from an epidural?
An epidural steroid injection is designed to spread — it bathes a region, which is useful for treatment and useless for pinpointing a level. A selective nerve root block does the opposite: a small volume of local anesthetic is placed at one specific nerve root under fluoroscopic guidance, with contrast confirming it stayed there. If your leg pain goes quiet, that root is implicated. Spread is the enemy of the test. Learn more: selective nerve root block.
How accurate is a selective nerve root block?
Moderately, and it is worth knowing the actual numbers rather than a reassurance. In a prospective controlled study of 105 blocks in 47 patients with radiculopathy from a single confirmed level, using a 70% pain-relief threshold, the block had a sensitivity of 57%, specificity of 86%, accuracy of 73%, positive predictive value of 77% and negative predictive value of 71%. Useful, clearly imperfect, and better than deciding from an MRI alone. Learn more: why contrast confirmation matters.
Why does the contrast picture matter during a nerve root block?
Because it explains the wrong answers. In that same study, false negatives traced to insufficient infiltration, insufficient passage of the injectate and intraepineural injection, while false positives came from the injectate overflowing out of the asymptomatic level into the epidural space or into the symptomatic level. Every one of those failures is visible on the contrast images. Without them, a result cannot be graded. Learn more: reading a diagnostic block result.
My MRI shows bulges at two levels. Which one is the problem?
That is precisely the question a selective nerve root block exists to answer, and it is common — multi-level degenerative change is the rule rather than the exception past middle age. Blocking one level at a time, with contrast documenting containment, is how a two-level or three-level MRI gets reduced to a single actionable target instead of a broad guess. Learn more: when the answer is not a disc at all.
What if the nerve root block does not help either?
Then the disc probably is not your generator, and that is a genuine result rather than a dead end. It moves attention to the structures that produce the same leg pain without involving a nerve root — the deep gluteal muscles, the sacroiliac joint, the hip joint and the cluneal nerves — each of which can be tested the same way. Learn more: hip arthritis or sciatica.
To discuss your own case, request an appointment through the appointment request form, call (314) 481-5000, or text (314) 886-5902. You can review the full range of pain treatments, read about the pain management doctors in St. Louis, or find both offices on the locations page.
Sources
- Brinjikji W, Luetmer PH, Comstock B, et al. “Systematic literature review of imaging features of spinal degeneration in asymptomatic populations.” AJNR American Journal of Neuroradiology. 2015;36(4):811–816. doi:10.3174/ajnr.A4173
- Yeom JS, Lee JW, Park KW, Chang BS, Lee CK, Buchowski JM, Riew KD. “Value of diagnostic lumbar selective nerve root block: a prospective controlled study.” AJNR American Journal of Neuroradiology. 2008;29(5):1017–1023. doi:10.3174/ajnr.A0955
Dr. Gurpreet Singh Padda, MD, MBA, MHP
