Your MRI Shows Three Bad Levels. Which One Hurts?

• Posted on: Aug 10 2026
• By: Amit Sharma, MD

A diagnostic nerve root block answers a question that imaging often cannot: when your MRI shows degeneration at several levels, which nerve is actually producing your leg pain?

This is a more common problem than most patients realize. A lumbar MRI in an adult with back and leg pain frequently shows a disc bulge at one level, foraminal narrowing at another, and arthritic change at a third. The report lists all of it. What the report cannot reliably tell you is which of those findings is actually generating symptoms and which are simply structural changes that happen to be visible.

That distinction matters enormously, because the treatment for a symptomatic L5 nerve root and an incidental L4 disc bulge are not necessarily the same.

Why Your MRI Can’t Tell You Which Level Hurts

Imaging is essential, but it has an important limitation in this setting. A systematic review examining MRI for lumbosacral nerve root compromise reported sensitivity of 0.25 and specificity of 0.92. The larger lesson is not that MRI is unreliable, but that an anatomical abnormality on MRI cannot by itself establish which nerve is responsible for a patient’s symptoms. Imaging works best when interpreted together with the history, neurological examination, and symptom distribution.

The opposite problem is also common. Imaging studies in people without pain have repeatedly demonstrated disc degeneration, protrusions, narrowing, and other structural abnormalities that may cause no symptoms at all. As the UK’s National Institute for Health Research evidence synthesis on selective nerve root blocks concluded, neither clinical findings nor radiological imaging have perfect diagnostic accuracy.

So we are often working with two imperfect sources of information. The examination narrows things down. The MRI narrows things down. But when several levels look abnormal and the symptom pattern is ambiguous, neither one necessarily closes the question.

This is exactly the gap a diagnostic block is intended to help fill. For a broader overview of how nerve root compression develops in the first place, our pinched nerve root guide covers the underlying anatomy and causes.

What a Selective Nerve Root Block Actually Is

A selective nerve root block (SNRB) is a targeted injection placed adjacent to a specific spinal nerve root as it exits the spine. It is typically performed under live fluoroscopic guidance, with contrast dye used to confirm needle position and medication spread.

Two features make a diagnostic block particularly useful:

• Small volume. A diagnostic block uses a deliberately limited amount of local anesthetic. Excessive volume can spread to neighboring nerve roots or the epidural space and reduce the specificity of the test.
• Confirmed spread. Contrast dye is injected first and observed under fluoroscopy. The contrast pattern helps confirm that the injectate is reaching the intended region before the diagnostic medication is administered.

Fluoroscopic image demonstrating targeted right L4 and L5 nerve root injections. Contrast helps confirm the location and spread of the injectate around the intended nerve roots.

This is where technique matters. An injection that spreads broadly through the epidural space may still reduce pain, but broader spread makes it much harder to determine which individual nerve root produced the response. If the purpose of the procedure is diagnostic, selectivity is the entire point.

Diagnostic Nerve Root Block vs. Transforaminal Epidural Injection

It is worth addressing a common point of confusion directly. A selective diagnostic nerve root block and a transforaminal epidural steroid injection may use a very similar fluoroscopic approach, and the terminology is sometimes used inconsistently.

The important distinction is the purpose of the procedure.

A diagnostic block is designed to isolate one suspected nerve root, using the smallest practical volume of local anesthetic so that the patient’s immediate response can be interpreted. A therapeutic transforaminal injection is intended primarily to reduce inflammation and pain and commonly uses corticosteroid with local anesthetic.

Precision matters in both procedures, but diagnostic selectivity becomes especially important when the procedure is being used to decide which spinal level is actually symptomatic.

Reading the Result Honestly

Here is where intellectual honesty matters, because diagnostic nerve root blocks can easily be over-interpreted.

A systematic review of diagnostic SNRB accuracy found sensitivity ranging from 57% to 100% and specificity from 10% to 86% across studies, with substantial methodological limitations in the available evidence.

That is a wide range. It means a selective nerve root block should not be viewed as the spinal equivalent of a laboratory test that returns a simple positive or negative answer.

Several things can muddy the result. Local anesthetic can spread beyond the target despite technically good needle placement. A patient with pathology at two adjacent levels may experience partial relief when either one is blocked. Longstanding pain may include mechanisms that extend beyond the peripheral nerve root itself. And pain reporting is inherently subjective.

The most useful response is therefore a concordant response: the pain that improves is the patient’s familiar pain, the improvement occurs in the expected anatomical distribution, and the timing corresponds to the expected effect of the local anesthetic.

When those pieces line up with the history, examination, and imaging, the block provides meaningful supporting evidence that the targeted nerve root is clinically important.

When they do not line up, the result should be interpreted cautiously.

A block that produces little or no relief can also provide useful information. It may redirect attention away from that particular level and prevent repeated treatment of a target that is unlikely to be responsible for the patient’s symptoms.

Where the Block Fits in the Treatment Ladder

Most patients with sciatica or other radicular pain should not begin with a diagnostic nerve root block.

First comes conservative care. Time, activity modification, targeted physical therapy, and appropriate medication improve many episodes of radicular pain, particularly when symptoms follow an acute disc herniation. Many disc herniations also decrease in size naturally over time.

Then comes therapeutic injection when appropriate. If the clinical examination and imaging clearly point to one level, a transforaminal epidural steroid injection may be the more logical next step because there is already a reasonably established therapeutic target.

A diagnostic block becomes most useful when genuine uncertainty remains.

• MRI shows abnormalities at two or more levels and the symptom pattern does not clearly favor one
• The examination and imaging findings do not agree
• Prior injections produced inconsistent results, raising the possibility that the correct level was never isolated
• Pain persists after previous spine surgery, where scar tissue and altered anatomy may complicate interpretation
• A surgical decision is being contemplated and identifying the correct level becomes particularly important

That last scenario is one reason diagnostic nerve root blocks have received significant research attention. Published studies have examined whether SNRBs can help identify the symptomatic level in patients being considered for lumbar decompression surgery.

A diagnostic injection should not determine a surgical decision by itself. But when history, examination, and MRI leave two plausible candidates, additional physiological information can sometimes help resolve the uncertainty before an irreversible treatment is chosen.

Conditions where multi-level ambiguity is especially common include foraminal stenosis and multi-level spinal stenosis, where several levels may look abnormal even though one nerve root appears to account for most of the patient’s symptoms.

The Therapeutic Side of the Procedure

A corticosteroid may sometimes be added to the anesthetic, giving the procedure a therapeutic component as well as a diagnostic one. Reducing inflammation around an irritated nerve root may provide longer-lasting pain relief and create an opportunity for rehabilitation or physical therapy to progress.

There is a tradeoff, however. Once steroid is added, relief lasting days or weeks is no longer purely diagnostic information. For diagnostic purposes, the patient’s response during the expected duration of the local anesthetic remains particularly important.

Pulsed Radiofrequency of the Dorsal Root Ganglion

For a much narrower group of patients with chronic radicular pain, a clearly identified nerve root, temporary improvement from targeted treatment, and persistent symptoms despite appropriate conservative care, pulsed radiofrequency (PRF) of the dorsal root ganglion may sometimes be considered.

PRF delivers radiofrequency energy in short pulses while limiting tissue temperature. Unlike conventional thermal radiofrequency ablation, the intention is neuromodulation rather than creating a destructive thermal lesion near a functioning spinal nerve.

The evidence is promising but remains incomplete.

One randomized triple-blind trial enrolled patients with lumbar radicular pain who first underwent a selective diagnostic nerve root block. Only patients demonstrating a positive response proceeded to randomization, illustrating an important principle: even when considering a more advanced treatment, identifying the appropriate nerve root came first.

A 2026 prospective observational study of 72 patients compared 4-minute and 8-minute DRG-targeted PRF protocols performed together with transforaminal epidural dexamethasone and bupivacaine. Both groups improved, with an overall association favoring the 8-minute protocol for pain and functional outcomes. However, treatment duration was selected clinically rather than randomly, and all patients also received epidural medication, so the study cannot establish that longer PRF exposure alone caused the difference.

For that reason, we view PRF as an option for carefully selected patients rather than a routine next step. Its potential usefulness depends heavily on identifying the correct symptomatic nerve first.

For the relatively small group of patients with persistent focal neuropathic or radicular pain despite appropriate treatment, dorsal root ganglion stimulation represents another option further along the treatment pathway.

Why Diagnostic Nerve Root Blocks Are Under-Used

Diagnostic nerve root blocks require more than simply placing a needle near a nerve.

The injection must be selective. Contrast spread must be interpreted carefully. The volume of anesthetic matters. The patient’s usual pain needs to be documented before the procedure, and the response afterward needs to be compared with the original symptoms.

In other words, the procedure produces information rather than simply delivering treatment.

That can make it appear to be an extra step. But when the diagnosis is genuinely uncertain, skipping that step may simply transfer the uncertainty downstream.

The cost of uncertainty can be repeated injections at the wrong level, unnecessary escalation, or treatment decisions based primarily on what looks worst on an MRI rather than what best explains the patient’s symptoms.

For carefully selected patients with persistent leg pain and more than one plausible abnormality on MRI, a diagnostic nerve root block can sometimes be the most direct way to determine what should be treated next.

Frequently Asked Questions

References

1. Tawa N, Rhoda A, Diener I. Accuracy of magnetic resonance imaging in detecting lumbo-sacral nerve root compromise: a systematic literature review. BMC Musculoskelet Disord. PMC
2. Beynon R, Elwenspoek MMC, Sheppard A, et al. The utility of diagnostic selective nerve root blocks in the management of patients with lumbar radiculopathy: a systematic review. BMJ Open. 2019;9:e025790. PMC
3. The diagnostic utility and cost-effectiveness of selective nerve root blocks in patients considered for lumbar decompression surgery: a systematic review and economic model. Health Technology Assessment. NCBI Bookshelf
4. NIHR Journals Library. Diagnostic utility and cost-effectiveness of selective nerve root blocks. Health Technology Assessment. 17(19). NIHR
5. Transforaminal epidural injection of local anesthetic and dorsal root ganglion pulsed radiofrequency treatment in lumbar radicular pain: a randomized, triple-blind, active-control trial. PubMed
6. Babaoğlu G, Şahutoğlu Bal N, Sabuncu Ü, et al. Duration-dependent outcomes of combined dorsal root ganglion pulsed radiofrequency and epidural steroid injection in chronic lumbosacral radicular pain. J Clin Med. 2026;15(2):708. PMC
7. Pfirrmann CW, Oberholzer PA, Zanetti M, et al. Selective nerve root blocks for the treatment of sciatica: evaluation of injection site and effectiveness, a study with patients and cadavers. Radiology. 2001;221(3):704-711. PubMed
8. Does transforaminal epidural steroid injection added to dorsal root ganglion pulsed radiofrequency treatment increase efficacy? 2024. PMC

Tagged with: Pinched Nerve Root, Radicular Pain, Sciatica, Selective Nerve Root Block

Posted in: News, Special Report