Suspected cauda equina syndrome

ACR AC Low Back Pain (2021); UK cauda equina MRI provision guidance (2023)

An emergency MRI question with one specific trap: CT of the lumbar spine cannot exclude cauda equina compression, so a CT request for this indication is a redirect rather than a protocol adjustment.

Back pain with bilateral radicular symptoms, saddle anaesthesia, new bladder or bowel dysfunction, reduced anal tone, or rapidly progressive bilateral leg weakness.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Red flags

These accumulate rather than exclude. Back pain with one of these is a different question from back pain.

Known cancer

Back pain in a cancer patient is a different question, and staging phase changes which contrast phases are needed.

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MRI Lumbar Spine — suspected cauda equina syndrome

MRI Lumbar Spine

What we'd amend, and why

The question is whether the nerve roots of the cauda equina are being compressed within the thecal sac, and answering it requires seeing the roots themselves against cerebrospinal fluid — soft tissue against fluid, which is exactly what MRI resolves and CT does not. A disc fragment, an epidural collection and a tumour can all be near-isodense to the thecal sac on CT, so a CT reported as showing no significant compression carries very little negative predictive value here. That is why a CT request for this question is redirected rather than protocolled: the study cannot fail safely.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(1)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.
Time-critical: screening happens alongside preparation, not before booking. If an implant cannot be identified rapidly, escalate to the MR safety expert immediately and in parallel — surgical decompression is a same-day decision and the delay is what causes the permanent deficit.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Suspected spinal infection — whole spine, with contrast

Role	Study & protocol	Why this answers the question
First line	MRI Whole Spine MRI Spine — suspected infection / discitis usually appropriate	An epidural abscess produces the same syndrome and is a surgical emergency for an entirely different reason, and two things about the study change with it. Gadolinium stops being optional — the collection is defined by its enhancing rim, and an unenhanced study can confirm discitis while missing the compartment that decides the operation — and the coverage extends, because skip lesions are characteristic and the drainable level is regularly not the painful one. Both are cheap at the time of booking and expensive to discover afterwards in a patient who is losing sphincter function.

Known malignancy — the coverage extends above the conus

Role	Study & protocol	Why this answers the question
First line	MRI Whole Spine MRI Whole Spine — suspected metastatic cord compression usually appropriate	A cauda equina syndrome in a patient with cancer is a metastatic cord compression question wearing different clothes, and the difference is entirely one of extent. Deposits are frequently multilevel and non-contiguous, a conus or thoracic lesion produces a very similar clinical picture with a different surgical approach, and the level that defines the radiotherapy field is regularly not the one causing the saddle anaesthesia. Acquiring the whole spine at the outset costs sagittal sequences and a few minutes; discovering the need afterwards costs a second emergency attendance in a patient whose function is the thing being raced for.

The urgency banding and the treatment sequence belong to the metastatic spinal cord compression card; the emergency is the same one.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Lumbar Spine MRI Lumbar Spine — suspected cauda equina syndrome usually appropriate	The question is whether the nerve roots of the cauda equina are being compressed within the thecal sac, and answering it requires seeing the roots themselves against cerebrospinal fluid — soft tissue against fluid, which is exactly what MRI resolves and CT does not. A disc fragment, an epidural collection and a tumour can all be near-isodense to the thecal sac on CT, so a CT reported as showing no significant compression carries very little negative predictive value here. That is why a CT request for this question is redirected rather than protocolled: the study cannot fail safely.
Second line	MRI Whole Spine MRI Whole Spine — suspected metastatic cord compression	Extended coverage where the presentation could be explained by a lesion above the conus — a sensory level, upper motor neurone signs, or a known malignancy. Acquiring it at the outset costs a few minutes; discovering the need afterwards costs a second emergency attendance.

Where MRI genuinely cannot be delivered locally within a clinically useful time, the correct action is transfer to a centre that can, not substitution of a study that cannot answer the question.

Pitfalls

Accepting a CT lumbar spine as the answer. It cannot exclude cauda equina compression, and a normal CT falsely reassures the team that referred the patient.
Restricting coverage below the conus and missing a conus lesion, which produces a similar clinical picture with a different cause.
Missing an epidural collection or abscess, which is a surgical emergency for a completely different reason and can be subtle without contrast.
Reporting canal stenosis without stating whether the cauda equina roots are compressed and crowded, which is the finding the surgeon acts on.
Treating incomplete or evolving symptoms as less urgent. Retention is a late sign, and waiting for it is waiting for the deficit that does not recover.

Priors — what to pull first

Prior lumbar imaging distinguishes a new fragment from longstanding degenerative change, which is otherwise the hardest call in this study.
After recent lumbar surgery, the operative note is essential: post-operative haematoma is a distinct and time-critical cause of the same syndrome.

What makes a good request

The diagnosis is made or excluded by MRI at the admitting hospital, and the specific guidance in the United Kingdom is that this should be available around the clock rather than deferred to a routine list or to a transfer the following morning.
An optimised CT protocol has been shown to predict MRI findings well enough to serve as a triage tool where MRI access is limited, but it is explicitly a triage adjunct for selecting who is transferred, not a test that excludes the diagnosis.
Coverage runs from the conus to the sacrum. If the history raises the possibility of a higher lesion — a sensory level, upper motor neurone signs, a cancer history — extend to whole-spine coverage in the same attendance rather than repeating the study later.

Scoring this once it is done

The classification and risk tools this question ends in.

ASIA Impairment Scale (AIS) with Frankel Mapping Meyerding Grading (Spondylolisthesis)

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

MRI Spine — suspected infection / discitis MRI Whole Spine — suspected metastatic cord compression MRI Lumbar Spine — suspected cauda equina syndrome

Confirm locally

MRI Lumbar Spine — suspected cauda equina syndrome: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.