Your next patient has had back pain for 3 weeks; over 2 days both legs have become weak and heavy. Here's how the cauda equina syndrome case unfolds and what to pick up on the way.

An everyday back pain story that drifts into both legs over 48 hours is exactly how a cauda equina syndrome case can slip past you in the history. Patients often under-report saddle symptoms or bladder change, and urinary issues are written off as opioids.

Your next patient has had back pain for 3 weeks, and over 2 days both legs have become weak and heavy. This page walks through how the case presents, what to pick up in the history, the red flags to ask every time, and the bedside tests that decide it: a post-void residual bladder scan and a PR examination for anal tone and perianal sensation. It sets out the teaching points and nothing else.

How the case presents

Your next patient has had back pain for 3 weeks; over 2 days both legs have become weak and heavy. In their own words: "Back pain for a few weeks, and now both legs have gone weak."

Observations on arrival:

Observation	On arrival
Heart rate	82 beats per minute
Blood pressure	134/82 mmHg
Respiratory rate	16 breaths per minute
Oxygen saturation	98%
Temperature	36.7 °C

What to pick up in the history

These are the findings the case is built around. Each one moves you towards or away from cauda equina syndrome, and the scorecard checks whether you found it.

Sciatica now in both legs, previously one only: bilateral radicular pain signals compression of multiple roots rather than a single disc radiculopathy.
Numbness around the perineum and back passage: saddle anaesthesia is the defining sensory red flag of cauda equina syndrome.
Two days unable to pass urine, only dribbling: retention with overflow means an established, time-critical cauda equina syndrome needing immediate MRI.
Cannot feel the bladder filling or emptying: loss of bladder sensation is the earliest and most reliable autonomic sign of the syndrome.
Both legs weak and heavy, worse over 48 hours: progressive bilateral lower limb weakness confirms a rapidly evolving compressive lesion.
Cannot feel when he needs to open his bowels: bowel sensory loss is another cauda equina red flag and marks progression towards a complete syndrome.
He blames the urinary problem on his codeine: A plausible false explanation that must never be accepted without a bladder scan and rectal examination.
New erectile difficulty over the past two days: new sexual dysfunction is a sacral root red flag completing the cauda equina picture.

Red flags you must not miss

The case marks these as must-elicit. They only come out if you ask.

Sciatica now in both legs, previously one only.
Numbness around the perineum and back passage.
Two days unable to pass urine, only dribbling.
Cannot feel the bladder filling or emptying.
Both legs weak and heavy, worse over 48 hours.
The numb area covers the skin round his back passage and between his legs, as if he were sitting on a saddle, and he cannot feel it properly when he touches it.

Investigations and what they showed

MRI lumbar spine: A large central disc prolapse (extrusion) severely compressing the thecal sac and cauda equina, obliterating the CSF around the nerve roots. Why it helps: urgent MRI is the definitive investigation and must be obtained immediately, out of hours if necessary. Nothing about the clinical picture justifies waiting until morning.
MRI whole spine: Axial images confirm the canal is filled by the extruded disc with effaced nerve roots; whole-spine imaging excludes a second, higher compressive lesion. Why it helps: whole-spine imaging is preferred when a compressive lesion is suspected, to exclude additional levels (especially if malignancy or infection is possible).
Post-void residual bladder scan: A large post-void residual volume with a distended bladder, objective confirmation of retention with overflow. Why it helps: A quick, decisive bedside test, a large post-void residual in a patient with back pain and saddle symptoms strongly supports cauda equina syndrome and mandates immediate MRI and referral.
Inspect the abdomen: A smooth, rounded, dome-shaped swelling rising out of the pelvis above the pubic bone, a grossly distended bladder, which he cannot feel. Why it helps: this is the one visible sign of the emergency.
Inspect the perianal area and anus: The perianal skin looks normal to the eye; the abnormality is felt, not seen, reduced sensation in the saddle area and reduced anal tone on rectal examination. Why it helps: saddle anaesthesia and lax anal tone are the defining signs of cauda equina compression. They are only ever found by deliberately testing for them, never by inspection, which is why they are so often missed.
Inspect the patient's gait: He walks slowly and unsteadily, both legs heavy, holding on to the trolley for support. Why it helps: bilateral rather than one-sided leg weakness points to compression of multiple nerve roots at the cauda equina, not a single prolapsed disc pressing on one root.
Inspect the patient's posture and position: He shifts constantly and cannot get comfortable, keeping his back rigid and avoiding bending. Why it helps: pain behaviour tells you how much he is suffering, but note that he plays down the bladder symptoms and blames them on his codeine, the danger sign has to be actively asked for.

Imaging findings

CT myelogram: A complete (or near-complete) block of the intrathecal contrast column at the compressed level, the column is pinched into an hourglass waist with dense contrast pooling below the block, on a background of bony central canal stenosis.

Why the diagnosis is cauda equina syndrome

The decisive diagnosis is cauda equina syndrome, compression of the lumbosacral nerve roots, here by a large central disc extrusion. the central lesson is that it hides inside ordinary mechanical back pain, and the diagnosis depends entirely on actively asking the red-flag questions that patients do not volunteer because they are embarrassing or attributed to painkillers: bilateral sciatica, saddle (perineal) anaesthesia, bladder dysfunction (loss of sensation, poor stream, retention with overflow dribbling), bowel dysfunction/reduced anal tone, and new sexual dysfunction. Never accept 'it's the codeine' as an explanation for urinary retention in a patient with back pain, check a post-void residual bladder scan and perform a PR for anal tone and perianal sensation. The definitive investigation is an immediate MRI (whole spine preferred), it must not wait for the morning list; a normal X-ray excludes nothing. 'Prolapsed intervertebral disc' is the correct anatomical umbrella, but the syndrome, not the disc, drives the emergency.

Cauda equina syndrome at a glance

What it is

Cauda equina syndrome is compression of the bundle of nerve roots that continue below the spinal cord's end, around the first lumbar vertebra. These roots carry sensation and motor control to the legs, and, critically, control of the bladder, bowel and sexual organs. Squeezing this bundle produces a mixed picture of leg symptoms plus loss of the pelvic functions that make the syndrome a true surgical emergency rather than routine back pain.

Pathogenesis

Most commonly a large central disc fragment prolapses into the spinal canal and mechanically compresses the nerve roots against the bony canal wall. Because the cauda equina roots are peripheral nerves (lower motor neurons) rather than spinal cord tracts, compression causes flaccid weakness and reduced reflexes rather than spasticity. Sustained pressure causes venous congestion and ischaemia of the nerve roots, and the longer compression continues, the more the damage moves from reversible to permanent.

Who it affects

Rare: roughly 1 to 3 cases per 100,000 people per year. It occurs mainly in working-age adults, often in those with a pre-existing history of back pain or sciatica, and slightly more often in men. Any patient with severe low back pain and leg symptoms is a potential case until the red flags are actively excluded.

How serious it is

Cauda equina syndrome is a surgical emergency. Delay converts a recoverable compressive injury into permanent incontinence, sexual dysfunction and lower-limb weakness. It is one of the most common sources of clinical negligence claims in medicine, almost always because the red-flag questions were not asked or the imaging was delayed.

Causes and risk factors

The overwhelming majority follow a sudden large central lumbar disc prolapse, usually at L4/5 or L5/S1. Less common causes include spinal stenosis, vertebral fracture, spinal tumour (primary or metastatic), epidural abscess or haematoma. A history of chronic back pain, heavy lifting, or a recent fall raises suspicion, but the syndrome can also be the first presentation of malignancy or infection.

Investigations and lab tests

Two decisive bedside tests come first: a post-void residual bladder scan (a large residual volume signals painless retention with overflow) and a PR examination assessing anal tone and perianal sensation. Saddle anaesthesia, reduced anal tone and bilateral leg signs together are strong evidence for the diagnosis. The definitive investigation is an immediate whole-spine MRI, arranged out of hours if necessary, since it must never wait for a routine morning list.

At a glance: Bedside: post-void residual bladder scan, PR exam for anal tone and perianal sensation. Definitive: immediate MRI whole spine, not a plain X-ray.

Teaching points

Cauda equina syndrome hides inside ordinary back pain, you will only find it if you ASK the red-flag questions every time.
The red flags are: bilateral sciatica, saddle (perineal) anaesthesia, bladder dysfunction (loss of sensation, poor stream, retention with overflow), bowel dysfunction/reduced anal tone, and new sexual dysfunction.
Patients under-report these symptoms, they are embarrassing, and urinary problems are commonly blamed on opioids. never accept 'it's the codeine' without checking.
Two decisive bedside tests: a post-void residual bladder scan and a PR examination for anal tone and perianal sensation.
The definitive investigation is an immediate MRI (whole spine preferred), out of hours if necessary; it must not wait for a routine morning slot.
A normal lumbar X-ray excludes nothing, it cannot show the disc, the thecal sac or the compressed nerve roots.
Distinguish cauda equina (lower motor neuron: flaccid weakness, areflexia, saddle anaesthesia) from spinal cord compression (upper motor neuron: hyperreflexia, upgoing plantars, a sensory level).
Delay causes permanent incontinence, sexual dysfunction and lower-limb weakness, and cauda equina syndrome is a leading source of clinical negligence claims.

Practise a similar case

Reading this case is not the same as driving the questions yourself when the clock is ticking. It is easy to skim past the detail, like numbness around the perineum and not feeling the bladder fill, or to assume the dribbling is the codeine. Try another simulated neurology patient and take the history yourself.

Here's one from the same specialty, with the diagnosis left for you to find. Your next patient is 41, with 2 weeks of worsening headache, a weak right arm, and a fit this morning. Take this patient's history in Diagnosica. It's free to start, no card needed.

Cauda equina syndrome case presentation: history, findings and tests