Cauda equina syndrome

In one minute

Cauda equina syndrome is compression of the bundle of nerve roots at the bottom of the spinal canal that serve the bladder, bowel, genitals and legs. The usual cause is a large lumbar disc herniation. It is rare, one to three people per 100,000 each year, and it strikes people in the middle of life more often than the old. The diagnosis is made by an emergency MRI and the treatment is an emergency operation to take the pressure off. Recovery of the bladder and bowel depends more on how much function was left when the patient reached hospital than on anything else, which is why the hours matter.

What it is

The spinal cord ends at about the first lumbar vertebra. Below it, the nerve roots for the legs, bladder, bowel and sexual organs hang down inside the canal in a loose bundle that anatomists named the cauda equina, horse's tail. Anything that fills the lower canal quickly can crush these roots together: a large central disc herniation, a blood clot after surgery or injury, an abscess, a tumour, a fracture, or severe stenosis on top of a smaller herniation. The StatPearls review attributes about 45% of cases to a herniated disc, and the GIRFT (Getting It Right First Time, NHS England) pathway describes a large disc prolapse as the commonest cause.

Two forms are distinguished. In incomplete cauda equina syndrome (CESI) the patient has altered saddle sensation and some bladder symptoms, such as difficulty starting or loss of the urge, but can still pass urine. In cauda equina syndrome with retention (CESR) the bladder has lost its nerve supply: it fills without sensation and overflows. The incomplete form is the one where time buys the most, and the British standards ask for every threatened, partial or complete case to be investigated as an emergency.

Causes and risk factors

The population study from Scotland found an incidence of 2.7 per 100,000 per year, higher than earlier European estimates of 0.3 to 0.6, with the highest rate in women aged 30 to 39. The GIRFT pathway gives one to three per 100,000. The StatPearls review estimates that about 3% of lumbar disc herniations lead to cauda equina syndrome.

There is no established list of risk factors beyond the causes themselves. It can follow a first herniation or a recurrent one, and, rarely, spine surgery or injury through bleeding into the canal; the StatPearls review lists epidural haematoma, abscess, tumour, trauma and stenosis among the other causes. There is no way to predict which sciatica will become cauda equina syndrome, which is why every patient with a herniation should know the warning signs.

How it develops

The sacral roots at the centre of the bundle carry the nerves of the bladder, the rectum, the genitals and the skin of the saddle. A fragment that fills the canal presses on them first and hardest. The bladder loses its sensation of filling, then its ability to empty; urine builds up and eventually overflows, so that leaking is a late sign, not an early one. The rectum loses the sense of fullness and then its control. Sensation in the saddle fades. The leg roots at the edge of the bundle are involved too, so sciatica is usually on both sides and the feet can weaken. A root crushed for long enough is thought to lose fibres that no operation brings back; the aim of emergency surgery is to save the ones still working.

Symptoms

Back pain with sciatica is the usual background. Bladder symptoms, saddle numbness, bowel symptoms and, in men, loss of erection or ejaculation are added to it in varying combinations, and GIRFT stresses that no single symptom or combination has good diagnostic accuracy. The pattern that should raise the alarm is sciatica in both legs, or in one leg that has moved to the other, together with any change in the bladder, bowel or saddle sensation. The GIRFT pathway lists difficulty starting to pass urine or a numb stream, altered feeling in the saddle, severe or progressive weakness of both legs, loss of the feeling of rectal fullness and sexual dysfunction as the symptoms that require an emergency MRI when they began within the last two weeks. Sudden sciatica in both legs without these symptoms is a warning sign and needs urgent, not emergency, assessment.

Painless retention, a full bladder that cannot be emptied and does not hurt, is the sign of the established syndrome and a late one; GIRFT notes that patients who present with it have a poorer prognosis, though around 70% still benefit from decompression.

Diagnosis

The British standards state that the reliability of clinical diagnosis is low and that the threshold for an emergency MRI should therefore be low. The examination records strength and sensation in the legs, sensation in the saddle area, and the bladder. GIRFT advises measuring the urine left after voiding with a bladder scanner: a residual over 200 ml makes cauda equina syndrome about 20 times more likely, though 60% of patients who needed emergency surgery had a residual under 200 ml, so a normal scan does not rule it out. A patient who cannot pass urine at all and has more than 600 ml on the scan is catheterised to protect the bladder.

MRI is the test, and the British standards say it must be done as an emergency at the hospital where the patient presents, ahead of routine scans, without waiting to discuss the case with a spinal unit; GIRFT sets a standard of within four hours of the request, with a single sagittal T2 sequence (the MRI sequence on which fluid looks bright) sufficient to screen for compression. Where MRI is impossible, CT (computed tomography) or CT myelography (CT with contrast dye in the spinal fluid) is used.

Most people scanned for suspected cauda equina syndrome do not have compression; the Practical Neurology review puts it at 70% or more. Those patients are not dismissed: their pain and bladder symptoms are treated, and the standards ask for a plan that may include scanning the rest of the spine and referral to a continence service.

Treatment without surgery

For compression seen on MRI there is no non-surgical treatment; the pressure is removed by an operation. For patients whose scan shows no compression, treatment addresses the cause of the pain and the bladder symptoms, which are often triggered by severe pain, medication and anxiety rather than by nerve damage, and they recover with pain control, a catheter for a time if needed, and follow-up.

When surgery is considered

Surgery is considered as soon as cauda equina compression is confirmed. The British standards say that nothing is to be gained by delaying surgery, that it should be undertaken at the earliest opportunity, and that any reason for delay must be documented. GIRFT classes the incomplete form as an emergency to be operated as quickly as possible and, for patients who present already in painless retention, requires surgery within 24 hours of the MRI, noting that around 70% of these patients still benefit from decompression.

The evidence on how fast is fast enough is worth stating plainly. The meta-analysis of 2000, 322 patients, found significantly better recovery of sensation, strength, bladder and bowel when decompression was done within 48 hours of onset compared with later, and no additional advantage for surgery within 24 hours over 24 to 48 hours. The 2026 meta-analysis of 15 studies and 26,627 patients reached the same conclusion: decompression within 48 hours roughly doubled the odds of urinary recovery, the effect was strong in the incomplete form and not significant once retention was established, and there was no consistent benefit from operating within 24 hours rather than 24 to 48. The British prospective cohort of 621 patients, in which the median time to surgery was three days and only 32% were operated within 48 hours, found no association between time to surgery and outcome at one year, with bladder function at presentation the dominant factor. The sources agree on the practical point: bladder status on arrival is what predicts recovery, and the way to arrive with a working bladder is to come early.

Surgical options

The operation is a decompression: removing the bone and ligament over the compressed segment and the disc fragment or other mass beneath it, until the nerves are free. GIRFT lists total laminectomy, hemilaminectomy and laminotomy as acceptable techniques, and asks for the bladder to be catheterised before the operation starts. The choice between them, and whether the operation can be done endoscopically, depends on the size and position of the fragment, the width of the canal and the surgeon's judgement of what will decompress the nerves completely in that patient. A small incision is not the objective; a complete decompression is. After surgery, patients with continuing bladder or bowel symptoms are referred to continence and urology services, and the British standards ask that this be prompt.

How we treat it at spine.uz

We want to be clear about what our free remote MRI opinion is for. A doctor from the team replies by voice within 48 hours, and that is the right service for a disc herniation, stenosis or a slipped vertebra. It is the wrong service for cauda equina syndrome, where 48 hours is the whole window. If you have the symptoms in the callout at the top of this page, go to the nearest hospital with an MRI scanner and an emergency department.

Once the emergency has been dealt with, the remote opinion is the right tool again: send the scans and the operation report for a second opinion on whether anything remains compressed and whether a further, planned operation is needed for the underlying disc or stenosis. The team treats the full range of surgical spine pathology, operates on all levels, and does planned decompressions endoscopically through a 7 mm incision where that is the right tool. Every patient receives a written conclusion from the team. Across 27,600+ endoscopic and spine operations, the team's complication rate is well below 1%.

Recovery and outcomes

Recovery is real but often incomplete. In the British cohort, 31% needed a catheter before surgery and 13% at discharge, and function improved even in those who arrived with retention. In the StatPearls review, at about two months after surgery, 47.7% still had bladder problems, 41.8% bowel problems, 53.3% sexual dysfunction and 56.6% saddle numbness. The 2026 meta-analysis found that the advantage of early surgery was largest in the first months and narrowed by a year, when bladder status at presentation was the dominant predictor.

Bladder, bowel and sexual function can keep improving for many months, and they need active management: a catheter or self-catheterisation while the bladder recovers, bowel routines, and referral to continence, urology and, where needed, sexual health services. In the British cohort, 65% of patients needed additional healthcare services during the first year, which is the honest measure of how much support this recovery takes.

Living with it and prevention

Anyone with a known disc herniation, and anyone recovering from one, should know the warning signs on this page and where the nearest hospital with an MRI scanner is. The GIRFT pathway asks clinicians to give patients with back pain a safety-netting card and video for exactly this reason, and this page is our version of that card. After cauda equina syndrome, keep working with the continence services, keep moving within the limits of pain, and report any return of saddle numbness, bladder change or two-sided sciatica the same day, because a recurrent herniation can do it again.