During home rehabilitation visits, families have described a situation like this. An older person had a lumbar decompression operation a few days earlier. On the morning of the visit, while the therapist was teaching exercises, the patient said the area that touches a seat cushion had lost all feeling and that he had not been able to pass urine for most of the day. The family asked whether this was just ordinary numbness from nerve compression that would settle with more rest. The therapist stopped all exercise training at once, because this group of symptoms appearing together is not ordinary numbness. It is a warning sign of a surgical emergency.
The most important point first: cauda equina syndrome is the most urgent red flag before and after spine surgery. Incontinence of urine or stool, difficulty passing urine, saddle numbness or weakness in both legs is not ordinary back pain. It is a surgical emergency that needs a call to 999 or a direct trip to the emergency department, and nerve function can be saved only if decompression surgery is carried out within 48 hours (Ahn 2000; Rider 2023). This page does not repeat the general list of red flags. It concentrates on three questions: how to tell this particular emergency apart at home, why every hour counts, and at which stage there is still a chance of recovery.
This is not ordinary back pain that can be watched for a while. A single symptom has low sensitivity and is easily missed at the start, but if cauda equina syndrome is treated late the outcome is often poor. The diagnosis has to be judged from the whole group of symptoms together, and it needs a surgical consultation and urgent decompression surgery (Long 2020). If any of the signs below appear, treat it as an emergency at once, and do not decide to wait until the next follow-up visit.
The four red flags of cauda equina syndrome
Learn the four core red flags first. If any one of them appears together with back or leg symptoms, the patient must be examined at once. Many studies have shown repeatedly that these red flag symptoms have high specificity but low sensitivity (sensitivity 0.19 to 0.43, specificity 0.62 to 0.88; Dionne 2019). In simple terms, when these symptoms appear, there is very likely a serious problem and the patient must be checked immediately. Yet the absence of these symptoms does not mean the patient is completely safe, so clinical practice is to err on the side of caution.
- Difficulty passing urine, or incontinence of urine or stool: unable to pass urine, urinary incontinence or loss of bowel control
- Saddle numbness: loss of feeling in the perineum and the areas that touch a seat when sitting (including the buttocks and inner thighs)
- Sciatica in both legs or weakness in both legs: both legs go weak or numb at the same time
- Reduced anal sphincter tone: weak straining when opening the bowels, or no sensation of needing to
These four symptoms do not appear in fixed proportions or in a fixed order. A study of 256 patients with suspected cauda equina syndrome found that the most common presentation was severe bilateral neurological deficit (50.8%), followed by bladder and bowel incontinence (35.2%), bilateral sciatica (21.9%) and initial difficulty passing urine (17.6%) (Zeb 2023). Bladder and bowel changes are therefore a common warning sign that must not be ignored. For the everyday rehabilitation rules, such as which movements to pause in the early period after surgery and when activity can resume, which is a separate care topic, see movements to avoid and activity restrictions after spine surgery.
Why surgery within 48 hours of onset matters
Cauda equina syndrome is regarded as a critical emergency mainly because saving the nerves runs against a countdown. The later decompression surgery is carried out, the harder it is to recover the damaged function. Many medical studies agree on 48 hours as the key dividing line.
It is worth noting that the point is not to chase “the sooner the better” by a few hours. Clinical evidence shows that surgery within 24 hours is not necessarily better than surgery between 24 and 48 hours. The real line is not to go beyond 48 hours (Ahn 2000). The starting point of the diagnostic clock often begins with saddle numbness, which is present in as many as 93% of patients with cauda equina syndrome. Ideally an MRI scan should be completed within one hour of presentation and surgery carried out within 48 hours (Rider 2023). This countdown is exactly why, once saddle numbness is noticed, the attitude of waiting a little longer to see must never be taken.
The two stages of cauda equina syndrome: CESI and CESR
Although both are cauda equina syndrome, the condition falls into two stages with very different outlooks. The approach most often used clinically, and the most useful for prognosis, is to divide cauda equina syndrome into CESI and CESR (this is the most widely used classification among 17 definitions in a review of 212 papers; Lavy 2022). Understanding the difference explains why seeking help early is the key to preserving bladder function.
- Some control of passing urine is still present
- Only the sensation changes: needing to strain, a weaker stream, no urge to pass urine
- It is an emergency: go to the emergency department as soon as the symptoms appear
- This is the stage in which time can still be gained
- Loss of bladder control and painless urinary retention
- Overflow incontinence: urine leaks only once the bladder is overfull
- Usually a later stage, and the after-effects are often irreversible
- Help should not be sought only at this point
This dividing line also carries an important warning. Todd (2017) reviewed the recommendations of 37 clinical guidelines and pointed out that about two thirds of the symptoms traditionally listed as red flags (such as painless urinary retention and complete loss of saddle sensation) actually reflect a later stage, often irreversible, of cauda equina syndrome. Strictly speaking they should be regarded as white flags, because by the time these symptoms appear it is often already too late. What gives a real chance of saving nerve function are the earlier presentations, such as bilateral nerve root symptoms or a change in urinary habit that has only just been noticed (Todd 2017). In other words, when the patient cannot pass urine yet feels no pain, the warning is already late, and no further delay is acceptable. If the patient’s main symptom is only a one-sided leg pain or numbness left over from the operation, rather than the acute bilateral neurological deficit described above, the nature of the problem and the way it is triaged are quite different. See physiotherapy after spine surgery for the wider picture of recovery.
Sensitivity and specificity of the red flags
A clinical contradiction has been mentioned several times above. The sensitivity of the red flag symptoms is low (a single symptom is easily missed), but their specificity is quite high (when they appear, there is usually a serious problem). That is why the medical approach is to err on the side of caution. Rather than waiting passively for all the typical symptoms to appear one by one, objective tests are used early to trigger a deeper investigation.
Diagnostic features of the red flags
- The sensitivity of red flag symptoms is only 0.19 to 0.43, so looking at a single symptom easily leads to a missed diagnosis (Dionne 2019)
- Specificity reaches 0.62 to 0.88, which means that when these symptoms appear, there is usually a real problem (Dionne 2019)
- Saddle numbness occurs in as many as 93%, and is an important starting point for counting the critical time for rescue (Rider 2023)
Objective clinical test
- Post-void residual volume (PVR) can help with clinical triage
- When PVR is greater than 200 mL, sensitivity is as high as 94% (Alshahwani 2021)
- Once the value is exceeded, an urgent MRI scan is arranged at once, without waiting for all the symptoms to appear
In other words, measuring the residual urine volume with a bladder scan is a simple and objective test that gives a clear line while symptoms are still vague. When PVR is greater than 200 mL, its sensitivity is as high as 94%, which is enough to serve as an objective threshold for triggering an urgent MRI scan (Alshahwani 2021). This is also the common conclusion of nine international guidelines: an urgent MRI scan is needed once a red flag symptom is found, eight of the guidelines list sciatica as an item to assess, and five include measurement of residual urine volume in routine assessment (Hennessy 2025).
Steps for urgent transfer to hospital when symptoms appear
If a family member at home shows any of the red flags above, do not spend time searching online and do not wait for the next follow-up visit. Act at once according to the steps below. International guidelines agree that an urgent MRI scan must be arranged as soon as a red flag symptom is seen (Hennessy 2025), and ideally the scan should be completed shortly after presentation (Rider 2023), so every minute counts.
If cauda equina syndrome is suspected, family members and caregivers can act at once as follows:
- Step 1: call 999 for an ambulance at once—Suspected cauda equina syndrome is a surgical emergency, and an ambulance should take the patient straight to the emergency department. Do not drive there yourself, and do not wait for an outpatient appointment.
- Step 2: note the time the symptoms began—The 48-hour countdown for rescue starts from the moment the symptoms appear (Ahn 2000). It is therefore very important to tell the medical staff clearly when the patient could no longer pass urine or when both legs began to feel weak, as this matters for triage in the emergency department.
- Step 3: state the red flags clearly at the emergency department—On arrival, tell the triage nurse clearly that the patient has bladder or bowel problems, numbness where the body touches a seat, and weakness in both legs, so that the hospital can arrange an urgent MRI scan as soon as possible (Hennessy 2025).
- Step 4: pause eating and drinking (if surgery may be needed)—The patient may need urgent decompression surgery. On the way to hospital or on arrival, the family should ask the medical staff whether the patient needs to stop eating and drinking at once, so that the arrangements for surgery are not delayed.
Local routes to medical help (worth saving for reference). Suspected cauda equina syndrome is a surgical emergency in which every minute counts. Call Hong Kong emergency services on 999 for an ambulance to the emergency department, and do not wait for an outpatient appointment. All 18 Hospital Authority accident and emergency departments (A&E)in Hong Kong provide a 24-hour service and can arrange an urgent MRI scan and referral for a neurosurgical consultation. The nearest A&E can be found on the Hospital Authority website (www.ha.org.hk).
Why to keep watching for recurrent compression after surgery
Even after the patient has had decompression surgery, families and caregivers must not relax. Cauda equina syndrome itself is easily affected by a delay in diagnosis, and after surgery the area can again be affected by a haematoma, swelling or a recurrent disc herniation, causing recurrent compression and bringing back the critical symptoms (Long 2020). The principles of monitoring after surgery are therefore the same as before. If the patient’s legs become weaker, difficulty passing urine appears for the first time, or saddle numbness returns, it must be treated as a new emergency, and vigilance must not be relaxed because the patient has already had an operation (Zeb 2023).
In home rehabilitation assessments, the most common mistake is that the family takes new difficulty passing urine or saddle numbness after surgery to be general weakness after an operation, and prefers to watch at home. The overview article on neurological warning signs and complications after spine surgery already sets out the six red flags one by one. What this page adds is the habit of active screening during follow-up after surgery, because the sensitivity of red flag symptoms is low and, if the family waits passively for all the symptoms to appear, the chance is often already lost. For this reason, at each follow-up after surgery, a physiotherapist checks the wound and the level of pain and also asks regularly, one item at a time: “Can you pass urine normally?” “Is there numbness where you touch the seat?” “Are both legs going weak at the same time?” “Is there any incontinence of urine or stool?” Only active screening item by item can catch CESI while it is still at an early stage.
At the same time, the physiotherapist turns the technical clinical red flag indicators into everyday words the family can understand, and teaches the family to remember these “four questions”: “Can you pass urine normally?” “Is there numbness where you touch the seat?” “Are both legs going weak at the same time?” “Is there any incontinence of urine or stool?” If any one of them is abnormal, call 999 at once. Only by extending the monitoring into everyday life, beyond the therapist’s visits, can this time-critical surgical emergency be guarded against.
The physiotherapist’s role in monitoring after surgery
If the family is caring for an older person who has just had spine surgery and wants professional help to tell general weakness after an operation from the red flags of cauda equina syndrome, the home physiotherapy assessment provided by Nova Health’s registered physiotherapists can help. At every visit, the therapist screens one by one the patient’s urinary status, saddle sensation, strength in both legs and anal sensation, and if a red flag is found, helps to arrange immediate triage to hospital. It must be stressed again that for the critical symptoms of the four red flags, the family should always call 999 first or go straight to the emergency department, and should never wait for the next home visit, because every hour of delay directly affects whether the damaged nerve function can be saved.