Cauda equina syndrome: urinary retention and overflow incontinence
The typical picture in cauda equina syndrome (compression of the nerve roots below the end of the spinal cord) is overflow incontinence: a bladder that cannot empty overfills and leaks.
Remember
In cauda equina syndrome, damage to the sacral nerve roots (S2 to S4) removes the parasympathetic drive to the detrusor (the muscular bladder wall), so urine is retained and then spills over. The typical picture is overflow, not urgency.
The spinal cord ends at about the L2 level. What carries on below it is a bundle of lumbar and sacral nerve roots (the nerve roots of the lower back), the cauda equina, which is Latin for horse's tail. In cauda equina syndrome these roots are compressed, and it is a medical emergency.
The sacral roots (S2 to S4) carry the parasympathetic supply (the rest-and-digest branch of the autonomic nervous system) to the detrusor, the muscular wall of the bladder. Those nerves make the detrusor squeeze. Sensory fibres in the same roots report that the bladder is filling.
When compression cuts off the parasympathetic drive, the detrusor cannot contract properly. The bladder keeps filling but does not empty, which is urinary retention, the bladder not emptying properly. Sensory fibres can be damaged too, so a patient may not feel the bladder filling, and it overfills further. Once the pressure inside finally overcomes the resistance of the urethra (the outlet tube), urine leaks out passively. That is overflow incontinence: urine spilling from a bladder that is stretched and cannot empty. Urinary retention and overflow incontinence are late signs of cauda equina syndrome, so dribbling is a reason for urgent action, not reassurance.
The damage is to peripheral nerve roots (a lower motor neurone injury, meaning damage to the nerve cells that run out from the spinal cord to the target), so the bladder is underactive, not overactive. An upper motor neurone problem (damage in the brain or spinal cord) typically gives the opposite pattern, an overactive bladder, which can cause urgency with leakage.
Key facts
- Cauda equina syndrome
- compression of the nerve roots below the lower end of the spinal cord, and a medical emergency.
- Parasympathetic supply to the detrusor
- carried by the sacral roots (S2 to S4); losing it means weak contraction and incomplete emptying.
- Overflow incontinence
- passive leakage from an over-full bladder, together with urinary retention.
- Sensory loss
- a patient may not feel the bladder filling, which lets it overfill further.
- Pudendal nerve (S2 to S4)
- a somatic motor nerve (one that supplies voluntary muscle) to the external urethral sphincter, the ring of voluntary muscle that holds urine in; losing it can weaken closure and cause leakage from a bladder that is not overfull.
- Upper motor neurone damage
- in the brain or spinal cord it typically gives the opposite pattern, an overactive bladder, which can cause urgency with leakage.
- Red flags
- pain down both legs (bilateral sciatica), numbness of the area that would touch a saddle (saddle anaesthesia) and loss of urinary or faecal continence should prompt urgent assessment.
Exam traps
- Overactive sympathetic nerves do not produce a sustained detrusor contraction, because their job is to relax the detrusor so that the bladder can store urine.
- Urge incontinence (leakage linked to an overactive detrusor and a sudden compelling desire to pass urine) with heightened bladder sensation does not explain overflow, because bladder sensation is more often reduced in cauda equina syndrome, so a patient may not feel the bladder filling.
- Stress incontinence, leakage when exertion raises abdominal pressure, comes from weak pelvic floor muscles and urethral support, and does not explain the overflow incontinence that follows sacral nerve-root compression in cauda equina syndrome.
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