Reflex bladder after complete mid-spinal cord transection
A complete mid-spinal cord transection, which cuts the descending pathways, later leaves a reflex bladder: the detrusor (bladder wall muscle) contracts involuntarily against a sphincter that fails to relax.
Remember
Once the early phase has passed, a complete mid-spinal cord transection leaves a bladder that contracts by reflex against a sphincter that does not relax, so urine cannot be passed effectively.
Memory aid
the engine runs but the gate stays shut
The engine is the strong reflex contraction of the detrusor, and the gate is the sphincter that does not relax.
Normally the brainstem coordinates emptying. The medial micturition centre (a group of neurones in the brainstem that starts voiding) stimulates parasympathetic neurones (the nerve cells that drive the detrusor), so the detrusor contracts, and at the same moment it relaxes the sphincters.
A complete transection above the sacral cord (the lowest part of the spinal cord, which supplies the bladder) cuts the descending pathways, so the sacral cord is cut off from the brain. Descending inhibition (restraining signals sent from the brain and brainstem down the spinal cord) is lost, but the sacral reflex arc stays intact.
At first there is spinal shock (a temporary phase after a severe cord injury in which the reflexes below the injury are depressed, settling over days to months). The bladder is flaccid (floppy and unable to contract) and retains urine.
When shock settles, a reflex bladder develops. Bladder stretch triggers involuntary detrusor contractions, but the signal that relaxes the external urethral sphincter (the ring of voluntary muscle around the urethra) is cut, so the sphincter does not relax in step. This is detrusor-sphincter dyssynergia (the bladder muscle contracts while the sphincter stays tight, so the two are out of step).
The bladder therefore squeezes against a closed outlet, so pressure rises and urine is retained. Emptying is therefore incomplete and the patient has leakage; the reflex bladder also empties involuntarily and without warning, since sensory signals no longer reach the brain. Catheterisation (passing a tube into the bladder to drain it) relieves the pressure.
Key facts
- Level
- above the sacral cord, the reflex arc is intact, but brain stem coordination is lost.
- First phase
- spinal shock gives a flaccid bladder that retains urine.
- Later phase
- there are involuntary detrusor contractions, and the sphincter does not relax with them.
- Urodynamics
- measuring bladder and sphincter pressures during filling and voiding shows the detrusor and the external urethral sphincter contracting together.
- Contrast
- damage to the sacral cord or the peripheral nerves typically gives a flaccid bladder, with an underactive detrusor instead of reflex contractions.
- Management
- catheterisation relieves the pressure and protects the kidneys.
Exam traps
- Involuntary detrusor contractions do return once spinal shock settles, but emptying is not automatic, because the transection disconnects the brainstem micturition centre that relaxes the sphincter in step with the detrusor.
- A flaccid bladder with absent contractions does not fit once spinal shock has settled, because the sacral reflex arc below a mid-spinal injury is intact, so reflex detrusor contractions return.
- A permanently open sphincter does not fit, because after a complete mid-spinal cord transection the sphincter fails to relax rather than failing to close, so urine is held under high pressure rather than leaking continuously.
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