Situation: A 30-year-old man with a complete spinal cord injury at the sixth cervical (C6) level from a fall 3 months ago is on a rehabilitation unit. Spinal shock has resolved. His bladder is emptied by intermittent catheterization every 4 hours. His usual blood pressure (BP) is 96/60 mmHg. He asks why urine sometimes leaks between catheterizations even though he cannot feel when his bladder is full. Which explanation is accurate?

An injury above the sacral micturition center (S2–S4) leaves the sacral reflex arc intact but cut off from the brain. The bladder fills and contracts reflexively without sensation or voluntary control, causing reflex incontinence, an upper motor neuron (spastic) bladder. Destruction of the sacral center, as in conus or cauda equina injury, instead causes a flaccid bladder with retention and overflow.

Core mechanism
A complete injury at C6 lies well above the sacral micturition center (S2–S4). The local reflex arc in the sacral cord remains anatomically intact, but its connection to the cerebral cortex and pontine micturition center is interrupted. Because spinal shock has resolved, the sacral reflex arc becomes hyperactive and can generate bladder contractions on its own. The man cannot feel bladder fullness and cannot voluntarily start or stop voiding, so when the bladder reaches a threshold volume, it contracts reflexively and urine leaks between scheduled catheterizations. This pattern is called reflex incontinence or an upper motor neuron (spastic) bladder.

The key distinction is that the sacral reflex arc is preserved but disconnected from higher centers, producing an overactive bladder without sensation or voluntary control. If the injury had destroyed the sacral center itself, as in conus medullaris or cauda equina injury, the bladder would be flaccid and would not contract reflexively; instead, urine would accumulate and overflow. The leakage described here is not overflow from a flaccid bladder and is not caused by weak sphincter muscles or catheter irritation.

Feature	Injury above S2–S4 (e.g., C6)	Injury at S2–S4 or below (conus/cauda equina)
Bladder type	Upper motor neuron, spastic	Lower motor neuron, flaccid
Detrusor activity	Reflex contractions, uninhibited	Areflexic, no effective contraction
Continence pattern	Reflex incontinence between catheterizations	Retention with overflow incontinence
Sensation of fullness	Absent	Absent or reduced
Sphincter behavior	May show detrusor-sphincter dyssynergia	Low tone, open sphincter

Watch out! Do not confuse reflex incontinence with overflow incontinence. Overflow occurs when a flaccid bladder cannot contract and urine leaks only after extreme distention. In this C6 injury, the bladder actively contracts on its own, which is why leakage is episodic and related to bladder filling rather than to coughing or movement.

Key point! The level of spinal cord injury determines bladder behavior. Lesions above the sacral micturition center spare the reflex arc but remove cortical inhibition, producing a spastic bladder. Lesions at or below S2–S4 destroy the reflex arc and produce a flaccid bladder.

Intermittent catheterization every 4 hours is used to empty the bladder on a schedule and reduce episodes of reflex incontinence, but it does not abolish the underlying reflex contractions. Urodynamic evaluation is important in these patients because it can confirm uninhibited detrusor contractions and guide additional management such as antimuscarinic medication or other adjunctive strategies ^[1][2]. Even with a regular catheterization schedule, reflex bladder contractions can still occur between catheterizations, especially as bladder volume increases ^[4]. Achieving a balanced bladder with intermittent catheterization is possible in many patients with upper motor neuron lesions, but continence is not guaranteed in every case ^[3].