So What Happens in Disc Herniations?
The pressure on the sacral (S2-4) roots interrupts the visceral sensory fibers that fire when the detrusor muscle is being stretched, and the patient loses the ability to sense that the bladder is full. The stretching of the detrusor muscle may also make it less competent to contract (a decrease in smooth muscle tension with excess stretch). The parasympathetic motor fibers that are carried in the S2-4 roots on their way to the bladder via the pelvic nerve are also affected.
The ultimate result is an areflexic, flaccid, lower motor neuron bladder
The bladder fills with urine, but the patient is unable to feel it and unable to void because there is no reflex activity to stimulate emptying. Overdistension results, and the incontinence pattern is one of overflow.
As An Aside, What About Spinal Cord Injured Patients?
Their pattern of bladder dysfunction is different. The input and output reflex arcs of the S2-4 reflex voiding center are intact, but the connections with higher centers are disrupted, causing the loss of the sensation to void and loss of voluntary, coordinated control over the center.
When the bladder fills, the signals are sent back to the cord, and reflex parasympathetic signals are sent to the bladder. This causes independent, uncontrolled bladder contraction – the hyperreflexic, spastic, upper motor neuron bladder.
To make matters worse, the sphincters may be spastic, making it impossible for the spastic detrusor muscle to push urine out; this also results in over-distension of the bladder.