Urodynamics and Voiding Dysfunction
Core concepts
This domain tests whether you can interpret a trace and whether you know when the test is indicated.
Indications: urodynamics is not routine before every incontinence operation; it is indicated when the diagnosis is unclear, in neurogenic bladder, before re-operation, and when symptoms and findings conflict.
The study: understand filling cystometry (compliance, detrusor overactivity, first sensation, capacity), pressure-flow study (detrusor pressure at maximum flow, maximum flow rate), and the voiding phase. Know the definitions: detrusor overactivity is involuntary detrusor contraction during filling; detrusor sphincter dyssynergia is involuntary sphincter contraction during detrusor contraction; poor compliance is a rising detrusor pressure with filling.
Artefacts: the commonest trap is misreading artefact as pathology - a cough spike that does not reproduce on the detrusor channel, a rectal line that has fallen out, or a flow rate measured without adequate privacy. Always check that the abdominal and vesical lines track together on a cough.
Neurogenic bladder: know the difference between upper motor neuron (spastic, dyssynergic, risk of upper tract damage) and lower motor neuron (areflexic, retention) patterns, and that the priority in spina bifida and spinal cord injury is protecting the upper tracts.
Voiding dysfunction in men: obstruction vs detrusor underactivity; the pressure-flow plot separates them.
Worked rule of thumb
A high detrusor pressure with a low flow rate indicates obstruction; a low detrusor pressure with a low flow rate indicates underactivity. The trap answer is to label every slow flow as obstruction.
Exam traps
- Treating artefact as detrusor overactivity.
- Ordering urodynamics reflexively before primary surgery.
- Missing detrusor sphincter dyssynergia in a neurogenic patient.
- Confusing underactivity with obstruction.