Urodynamic Studies
Urodynamic studies measure how the bladder stores and empties urine, by recording pressures and flow while the bladder is filled and emptied. They separate bladder problems from outlet problems, and they explain symptoms that history and examination cannot.
The test combines several recordings. Cystometry measures bladder pressure during filling through a fine urethral catheter, with a second catheter in the rectum or vagina to record abdominal pressure. The detrusor pressure is the bladder pressure minus the abdominal pressure. A pressure flow study follows when the patient voids, and it relates detrusor pressure to flow rate.
Additional parts include urethral pressure profilometry, leak point pressures, electromyography of the pelvic floor, and videourodynamics, which adds contrast and fluoroscopy to show the bladder neck, reflux and the shape of the urethra.
The bladder is filled with warmed saline at 20 to 50 mL per minute (slower in neurological disease or small bladders). The patient reports first sensation, first desire to void and strong desire, and is asked to cough, strain and change position to provoke leakage. The test takes 30 to 60 minutes.
Urodynamics changes the diagnosis in a large share of patients with mixed or complex symptoms: it finds detrusor overactivity in about 50 to 60% of men and women with refractory urgency, and it shows bladder outlet obstruction in about 50% of men with lower urinary tract symptoms before surgery. A simple symptom history is wrong about the underlying mechanism in up to a third of women with incontinence.
Indications.
- Failure of first treatment for urinary incontinence or lower urinary tract symptoms.
- Before surgery for stress incontinence or prolapse, when the diagnosis is unclear, the symptoms are mixed, or there has been previous surgery.
- Suspected bladder outlet obstruction in men before surgery, especially when the flow rate is not clearly low or the prostate is small.
- Neurogenic bladder (spinal cord injury, spina bifida, multiple sclerosis, Parkinson disease, diabetes): to assess risk to the upper tracts.
- Recurrent urinary infection, hydronephrosis, or renal impairment of unknown cause with lower tract dysfunction.
- Voiding dysfunction, underactive bladder and urinary retention.
- Persistent bladder symptoms in children, including daytime wetting, with abnormal imaging.
Not indicated as a routine test in uncomplicated stress incontinence or in men with clear symptoms and a typical flow pattern, when the result will not change management.
Contraindications and precautions.
- Active urinary tract infection: the test is delayed and the infection treated, since catheterisation can cause bacteraemia.
- Recent urological surgery or injury to the urethra.
- Patients at risk of autonomic dysreflexia (spinal lesion at T6 or higher): blood pressure is monitored, filling is slow, and the test stops at the first sign of headache, sweating or hypertension.
- Bleeding disorder or severe immunosuppression: caution.
- Prophylactic antibiotics are used only in selected high risk patients, such as those with heart valve disease, a recent joint prosthesis, or a neurogenic bladder with recurrent infection.
- Patients with latex allergy need a latex free catheter.
Preparation and technique.
The patient is asked to void before arrival, and a flow test and post-void residual are done first. A bladder diary and a recent urine culture are reviewed. Anticholinergic and beta-3 agonist drugs, and alpha blockers, are usually stopped for a period before the test when the question is about their effect, or continued when the question is about control on treatment.
The patient is seated or standing for filling, since some provocation only happens upright. The catheters are zeroed at the level of the upper edge of the pubic symphysis, and the quality of the traces is checked with a cough, which should give equal spikes in the bladder and abdominal lines. Filling continues to a strong desire, to the point of leakage, or to a set maximum volume, and the patient then voids in private, with the catheters left in place.
