12 - Ultrasound as a tool in urodynamics

Summary

Introduction

Ultrasound of the bladder is commonly used in clinical practice as a non-invasive estimate of bladder volume when assessing post-void urinary residual. The advantages of ultrasound as a technique are that it is non-invasive, risk free and is easily applied and accessible when compared with traditional techniques, which are comparatively time consuming, invasive and involve exposure of the patient to X-rays.

Equipment

Two types of ultrasound equipment are available:

1. □ dedicated bladder scanner: a mechanical sector probe to calculate a volume (although this may pick up artefact: any fluid-filled structure such as postnatal lochia or ovarian cyst)

2. □ standard linear array transabdominal or transvaginal ultrasound: this estimates volume using an equation − height (cm) × width (cm) x depth/0.7 (cm). Estimated volume is unreliable at higher volumes because of a non-linear increase as urinary volume increases.

Indications for ultrasound in urodynamics

Bladder volume

Ultrasound can be used to measure bladder volume; for example, estimation of residual volume.

Urethral hypermobility

Ultrasound of the bladder neck can be used to assess urethral hypermobility (Figure 12.1). Increased bladder neck mobility is associated with stress urinary incontinence and can be measured ultrasonically using an X-Y coordinate system. The position of the bladder neck in terms of rotational angle and descent of the bladder neck can be determined (Figure 12.1). This method has a specificity of 83.1% and positive predictive value of 67.6% for urodynamic stress incontinence.