Ultrasound shear wave elastography cannot discriminate between low- and high-pressure neurogenic bladders

Abstract

Background: Patients with neurogenic bladders are monitored with renal bladder ultrasounds and video urodynamics studies (VUDS) to assess upper urinary tract injury. Ultrasound shear wave elastography (US-SWE) can assess tissue stiffness. If bladder compliance is affected by bladder wall fibrosis and stiffening, then high-pressure bladders may be detectable by US-SWE therefore reducing the need for VUDS in some patients.

Objective: To determine if US-SWE can differentiate between low- and high-pressure bladders and hence be used for noninvasive assessment of neurogenic bladder in children.

Study design: Prospective study of patients with neurogenic bladder undergoing clinically indicated VUDS between February and July of 2017. During VUDS, bladder wall US-SWE was measured at different filling percentages of estimated bladder capacity (EBC). The bladders were divided into cohorts according to the detrusor muscle pressure (P_det) reached at the maximum bladder capacity: normal (1 to <15 cmH₂O) and abnormal (≥15 cmH₂O) pressure. T-test was used to compare elastography values at different bladder volumes and P_det; (statistical significance set at p < 0.05).

Results: 30/31 enrolled children completed the protocol. With an empty bladder, as well as at all other bladder volumes, no relationship was observed between P_det and mean SWE of the anterior or posterior bladder wall. At maximum bladder capacity, there was no difference between mean SWE values of the anterior or posterior bladder walls in those with normal pressure, 2.97 m/s (SD ± 0.82) and 1.96 m/s (SD ± 0.75), compared to those with abnormal pressures 3.08 m/s (SD ± 0.84) and 2.39 m/s (SD ± 0.96), p = 0.75 and p = 0.2, respectively.

Discussion: We found no difference between SWE values of either the anterior or posterior bladder wall in neurogenic bladders with normal and abnormal filling pressures measured during VUDS. Our study differs from previously reported studies with more positive results in that our cut-off for abnormal bladder pressure was (P_det ≥15 cmH₂O). This is lower than the more commonly used leak point pressure of 40 cmH₂O because our practice is to intervene earlier. Moreover, SWE would be most useful if it can identify changes before the bladder has reached such severe conditions, to allow for early intervention. Additional differences between ours and other studies include US manufacturer and younger age of the participating children.

Conclusions: US-SWE, while feasible, was not able to discriminate between low- and high-pressure bladders. Moreover, US-SWE did not show significant correlation with the current gold standard, VUDS.

Keywords: Elastography; Neurogenic; Ultrasound; Urinary bladder; Urodynamics.