Magnitude and Structure of Urodynamic Change After Posterior Urethral Valve Ablation: An Effect-Size and Principal-Component Analysis of Three Surgical Techniques

Background. Comparisons of surgical techniques for posterior urethral valve (PUV) ablation usually rest on p-values for pre- versus postoperative means. A p-value confounds the size of a change with the size of the sample and gives no information about how the urodynamic variables move together, which is precisely what distinguishes a technique that relieves obstruction from one that merely shifts a single parameter.

Objective. To quantify the magnitude of postoperative urodynamic change on a scale-free metric, to compare the three techniques on postoperative values directly, and to characterise the dimensional structure of the urodynamic profile.

Methods. Aggregate urodynamic data from 142 boys operated on for PUV were analysed: endoscopic valve resection (n = 32), polyethylene valvulotome ablation (n = 49), metallic valvulotome ablation (n = 61). Eight variables were recorded before and after surgery. Standardised mean differences (Hedges' g) with 95 % confidence intervals were computed for every variable and group. Postoperative values were compared between techniques by Welch tests. The 6 × 8 matrix of group–phase profiles was subjected to principal component analysis on standardised variables.

Results. Effect sizes were very large throughout (|g| > 1.9 for every variable except maximum bladder capacity). Effective urethral cross-section showed g = 4.15, 8.37 and 6.25 in groups I, II and III; the urethral resistance coefficient showed g = −2.54, −5.91 and −3.75. Postoperative urethral resistance differed significantly between all three techniques, with metallic valvulotome ablation lowest (0.05 ± 0.02; d = 1.50 versus polyethylene, p < 0.001), whereas post-void residual volume did not differ between techniques (p ≥ 0.12 for all pairs). Principal component analysis recovered a single dominant axis accounting for 86.1 % of variance, with post-void residual, wall thickness and urethral resistance loading against flow rate, cross-section, detrusor pressure and detrusor tension. Displacement along this axis was largest for the metallic valvulotome (5.30 units) and smallest for the polyethylene valvulotome (4.03).

Conclusions. The urodynamic response to valve ablation is essentially one-dimensional. Post-void residual volume normalises equally well with all three techniques and is therefore uninformative for technique comparison; urethral resistance and flow rate discriminate between them.

Posterior urethral valve, bladder outlet obstruction, urodynamics

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