Effects of confounding variables on outcomes of incontinence surgery must be considered

EDITOR—Black et al's study highlighted difficulties that those who assess the treatment of urinary incontinence may encounter.1 The different surgical procedures were considered together, which makes it impossible to interpret cure rates or postoperative morbidity. Previous continence surgery was not reported, although this affects the results of surgery.

Black et al state that calculations of sample size were inappropriate as the objective was to provide preliminary estimates of variables and to generate hypotheses. They found no significant difference in outcome between women who had preoperative urodynamic testing and women who did not. On the basis of the objective cure rates reported in the literature, 2 3 however, two groups of 220 women would be needed to show that urodynamics does not make a difference to the outcome of surgery, assuming a power of 0.90 and a significance level of 0.05.4 Black et al's study is too small to show this and does not mention the type of testing or the reasons why some women had tests and others did not. Those who did may have had previous continence surgery, which is known to reduce the cure rate.

The statement that urgency and urge incontinence should not be considered contraindications to surgery has no basis from this study. Unfortunately, 93% of women with detrusor instability have these symptoms, and they would not benefit from a continence procedure. The women who were not operated on after urodynamics have not been mentioned.

In Black et al's study only 28% of women were found to be continent one year after surgery. This contrasts with a symptomatic follow up study carried out at our institution on women who had undergone preoperative urodynamics and then had continence surgery performed by us or the original referrer. We assessed 201 women with a postal validated disease …