Hypothesis / aims of study
Since many years mid-urethral sling (MUS) placement has become the treatment of choice for stress urinary incontinence (SUI). Following a MUS placement, urethral distortion and kinking have been observed, resulting potentially in a reduced effective cross-section of the urethra which can induce voiding dysfunction and a state of chronic outlet obstruction; that situation is not an uncommon complication. We used the VBN mathematical model [1] to develop an algorithm based on a non-invasive flow (NIF) which could be used post-MUS to gauge the risk of such an obstructive situation developing over time.
Study design, materials and methods
In the VBN micturition model, the detrusor contractility is characterized by the parameter k and the urethral obstruction by the parameter U. The standard model (flow-controlling zone at meatus) was modified to simulate a narrowing due to a urethral compression at 1.5 cm of the bladder neck. Data derived from NIF are: filling volume (Vini) = voided volume + post void residual (PVR, obtained by bladder scan) and maximum flow (Qmax). In the absence of intubated pressure-flow recordings, values of k and U cannot be precisely evaluated. But one infinity of couples [k,U] can restore Qmax which value is a compromise between k and U; a given Qmax needs high values of k and U or low values of the parameters. To restore PVR, introduction of a fading of detrusor excitation is needed. In previous study we have shown that,in most voidings, such condition occurs after the Qmax [2]. We have to identify the value kmin of k giving the lower possible value of U which is 0; kmin will be the characteristic identifier for each woman. Therefore, an increase of kmin would imply an increase of U, thus suggesting an element of sling obstruction.
Interpretation of results
Today, evaluation and diagnosis of bladder outlet obstruction rely upon flow test and PVR without any quantification. In addition one can observe variable presentation: urgency, straining to void, weak urinary stream, dysuria, urinary tract infection and so on. Invasive investigation is frequently proposed. Our method has the great merit to be non-invasive. Computations have been tested on a small sample, unfortunately non significant. We had been able to evaluate k and kmin for each woman without encountering any unexpected difficulty. This would mean that the theory is reliable. But this conclusion would be credible only if the method will be applied to a large population.