Is There Any Role of Bladder Outlet Obstruction Index and Bladder Contractility Index in women?

In a patient with voiding dysfunction, regardless of any age or sex, it is important to ascertain contribution of detrusor underactivity (DUA) and/or bladder outlet obstruction (BOO) towards the symptomatology (yes, both can and often do coexist). As easy as it may sound, diagnosis of DUA, and sometimes BOO, may be challenging and has intrigued researchers for long time.

Landmark research-work by Werner Schaefer, Paul Abrams, Abraham Griffith and others led to development of the BOO index (Abrams-Griffith number) defined as PdetQmax – (2XQmax) and the Bladder Contractility Index (BCI) PdetQmax + (5XQmax) for numerical quantification of BOO and DUA, respectively. Moreover, these researchers also constructed phenomenal pressure flow nomograms based on these values depicting the status of detrusor contractility as well as BOO on the same graph (the ICS nomogram, the Schaefer nomogram). It is important to emphasize that the research work was mostly done enrolling men over 50 years of age with clinical diagnosis of benign prostatic enlargement. This caveat has been reiterated in later studies while applying these equations in other patient-groups. Nevertheless, these have proved to be useful in other VD groups among men.

The real difference between basic physics and physiology of lower urinary tract function between men and women is that of urethral resistance, which is substantially lower in women due to shorter length. In urodynamic terms, for the same Qmax (at a given volume), PdetQmax would be substantially lower in a woman than male counterpart. In fact, some women may be able to void simply by urethral relaxation with minimal Pdet during voiding. In absence of comparative data on isovolumetric Pdet, it is not fair to assume that female bladders are weaker. It is ironic that despite similar basic physics and physiology, the concepts of BOOI and BCI have very rarely been examined by researchers in women (Gammie et al 2015). The most important work on BOOI in women was published recently by Solomon et al. (2018). They found that BOOI for women was optimized to PdetQmax – (2.2xQmax). Well, it couldn’t be closer to the definition of AG number for men; however, the cutoffs for obstruction were 20 points lower (men - >40 BOO, <20 no BOO. Women - >20 BOO, <0 – no BOO). Similarly, there has been virtually no research on quantification of DUA in women. The only available study was conducted on older women; the researchers found that the BCI of men (PdetQmax+5xQmax) would overestimate the bladder contractility (isovolumetric Pdet) by a large margin compared to measured value and they opined a better index would be (PdetQmax+Qmax) (Tan et al 2002). I believe this group of women were likely to have suboptimal detrusor function anyway.

To conclude, BOOI (the AG number) with lower cutoffs (by 20) is an important parameter to quantify BOO in women. As far as detrusor contractility is concerned, there is a void and there is a critical need to research the utility of the current definition of BCI (meant for men) in women.

References:

Abrams P. Bladder outlet obstruction index, bladder contractility index and bladder voiding efficiency: three simple indices to define bladder voiding function. BJU Int. 1999;84(1):14–5.

Gammie A, Kirschner-Hermanns R, Rademakers K. Evaluation of obstructed voiding in the female: How close are we to a definition? Current Opinion in Urology.  2015;25(4):292–295.

Lim CS, Abrams P. The Abrams-Griffiths nomogram. World Journal of Urology. 1995;13:34-39.

Schaefer W. Analysis of bladder-outlet function with the linearized passive urethral resistance relation, IinPURR, and a disease-specific approach for grading obstruction: from complex to simple. World Journal of Urology. 1995; 13:47-58.

Solomon E, Yasmin H, Duffy M, Rashid T, Akinluyi E, Greenwell TJ. Developing and validating a new nomogram for diagnosing bladder outlet obstruction in women. Neurourol Urodyn. 2018 Jan 1;37(1):368–78.

Tan TL, Bergmann M, Griffiths D, Resnick N. Detrusor contractility: order out of chaos. Neurourology and Urodynamics. 2002 April 1;21(4):44.

What cutoff of BOOI would be appropriate for diagnosis of bladder outlet obstruction in women?

As per above discussion, the Solomon-Greenwell formula (which is very close to Abrams-Griffith’s number) seems to be appropriate for calculation of BOOI and a value >20 is diagnostic of BOO. One caveat must be considered in this context – these nomograms and formulae were constructed with patients of obstruction with preserved bladder function. In women with DV/PRES, due to altered bladder-to-urethra and urethra-to-bladder reflexes bladder function is often compromised during voiding. Therefore, in such women, and those with detrusor underactivity of another etiology, BOOI may be lower than 20.

What cutoff of BCI would be appropriate for diagnosis of detrusor underactivity in women?

with the absence of any widely accepted / researched parameter for this index, the diagnosis of DUA in women remains more descriptive than objective (ICS definition - ‘a voiding contraction of reduced strength and/or duration, leading to prolonged or incomplete bladder emptying’). Clinicians and researchers are encouraged to examine the current definition prevalent for male population in their female patients with voiding dysfunction/ urinary incontinence undergoing urodynamics to be able to come to a meaningful conclusion.