The Schäfer linPURR nomogram is one of the methods to diagnose and grade bladder outflow obstruction (BOO), based on a pressure flow study (PFS), part of the urodynamic test. linPURR was originally proposed as a two-parameter classification system[1]. The maximum uroflow rate (Qmax) with the corresponding detrusor pressure (pdetQmax) and, the minimal urethral opening pressure (pmuo) were projected in the nomogram to grade BOO in men with -symptomatic- prostatic enlargement. A distinction was established between two types of BOO: Compressive BOO with both pdetQmax and pmuo above ‘normal’ was considered typical for BPH, while constrictive BOO was atypical, with a relatively low pmuo (see Fig 1). Based on Griffith’s distensible collapsible tube paradigm, this constrictive type represents a limitation of distension of the bladder outflow tract. Although differentiation between compressive and constrictive BOO is used in a later developed CHESS classification and the benefit of this differentiation is shown in a small study, correlations between clinical data and the type of obstruction are never studied. The aim of this study is to explore differences in age, prostate size, and free uroflowmetry with PVR between patients with a constrictive versus a compressive BOO.

PFS of ICS standard uroflowmetry test, performed with a water filled pressure system and corrected for flow and pressure peak artifacts, of 698 symptomatic, referred men with BOO (linPURR grade ≥ III or BOOI >40) without earlier LUT surgery, that underwent UDS with PFS  between 2003 and 2020, were included. PFS with 2<Qmax<35, Voided volume <100 ml were excluded. We determined compressive BOO when the actual pmuo, (PmuoAct) was within limits of +or- 20cm H2O from the pmuo predicted with linPURR (PmuolinPURR). PmuoAct is the average pdet associated with the flowrate between 1 and 0.5 ml/s at the end of voiding. We considered that this manner to determine PmuoAct was the best to reduce the pressure (with flow delay) -artifacts associated with assessment of pmuo. A PFS was considered constrictive if PmuoAct was more than 20 cmH2O lower than PmuolinPURR. This represents a difference, larger than the approximate width of one linPURR obstruction class. Differences of age, PVR, percentage voided, and prostate size, between the two types of BOO were investigated. In addition, differences between Qmax, PVR and void% between PFS and the free flow (FF) measurement were compared for the two obstruction types. Finally a sub-analysis was performed including only PFS with a bladder contractility index (BCI) >100, a normal detrusor voiding contraction strength.

510 PFS (73%) were found of the compressive type, while 154 PFS (22%) were found constrictive. 34 PFS (6%) had a PmuoAct which was >20 cmH2O higher than PmuolinPURR, and were excluded from further analysis, see Fig 2. Significant differences between constrictive and compressive were found for PVR and void%, with on average a larger residue and a smaller percentage voided for the constrictive obstruction. No significant differences were found for age and the difference between the free flow and the PFS (Mann-Whitney U-test p>0.05), although a non-significant difference in prostate size between compressive BOO and constrictive BOO was observed. There was no significant difference observed for the percentage prostate size <30 cm3.

For the subanalysis, 480 PFS (69%) met the criteria of BCI > 100. 355 PFS (74%) were found compressive, while 125 PFS (26%) were found constrictive. A difference with a higher significance (see table 1) compared to the whole group was found for PVR and void%. In addition, a significant difference in prostate size was observed, with the prostate size of patients with a compressive BOO (57 cm3) being significantly larger than those with a constrictive BOO (46 cm3). No significant differences were observed for age and the difference between the free flow and PFS. There was a significantly larger proportion of patients with a small prostate <30 cm3 within the constrictive BOO-cohort (35%) than within the compressive BOO-cohort (18%).

The mean prostate size was smaller for constrictive BOO; this may lead to the speculation that other factors than prostate size only, e.g., prostate structure, have a role in the pathophysiology of BOO in these patients. This can also lead to the speculation that a proportion of the currently used invasive treatment options could be less effective (or necessary) for the constrictive type, as those are mainly focused on the reduction of the prostatic size. In addition, the voiding efficiency was significantly lower in patients with a constrictive BOO, as a significant difference in PVR and void% was found. This difference was even larger in the sub-group of patients with a normal detrusor voiding contraction strength. A difference in treatment efficiency of men with BPH with a constrictive or compressive BOO was earlier observed when patients were analyzed according to their CHESS class.

There are significant differences in voiding efficiency and prostate size between compressive and constrictive BOO that may have relevance for individualized selection of management.

Schäfer, W.  The Contribution of the Bladder Outlet to the Relation Between Pressure and Flow Rate During Micturition. In: Hinman, F., Boyarsky, S. (eds) Benign Prostatic Hypertrophy. Springer, New York, NY. 1983; https://doi.org/10.1007/978-1-4612-5476-8_44.

Continence 7S1 (2023) 100871
DOI: 10.1016/j.cont.2023.100871