Bladder outflow obstruction (BOO) is present in a significant percentage of men with symptoms of lower urinary tract dysfunction. The ICS ‘Standardization of Terminology of Lower Urinary Tract Function: Pressure-Flow Studies of Voiding, Urethral Resistance, and Urethral Obstruction’, published in 1997 has introduced the pressure flow graph as the standard for the diagnosis and grading of outlet-; now ‘outflow’, -obstruction in men with prostate enlargement. The pressure flow (p/Q) graph in this manuscript shows the typical distension and collapse of the bladder outlet during voiding on an X-Y; pressure -flow graph. This graph is largely based on Griffiths distensible-collapsible tube –flow controlling zone -hydrodynamics-theory. Distension of the outlet to start flow in men, requires increase of detrusor pressure. Distension happens until maximum flowrate. After maximum flowrate (representing maximum distension) there is a continuation of flow and pressure in an (when idealized) equilibrium, until collapse of the outlet occurs when the detrusor pressure is lower than the outlet pressure. This typical pattern is closely associated with the prostate passive mass-volume effect on the outlet. Most of the parameters to grade BOO presented in the 1997 standardisation report are based on an average, ‘typical’ relation of distension and collapse. 
At the time of the 1997 report there was only one surgical treatment option for BPH with BOO being TURP, although transurethral thermotherapy was evolving. TURP typically aims to remove the entire amount of prostate gland tissue. Nowadays alternative treatments to relieve outflow obstruction become available, sometimes with purposely incomplete removal of the prostate adenoma tissue e.g. to preserve (antegrade) ejaculation. It is very likely that the recognition of not typical pressure flow relations/curves is gaining importance and clinical relevance.
A presentation at an earlier ICS annual meeting has shown that e.g. prostate middle lobe dynamics can be diagnosed during voiding, based on a specific pressure flow (X-Y graph-) pattern. The pattern reflects that not only mass effect is relevant in the obstructed micturition but that also prostate dynamics, specifically middle lobe kinking–in during the voiding is relevant in a proportion of men. This type of p/Q curve was recognized also in the 90s, however not linked to prostate middle lobe dynamics. There above, it is also recognized –in earlier publications- that some men have constrictive type of BOO where especially the outlet distension is limited, with relatively lesser effect on collapsibility. This type of BOO is quantifiable on the basis of the steepness of the p/Qcurve.
We summarize that, in theory, 3 subtypes of BOO can exist in symptomatic men with prostate enlargement; the classical compressive type; the constrictive type and the middle lobe type. Especially the middle lobe type may become of relevance when middle lope sparing interventions are selected, in cases where precisely the middle lobe is the predominant cause of voiding dysfunction. We present the prevalence of the various subtypes, because some of the nowadays evolving treatments may turn out to be more (or less) effective for one of the BOO subtypes.

We selected 876 measurements out of ±2500 consecutively referred male patients with LUTD aged 21-105 mean 66,5 years that underwent urodynamic testing and voided at least 100mL during pressure flow measurement in a representative manner. Excluded are measurements that showed underactive detrusor contraction since we considered that distension of the outlet is probably partially depending on the detrusor contraction or contractility. Although it is accepted that simple grading of BOO with the standard parameters is equally reliable in patients with detrusor underactivity, contractile force may be a confounder in the here presented detailed grading BOO, as distensibility may be underestimated in persons with weaker contraction.
P/Q analysis results were categorized according to steepness of the curve that was fitted to the lower pressure border of the curve; more steepness is representing a more constrictive type of obstruction. Furthermore the compressive type of curves (with a relatively good distension) were subdivided in standard compressive and prostate middle lobe dynamic compressive. 
A curve steeper than 0,5 cmH2O /(ml/s)2 was considered constrictive and a Pmuo (minimal pressure to maintain flowrate = curve foot point) was considered obstructive when >35cmH2O (which is the middle value of intermediate in the ICS pressure flow nomogram). A positive difference between the extrapolated standard graph's minimum pressure and the observed minimum voiding pressure was regarded as a sign of middle lobe dynamic compressive obstruction.

241 (27,5%) of the patients with normal detrusor voiding contraction (BCI 100-150; N 876)) had no BOO; 211(24,1%)  had intermediate/equivocal BOO and 424 (48,4%) had BOO, according to the ICS nomogram. Almost all patients without BOO or with intermediate BOO had a normal compressive type of pressure flow curve. In only 11 of these patients (2,4%), the curve was ‘steep’, according to the here above mentioned criterion, with a low foot-point. 
Of the patients with BOO 50,5% had a normal compressive type of obstruction and 49,5% has a constrictive type of obstruction, (with a steeper pressure flow curve). In 59% of men with a constrictive p/Q type the minimum voiding pressure was lower than the standard (compressive) curve predicted. This indicates that the distension is the flow limiting element, but also that a relatively low pressure (compared to the compressive type) may be sufficient to maintain flowrate. 
We observed that there is urodynamic evidence of prostate middle lobe kinking in 24,3% of the patients with a compressive type of obstruction. Deviant from the normal curve in these patients is that, as was reported earlier, a higher than standard minimum voiding pressure was observed (after the moment of maximum flowrate).

In the analyzed cohort of consecutive men with normal detrusor contractility, 51,6% had no BOO. The remaining 424 patients, with BOO, are subcategorized with precise pressure flow pattern analysis. The prevalence of two types of BOO and two subtypes in one of the type-categories, diagnosed on the basis of currently existing criteria, is reported. Almost 50% of these men had a constrictive  type of obstruction, where the flowrate is relatively low compared to the minimum required voiding pressure. We speculate that prostate volume mass effect is of lesser relevance here and that (any alternative) intervention without large prostate volume reduction may potentially be as effective as complete resection. Within the group of patients with a compressive type of obstruction ±25% had urodynamic evidence of prostate middle lobe kinking-in dynamics. It is known that these patients symptoms are relatively resistant to alpha blocking medication. We speculate that these patients will also do less good with management that preserves especially the middle-lobe prostate portion.

Technical evolutions allow alternatives to TURP in men with prostate enlargement and BOO. Not all of these alternatives aim at removal of the complete adenoma. Urodynamic staging and grading of dysfunction in men with signs and symptoms, not only leads to diagnosis of detrusor overactivity, detrusor underactivity and or absence of bladder outflow obstruction but diagnosis of outflow obstruction sub-type, when bladder outflow obstruction is (one of the) cause(s) of the symptoms, may be of help to reduce harm and or unnecessary interventions and be valuable to improve outcome of management. We have reported the prevalence of 3 types of bladder outlet obstruction in a large cohort of men with normal bladder contractility.