Fig. 36.1
The inner and outer layers of the bladder wall appear hyperechoic (white) and represent the mucosa with submucosa and adventitia, respectively. The detrusor seems hypoechoic (black) and is “sandwiched” between the hyperechoic lines of the adventitia and mucosa. BWT assessment consists in the measurement of all three bladder layers; DWT includes only detrusor
The optimal solution for the correct measurement is placing the probe’s beam perpendicular to the bladder wall, so that both the adventitia and the mucosa appear as straight and parallel lines. Video 36.1 shows the measurement technique of DWT proposed by Oelke et al.
A critical element to consider when making a measurement is that the BWT and the DWT depend on bladder filling in the range between 50 and 250 ml. Khullar et al. firstly showed no significant differences of BWT in almost empty bladder and those filled up to 50 ml [14]. Additionally, Oelke found that in healthy adult volunteers of both sexes, the DWT decreases rapidly between 50 and 250 ml of bladder filling (or up to 50 % of the capacity of the bladder), but then reaches a plateau with only small and insignificant differences between 250 ml and the maximum bladder capacity [15] (Fig. 36.2).
Fig. 36.2
Correlation between DWT and bladder filling volume in three unobstructed male volunteers. The DWT decreases rapidly up to a bladder filling of 250 ml and then reaches a plateau (Adapted from Oelke et al. [15])
Therefore, to avoid possible bias related to bladder filling, it is important to follow some recommendations: perform the assessment at the same volume, by inserting a catheter and filling the bladder until a preset value of 150 ml in the technique proposed by an Italian group coordinated by Prof. Tubaro [12] (Video 36.2), or use an empty bladder as proposed in female patients by Khullar V or use at least 250 ml of bladder volume (Oelke technique). The latter option could be the more advantageous because it is less invasive than that proposed by Tubaro et al. and easier to perform. Notwithstanding all these techniques, bladder volume is still a controversial issue in BWT/DWT assessment as detrusor thickness changes according to a different bladder volume (Video 36.3).
Kojima proposed an alternative approach to definitively overcome the influence of bladder filling [11]. The Japanese group adopted the UEBW as a parameter to evaluate detrusor thickness instead of BWT/DWT. They considered the bladder as a ball, and the volume of the bladder wall was calculated by subtracting the volume of intravesical bladder to total volume. The UEBW was obtained by multiplying this parameter with the specific weight (Fig. 36.3). However, the technique presents the disadvantage that small errors of the volume evaluation (since based on measurements in the third potency) have a great impact on UEBW.
Another important issue in BWT/DWT assessment is in which bladder site to perform the measurements. Several evidences showed that all parts of the bladder (dome, anterior, posterior or lateral walls) presented the same thickness in the same patient at the same degree of bladder filling; therefore, any portion of the bladder can be used for the assessment of the thickness [13]. However, some authors recommend performing the measurement at the level of the anterior wall. In fact, using ultrasound probes with high frequency and with high magnification, the anterior wall is the portion of the bladder better accessible. Finally, it is recommended not to be limited to a single measurement but performing at least three and calculating the BWT or DWT as the average of three measurements [13].
Applying all these recommendations, in selected centres, achieved excellent results in terms of reproducibility, since intraexaminer variability was less than 5 % and interexaminer ranged between 4 and 12 % [16, 17].
36.4 Bladder Wall Thickness (BWT)/Detrusor Wall Thickness (DWT) and Bladder Outlet Obstruction (BOO)
First in 1996, the Japanese group led by Kojima proposed a relationship between UEBW and BOO [7] assessed by pressure/flow study (PFS). They found that the mean UEBW was significantly higher in obstructed patients (mean UEBW 46.2 g) than in healthy ones (mean UEBW 29.3 g). Choosing a threshold of more than 35 g for UEBW to predict BOO, ROC analysis showed 87.9 % of diagnostic accuracy [11]. However, the study was conducted only on Asian patients; therefore, it is questionable that a threshold value of 35 g is applicable in other populations. Furthermore, all subsequent studies about UEBW were always conducted in Asian populations.
The first experience regarding the BWT assessment as tool to predict BOO in patients with prostatic hypertrophy dates back to 1998. Manieri et al. showed that obstructed patients presented a significantly higher BWT than those without BOO (mean BWT 5.0 versus 3.6 mm) and the degree of obstruction was directly correlated with the BWT [12]. A threshold value of 5 mm for BWT seemed the most accurate cut-off to distinguish between patients with or without BOO: patients with BWT less than 5 mm were not obstructed (Schaefer class < 2 at PFS) in 63 % of cases, while 88 % of those with BWT of 5 mm or more presented BOO (Schaefer class ≥ 3 at PFS). The BWT showed a diagnostic accuracy of 0.8608 at ROC analysis [12].
Afterwards, Oelke et al. successfully evaluated the correlation between BOO and DWT, assessed according to their technique. Using a threshold value of 2 mm, they correctly classified 95 % of men with BOO [18]. The same technique was adopted by Kessler et al., who measured the DWT in 102 patients with clinical benign prostatic hyperplasia (BPH). Using the threshold values of 2.0, 2.5 or 2.9 mm, they correctly identified 81, 89 and 100 % of patients with BOO, respectively [17]. In a further experience, Oelke et al. evaluated the DWT in 70 patients with lower urinary tract symptoms. The values of DWT in non-obstructed, equivocal, and obstructed patients, thus classified according to PFS results, were 1.33 mm (95 % CI, 1.17–1.48), 1.62 mm (95 % CI, 1.48–1.76) and 2.4 mm (95 % CI, 2.12–2.68), respectively. Moreover, they found a statistically significant difference of DWT (p < 0.001) between obstructed and non-obstructed patients and between patients with equivocal PFS for obstruction and obstructed patients. No significant differences were observed between non-obstructed and equivocal patients (p = 0.349) [19]. As described in their previous study, a threshold of 2 mm was the best cut-off for discriminating patients with or without BOO with the highest specificity (97.3 %) and positive predictive value (95.5 %). Despite these experiences seem to confirm a positive correlation between BWT and BOO, there are some controversial studies showing no associations. Blatt et al. found no significant differences of BWT in patients with BOO (n = 39) compared to those with normal PFS (n = 69) [20]. However, inexperience with the measurement technique, the inclusion in the study of patients of both sexes and with mainly low-grade BOO, inappropriate placement of markers and assessing the concomitant treatment with α-blockers may have led to these controversial results. Nevertheless, recently Franco et al. confirmed the effectiveness of BWT in predicting a condition of BOO. They included in the study one hundred male patients with LUTS. BWT was assessed according to Tubaro technique, and a threshold value of 6 mm was chosen to predict BOO. BWT demonstrated a diagnostic accuracy evaluated by ROC analysis of 0.845 (95 % CI 0.78–0.91), even better than other non-invasive methods of evaluation of BOO: maximum flow rate (Qmax) 0.779, the post-void residual (0.699) and prostate volume (0.626) [21]. Moreover, in the same study, Franco et al. evaluated the intravesical prostatic protrusion (IPP), another ultrasound parameter recently proposed as non-invasive technique for the assessment of BOO. The addition of IPP to BWT has determined the best diagnostic accuracy, reaching 87 % [21].
Table 36.1 lists the results obtained in terms of diagnostic accuracy in the various experiences mentioned above.
Table 36.1
Measurement techniques of BWT/DWT and outcomes
Studies | Bladder filling at measurement | Ultrasound approach (MHz) | Evaluated parameter | Threshold value | Outcome (AUC) |
|---|---|---|---|---|---|
Male patients with BOO | |||||
Manieri et al. [12] | 150 ml | TAUS (3.5) | BWT | BOO if BWT >5 mm | 0.860 (95 % CI not available) |
Oelke et al. [18] | Maximum bladder capacity | TAUS (7.5) | DWT | BOO if DWT >2 mm | 0.955 (95 % CI not available) |
Kessler et al. [17] | Maximum bladder capacity | TAUS (7.5) | DWT | BOO if DWT >2.9 mm | 0.88 (95 % CI 0.81–0.94) |
Oelke et al. [19] | ≥250 ml | TAUS (7.5) | DWT | BOO if DWT >2 mm | 0.930 (95 % CI 0.88–0.98) |
Kojima et al. [11] | 100–300 ml | TAUS (7.5) | UEBW | BOO if UEBW >35 g | 0.862 (95 % CI not available) |
Blatt et al. [20] | 200 ml | TAUS (10–5) | BWT | Not calculable | Not significant |
Franco et al. [21] | 150 ml | TAUS (3.5) | BWT | BOO if BWT >6 mm | 0.78 (95 % CI 078–0.91) |
Female patients with DO | |||||
Khullar et al. [25] | <50 ml | TVUS (5) | BWT | DO if BWT >5 mm | 0.940 (95 % CI not available) |
Lekskulchai et al. [27] | <50 ml | TLUS (8–4) | BWT | DO if BWT >5 mm | 0.606 (95 % CI not available) |
Chung et al. [27] | ≥250 ml | TAUS (8) | DWT
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