Transabdominal near infrared spectroscopy (NIRS) analysis of changes in concentration of oxygenated and deoxygenated hemoglobin during bladder voiding is an effective non-invasive screening tool for bladder outlet obstruction [1]. Bladder wall micromotion due to detrusor smooth muscle contractions during bladder filling are elevated in patients with overactive bladder [2]. We propose that NIRS may provide valuable information about bladder filling, including potential noninvasive, ambulatory, at-home estimation of bladder wall micromotion for overactive bladder assessment.  But, unlike NIRS assessment of skeletal muscle activity during exercise or brain activity, the distance from the NIRS probe to the target tissue in bladder applications could vary significantly with body mass index (BMI), bladder volume, and patient body position. Using NIRS to analyze bladder function requires an optically appropriate distance between the skin at the probe location and the anterior bladder wall. The objective of the present study was to determine if variations in the distance between the skin and the anterior bladder wall as a function of BMI, bladder volume or body position preclude use of bladder filling phase NIRS diagnostics for overactive bladder.

Eighteen women participated in this prospective bladder geometry study and completed a multiple-fill urodynamics protocol implemented using a Laborie Aquarius TT urodynamics system. An initial fill and void cycle was performed to determine cystometric capacity (CCap). During a subsequent fill, bladder ultrasound images were recorded at volumes of 40% CCap and 100% CCap using a GE Voluson E8 ultrasound system with a 3D convex 4–8.5 MHz transducer. All images were obtained by a trained ultrasound technologist. During a another fill, images were recorded at a volume of 40% CCap while participants were in four distinct body positions: supine, sitting at 45°, sitting upright at 90°, and standing. While in the supine and sitting positions, participants were positioned using a Sonesta electronically adjustable urodynamics chair/table, and the chair angle was confirmed for each body position using a protractor. Bladder images were analyzed using GE 4D View software as shown in Figure 1A. For each image, the depth from the ultrasound probe to the anterior bladder wall was measured in the transverse plane at the mid-line of the bladder (Figure 1A).

Women were divided into equal groups with relatively low and high BMI (26.6±1.8 kg/m^2 and 40.0±2.3 kg/m^2, respectively, median = 32.7 kg/m^2). The ages of the low and high BMI groups were not statistically different (41.2±6.3 years and 51.6±3.4 years, respectively). The depth from the ultrasound probe to the anterior bladder wall decreased significantly with increased bladder volume (Figure 1B, n=18, paired t-test, p<0.05), and the depth from the probe to the bladder wall was significantly greater in participants with higher BMI (Figure 1C, n=9 for each group, t-test, p<0.05). Body position changes did not significantly affect the depth from the probe to the bladder wall at a volume of 40% CCap (Figure 1D, n=18, paired t-test, p>0.05). The minimum and maximum depths were 0.8 cm and 6.3 cm, respectively, and only two participants with high BMIs (40 kg/m^2 and 54 kg/m^2) had a depth of greater than 4.5 cm for an image.

NIRS has been used to measure tissue oxygenation at depths of at least 4 cm [3]. The results of the present anterior bladder wall depth study indicate that application of NIRS for assessment of bladder filling should be feasible in most women, but could be limited in women with very high BMI.

The present data support the conclusion that variation in bladder wall depth due to changes in bladder volume and changes in body position will not prevent the application of NIRS for ambulatory assessment of bladder filling.

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