Date Published: September 18, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Andrew J. Macnab, Lynn S. Stothers, Babak Shadgan.
The current literature indicates that lower urinary tract symptoms (LUTSs) related to benign prostatic hyperplasia (BPH) have a heterogeneous pathophysiology. Pressure flow studies (UDSs) remain the gold standard evaluation methodology for such patients. However, as the function of the detrusor muscle depends on its vasculature and perfusion, the underlying causes of LUTS likely include abnormalities of detrusor oxygenation and hemodynamics, and available treatment options include agents thought to act on the detrusor smooth muscle and/or vasculature. Hence, near infrared spectroscopy (NIRS), an established optical methodology for monitoring changes in tissue oxygenation and hemodynamics, has relevance as a means of expanding knowledge related to the pathophysiology of BPH and potential treatment options. This methodological report describes how to conduct simultaneous NIRS monitoring of detrusor oxygenation and hemodynamics during UDS, outlines the clinical implications and practical applications of NIRS, explains the principles of physiologic interpretation of NIRS voiding data, and proposes an exploratory hypothesis that the pathophysiological causes underlying LUTS include detrusor dysfunction due to an abnormal hemodynamic response or the onset of oxygen debt during voiding.
Near infrared spectroscopy (NIRS) is an established noninvasive technique for monitoring changes in tissue oxygenation and hemodynamics in real time [1–4]. Simultaneous monitoring with pressure flow studies is recognized to add physiologic data of relevance in the evaluation of voiding dysfunction [5, 6]. A series of NIRS monitoring studies now suggest that during voiding changes can be detected in the detrusor microcirculation, which imply that abnormalities in hemodynamics or oxygen supply and demand occur in association with symptoms generated in several different situations, where there is voiding dysfunction [6–14]. These include bladder outlet obstruction (BOO) in males, nonneurogenic lower urinary tract dysfunction (NLUTD) in children, detrusor over activity (DO) in patients with neurogenic bladders due to spinal cord injury, and DO and over active bladder (OAB) in adult women. However, “More studies are needed to further define and validate uses for NIRS in urology” [5, 6]. Hence, this methodology report describes how urologists can conduct simultaneous NIRS monitoring of detrusor oxygenation and hemodynamics during UDS and summarizes the principles underlying physiologic interpretation of NIRS voiding data. Based on the published studies cited and literature referenced, the clinical implications of applying NIRS to study voiding dysfunction are outlined, the limitations of the technique discussed, and an exploratory hypothesis proposed that LUTS can result when an abnormal hemodynamic response or the onset of oxygen debt occurs in the detrusor during voiding.
The changes in NIRS parameters most relevant physiologically are as follows.The trend in total hemoglobin concentration (tHb), from which hemodynamic variations can be inferred. A positive or negative trend reflecting an increase or decrease in blood volume, respectively.Changes in the concentration of O2Hb and HHb, which add information to any variation in blood volume, and in the presence of hemodynamic stability, allow variations in oxygen supply and demand to be inferred. Stable or increasing O2Hb associated with comparable changes in HHb implies a balance in oxygen supply and demand, whereas an increase in HHb with no rise in O2Hb reflects an imbalance and when associated with a simultaneous fall in O2Hb indicates the onset of oxygen debt.
The reproducibility of data from NIRS bladder studies is an issue that needs to be addressed in future studies. It is important to recognize what the technique does and does not measure and that each void in each patient has unique elements, which complicates even intrapatient correlation. However, characteristic patterns of change and/or positive or negative trends in O2Hb and HHb, and tHb associated with specific voiding events, have been identified in cohorts of patients with conditions such as BOO, DO, OAB, and NLUTD. And good statistical correlations between NIRS data and parameters identified via UDS (diagnosis of BOO, OAB, and DO) have been reported [7, 11, 13, 14, 33].
NIRS monitoring can be done simultaneously with UDS pressure flow studies. The noninvasive optical methodology described for studying the detrusor is based on established principles and a body of research in other organs and tissues, as is the methodology explained for interpreting the potential physiologic relevance of NIRS data. Simultaneous NIRS and UDS studies done to date, additional urologic research using wireless NIRS [9–11, 24], and the body of NIRS studies in muscle [2–4, 23, 25–28] offer urologists an opportunity to evaluate where abnormal detrusor hemodynamics or oxygen supply and demand underlie voiding dysfunction due to BPH. This novel information has the potential to add to greater understanding of the pathophysiology underlying male LUTS because this is a condition which is recognized to be a heterogeneous clinical syndrome that probably has multiple causes of which BOO is only one [15, 16].