Evaluation of MVV is an important parameter in monitoring the diagnosis and treatment of children with LUTD [4]. There is no definite consensus on how to measure MVV [12, 13, 16, 17]. An accurate estimation of the MVV is important for several reasons. First, theoretically, if MVV is increased for any given condition, the number of voids can be reduced, provided that the 24-h VV remains similar. Second, changes in MVV provide an outcome measure by which treatment success or failure is assessed [18]. Third, the relationship between symptom severity, MVV, and BC provides a measure for understanding the pathophysiology [19]. In addition, an increase in AVV was evaluated as the primary outcome measure in the follow-up of therapy in patients with OAB. Assuming similar amounts of daily urine output, an increase in AVV reflects a decrease in frequency and an increase in habitual VV [20]. Finally, MVV and AVV provide useful information for developing diagnostic and therapeutic avenues in future research [21].
ICCS recommends keeping a minimum of a 48-h VD for adequate assessment of MVV and AVV. This time-consuming process requires motivation from the family and the child. UF is a reliable, simple, and noninvasive test. According to the ICCS, UF requires repetition for correct interpretation and reliability, as well as adequate VV (between 50–115% of EBC) [4]. It is well documented that uroflow is highly dependent on bladder volume (the larger the bladder volume, the greater the flow). For this reason, patients are often instructed to wait until the bladder is full before performing UF [22]. When the usual voiding pattern is achieved, the VV in UF is expected to be similar to that recorded in the VD. The concordance between VV measured in VD and UF is one of the parameters that show the physiological state of voiding. The greater the difference between these VV, the less physiological the urine flow [21, 23].
According to our study, although mean UF-MVV and UF-AVV were greater than VD-MVV and VD-MVV, VD-MVV and VD-AVV were greater in 42.1% and 23.1% of patients, respectively. Additionally, when calculating the difference between the two measurement tools as a percentage of the larger value, we found a discrepancy of the threshold difference of more than 30% (MVV/EBC) in 94 of 242 patients in the sample. This indicates that an MVV estimate can be inaccurate when only one measurement tool is used. MVV was comparable (threshold difference 0–10% (MVV/EBC)) in 62 of 242 patients between the two methods. When we examined the AVV threshold difference in UF and VD, we found a threshold difference discrepancy of more than 30% (AVV/EBC) in 83 of 242 patients in the sample, while AVV was comparable (threshold difference 0–10% (AVV/EBC)) between the two methods in 70 of 242 patients. These findings are important because the assessment of the patient's MVV and AVV through the use of a single VD or UF may be incomplete.
MVV is a representative parameter of maximum BC, rather than the usual and habitual physiological VV. Indeed, voiding may occur without or without a strong urge to void or large bladder filling volume, and voiding is often associated with other normal daily activities [24]. MVV may differ in UF compared to VD depending on environmental and psychological factors. While performing UF, the patient may experience an early or late urge to void due to the requirement of adequate VV for evaluation of UF or to avoid an incontinence episode [7,8,9]. These factors may limit the usefulness of UF alone for measuring MVV [4, 25, 26]. Rychik et al. showed that the mean VD-MVV (340.46 ± 147.83 ml) was higher than the UF- MVV (216.58 ± 152.11 ml) in patients with LUTS (p < 0,001) [27]. Contrary to this study, Ertberg et al.'s study of women with and without detrusor overactivity found no significant difference between MVV measured by VD and UF [17]. In a study comparing cystometry, UF, and VD in children with OAB, Uluocak et al. found MVV significantly higher in the VD and reported that VD was more reliable in predicting MVV due to its physiological structure and ease of application in all age groups [28]. Maternik et al. compared UF and VD in patients with LUTD. They showed no significant difference was observed between MVV in UF and VD in children with LUTD. This study showed that UF can be used as an alternative when diaries are unreliable or unavailable [29]. Byeong et al. compared MVV in UF with MVV in VD in patients with enuresis nocturna; MVV in UF was measured as 127.3 ± 60.9 ml and 152.6 ± 74.6 ml in VD. The mean MVV/EBC ratios did not differ significantly between the two measurement methods (55.6% vs 48.9%; p > 0.05) [30]. In another study comparing UF-VV and VD-VV in adults with LUTS, mean VV was 256.27 ± 122.95.ml and 217.66 ± 81.05 ml, respectively. UF-mean VV was significantly higher than VD-mean VV (P < 0.01) [15].
Maternik et al. reported no significant difference between the MVV obtained from the VD and UF in patients with several LUTD (MNE, OAB, and DV) [29]. Here, in our present study, the difference in MVV between the two methods was higher in patients with OAB and DV than in those with MNE. No significant difference was measured between the MVV obtained from VD and UF only in patients with MNE. This result suggests that UF can be used as an alternative to predict MVV in children with MNE for whom VD cannot be performed. This finding loses its clinical significance, as UF is not indicated in the investigation of patients with MNE. For the estimation of MVV in patients with OAB and DV, UF should be taken into account when MVV from UF is much higher than from VD. VD can be useful to identify how much UF reflects natural voiding. Maternik et al. also evaluated the effectiveness of the two methods in different age groups and genders. They did not observe differences in mean MVV between ≥ 10 years old and < 10 years old, or between genders [29]. In our study, we found no significant difference between the MVV obtained from VD and UF only in girls and patients younger than 12 years old. The more hesitation and embarrassment that boys and patients ≥ 12 years old may feel when asked to void in UF than other children may be a possible explanation for this condition.
Since the AVV more reflects the habitual VV, it was used in conjunction with the MVV for follow-up in studies of OAB [20]. To our knowledge, only MVV was evaluated when comparing UF and VD in the literature. In this study, in addition to the MVV obtained in UF and VD, AVV was also compared within itself. There was a significant difference in the mean MVV and mean AVV values obtained from the two measurement methods within all patients. We also measured significant differences between the AVV from VD and UF in different ages, sex, and LUTD subgroups. When we examined the MVV and AVV reported in the VD, no significant difference was found between VV in age and sex subgroups. On the other hand, MVV and AVV in UF were similar only in the sex subgroups. These results show the importance of evaluating the MVV and AVV in VD and UF together when evaluating patients with LUTD.
Limitations of this study were the lack of an objective measure of accuracy for 48-h VD measurements that were done at home and the exclusion of PVR in VD and UF evaluations. The further limitations of the study were the uncontrollable stress states that can alter BC in VD and UF and the relatively small number of children with MNE. The results indicating the value of performing both assessments in this subgroup will need to be verified by studies of larger cohorts.
