Hypothesis / aims of study
An increased post-void residual (PVR) is associated with lower urinary tract symptoms, although no general agreement exists on a cut-off value indicating from which point on an elevated PVR would contribute to urinary problems. Besides, there is an intra-individual variability in PVR measurement depending on the time of the day but also on differences in pre-void volumes.(1-2) Rather than absolute post-void residual volumes, the voided percentage (Void%) probably has more clinical relevance. Void% is defined by the ICS as the numerical description of voiding efficiency or the proportion of bladder content emptied. Although it correlates more strongly with Qmax than PVR, research on normative Void% in the context of initial assessment of urinary incontinence is currently lacking.(2)
In this study, the use of an extended frequency-volume chart with documentation of the PVR urine volume after each micturition over a period of 24 h was evaluated as method to describe the circadian rhythm of PVR and Void% in older women with urodynamic stress incontinence (USI) and/or detrusor overactivity (DO).
Study design, materials and methods
This single centre prospective observational study was conducted between December 2013 and December 2018 in a tertiary referral hospital. The study population consisted of a convenience sample of older female patients (age ≥ 65 years) who consulted for diagnostic work up of urinary incontinence. Exclusion criterion was a positive screening for dementia on the Mini-Cog and no diagnoses of USI and/or DO.
All measurements recorded on the frequency-volume urine charts with PVR (FVCPVR) were taken by nurses of the inpatient urology department for 24 h: voided volume (VV) (ml), incontinence (g) and PVR (ml; portable ultrasound scanner). Voided percentage (Void%) was calculated as Void% = (VV/(VV+PVR))*100%.
Patients completed validated questionnaires to evaluate frailty (Tilburg Frailty Indicator (TFI)) and independence in activities of daily living (Katz Index) .
All patients underwent invasive urodynamics and received their diagnosis based on the FVCPVR, urodynamics and clinical judgment of an urologist specialized in the field of functional urology.
The local ethical committee approved the study protocol and all patients provided their written informed consent.
Results are reported in terms of median and interquartile range. Non-parametric tests were used to compare unpaired (Mann-Whitney U test) and paired continuous variables (Wilcoxon test). Mean and maximum PVR, mean and minimum Void% and diurnal and nocturnal PVR and Void% were compared between and within women with USI, DO and DO plus USI. Fishers’ Exact was conducted to compare categorical variables. Spearman’s rho was used to measure rank correlation between nocturia and PVR and Void%. Statistical significance was defined as a p-value <.05.
Results
The median age of the 90 female patients was 76 years [IQR: 72-80]. All of them had a negative screening for dementia. The median total score on the Katz Index was 5 [IQR: 4-5], indicating a moderate impairment in activities of daily living. According to the TFI, 70% of all patients was classified as frail.
Data obtained from the FVCPVR showed that 20% of the patients had global polyuria (24h urine output > 40 ml/kg bodyweight), 60% had nocturnal polyuria (nocturnal urine volume > 33% of 24h urine volume in absence of global polyuria) and 67% suffered from nocturia (≥ 2 voids/night).
A comparison of FVCPVR characteristics in patients according to their urodynamic diagnosis is presented in Table 1. PVR (mean and maximum) and Void% (mean and minimum) were not different between the patients with the urodynamic diagnosis of USI, DO and DO plus USI.
Even between diurnal and nocturnal mean PVR and Void% no difference was found in the previously mentioned urodynamic diagnoses, showing that on average the patients emptied their bladder as effectively at night as during the day.
The maximum PVR in 24h was 92 ml [IQR: 42-177] and no difference was detected between maximum diurnal and nocturnal PVR in all patients and between diagnoses. Minimum Void% in 24h was 64% [45-83] in all patients. Minimum Void% was lower during daytime than during night-time in all diagnoses and no difference was observed between diagnoses (Figure 1).
For further analysis of the PVR and Void%, the first morning void was studied independently of the nocturnal voids. No difference was found between the first morning void and the mean diurnal and nocturnal PVR and Void% respectively.
In all diagnoses the minimum diurnal Void% was significantly lower than the Void% of the first morning void and the minimum nocturnal Void%. The Void% of the first morning void and minimum nocturnal Void% was not different, which suggests that the Void% of the first morning void is an indicator of the minimum nocturnal Void%.
No difference in PVR and Void% was found between patients with and without global or nocturnal polyuria, even so between patients who scored as non-frail or frail on the TFI. A weak linear relationship was found between nocturia and maximum nocturnal PVR (rs(89)= .304, p= .005) and minimum nocturnal Void% (rs(89)= -.354, p= .001), meaning that an increase in nocturia-episodes correlates with an increase in maximum nocturnal PVR and a decrease in minimum nocturnal Void%. A similar weak linear negative relationship was detected between mean nocturnal VV and nocturia (rs(89)= -.334, p= .002).
Interpretation of results
This is the first study in which PVR and VV was systematically measured after each void during 24 hours in older women with urinary incontinence. No circadian variation of mean PVR and Void% was found among and within the diagnoses USI and/or DO, which endorses the expert opinion that measurement of PVR is not routinely required as part of the initial assessment of urinary incontinence in older women.
Interesting is that higher PVR and lower Void% was measured in patients with higher burden of nocturia. The aetiology of this phenomenon could be explained by the hypothesis that inadequate bladder filling volume leads to less effective detrusor contraction during night-time.
As in all FVC recordings there must be an intra-individual variation between different days in PVR and Void% as well. Future studies should aim to validate these results in a three-day FVCPVR.