Nocturnal enuresis (NE) is defined as intermittent involuntary urinary incontinence displayed during sleep in children aged ≥5 years. NE is classified into monosymptomatic NE (MNE) and nonmonosymptomatic NE (NMNE). MNE refers to NE in children without any other lower urinary tract symptoms (LUTS), while NMNE means NE in children with any other LUTS and a history of bladder dysfunction. NE can cause lower self-esteem and a disturbance of social development. NE is a multifactorial disease. Main etiologies of NE include nocturnal polyuria (NP), small functional bladder capacity (FBC), an arousal disorder or a mixture of these factors. In children with NE, the evaluation of FBC can provide important information about NP and bladder capacity. Therefore, in children with NE, the evaluation of FBC is essential in the therapeutic approaches and monitoring of treatment response.

Despite many studies in NE, there is still considerable heterogeneity in evaluation methods and therapeutic approaches. The International Children’s Continence Society recommends the use of 48-hour frequency/volume (48-h F/V) charts for evaluating FBC. Maximal voided volume (MVV), which can be obtained using these charts, is known to represent FBC. The 48-h F/V chart is noninvasive and represents FBC of the everyday environment, but it must be performed for at least 48 hours to obtain objective and accurate MVV results. This is rather time-consuming for both parents and children, and obtaining reliable results may be difficult, especially in less motivated families. In addition, the chart evaluation does not recognize post-void residual volume (PVR); thus, this method may underestimate FBC. Uroflowmetry (UFM) with ultrasound PVR measurements and radionuclide cystography are alternative methods of assessing FBC. The disadvantage of these FBC measurement methods is their higher cost and unnatural situation. Maternik et al reported no significant difference between the FBC obtained from the 48-h F/V chart and the FBC obtained from UFM with PVR in patients with several LUTS.¹ Kang et al also reported a lack of significant variance in the FBC values obtained from the 2 measurement methods.²

Previous studies investigating the predictive factors of treatment response mainly focused on lower FBC, NP and arousal problems. Many children with NE have shown lower FBC compared to normal children.^2–5 In general, reduced FBC is the main factor not only in the prediction of response to desmopressin, but also refractory NE. However, according to the reports so far, there is controversy as to whether FBC is useful as a predictor of NE treatment response (see table).

Previous studies revealed the relationship between reduced FBC, NE severity and treatment resistance. Several studies have shown that FBC is reduced by up to 50% in children with NE. According to Kim’s study, 46.5% of all patients had reduced FBC for age, and the incidence of small FBC was increased in children with everyday wetting and everyday multiple wetting.³ In the Liu et al study, 33.9% of children with MNE had a low bladder capacity.⁴ Acosta et al reported that 85% of patients with NE showed an FBC less than 70% of the expected bladder capacity value.⁵ This is probably because the lower limit of the normal range is set at 70%. Kang et al reported that 68%-70% (according to the measurement methods) of patients had a small FBC for age, regardless of the NE subgroup.²

Several studies have highlighted the influence of FBC (MVV) on response to desmopressin. Kim reported a significant correlation between NE severity and FBC reduction degree.³ Yeung et al reported that the significantly small FBC patient group relapsed with decreased desmopressin response.⁶ Eller⁷ and Rushton⁸ et al reported that if FBC exceeds 70% of the age-adjusted norm, a good response to desmopressin could be expected. In the Shim et al study, when patients were stratified by treatment response, the mean FBC of patients with no response or partial response and complete response (CR) did not show a significant difference at baseline.⁹ However, in the univariable analysis, increased FBC (30% or more increase, 6 months after treatment cessation compared to baseline) was associated with decreased relapse of NMNE. Thus, practical consensus guidelines for the management of NE suggest that a reduced FBC for age is associated with a lower response rate to desmopressin.

On the other hand, several studies suggest that FBC does not predict the outcome of NE treatment. Chang and Yang found no significant association between a reduced FBC and response to medical treatment.¹⁰ Instead, they confirmed that elevated PVR and NP were significant predictors of medical treatment. In the Liu et al study, FBC was not a predictive factor of the response to desmopressin.⁴ Only the initial response to low-dose desmopressin was a positive predictor of greater therapeutic success. Kang et al evaluated whether FBC differs among subgroups of NE patients and can be used to predict treatment response.² They concluded that children with NE had diminished FBC in both 48-h F/V charts and UFM with PVR. However, they found no difference in FBC by NE type or treatment outcome. Therefore, FBC may not be helpful in distinguishing NE types or predicting treatment responses.

In conclusion, NE is a complex disease that stems from many etiological factors. FBC is calculated to confirm the NE characteristics and determine the treatment method. The 48-h F/V chart and UFM with PVR are reliable methods for measuring FBC. Children with NE, particularly those with severe NE, had small FBC on the 48-h F/V charts and UFM with PVR, regardless of the measurement method or NE subgroup. Thus, a small FBC is a common sign of NE, but so far, it is difficult to draw the conclusion that FBC can predict NE treatment outcomes.