Introduction Bladder overactivity is poorly defined by the ICS. Descriptive terms like 'a disorder of the filling/storage phase', 'involuntary bladder contractions, while the patient is attempting to inhibit', 'detrusor hyperreflexia' or 'detrusor instability' are today's confusing clinical practice. In this study "Bladder overactivity is a dysfunction of the bladder, in which a subject has no or decreased control over sudden occurring contractions of the musculus detrusor, in such a way that this leads to premature passing of urine".

Aims of study
In this study we investigated the applicability of Ambulatory CystoMetry (ACM) to assess efficacy of therapeutic modalities in patients suffering from bladder overactivity.

Methods
ACM is performed according to ICS standards [1] with event markers at the actual start and end of it, at initiation and cessation of voiding (entering, leaving toilet). Episodes of urgency, time and volume of fluid intake (at least 200 ml/h), voiding as well as time and leakage volume of pad and pad change are registered in a diary. Duration of an average ACM is approximately 6 hours, taking place between 9:00 a.m. and 4:00 p.m. Gaeltec equipment and microtip catheters are applied. There are no special provocative tests defined. After installation and instruction patients may go home. At the end of the 6-hour period they return to the urodynamic labobatory to finish the investigation. Results of the ACM are: total monitoring time, number of contractions during the filling phases and (mean) duration and amplitude of these contractions. The diary provides number of micturitions, the mean volume voided, the total volume drunk. Data of results are then used to calculate the Detrusor Activity Index (DAI, [2]); the theoretical range of the outcome of this quantitative model is lying between zero (non-overactive) and 1 (severe overactive bladder). Construct validity of the DAI has been investigated by the sensitivity /specificity with respect to correctly classified detrusor overactivity, the ICC_R (test-retest reliability) and the ICC_A (inter assessor agreement). The padtest (urineloss per hour during the A0CM) was also evaluated by the ICC_R. Finally, we compared the DAI (loss/hour, mean volume) as efficacy outcome in clinical trials of treatment modalities for overactive bladders with conventionally applied outcome variables like incontinence episodes/week, frequency/24-hours, mean volume/micturition and number of pads/24-hours. For this purpose we used the data from a recent trial (n=1022) investigating the efficacy of tolterodine [3] and from a recently finished own trial (n=28) investigating the efficacy of functional electrostimulation (FES). The effect measured by the different outcome variables was translated into a so-called 'standardized effect' (St.Eff.) in order to enable a proper comparison. If the St.Eff. for two groups is lying at 0.20 it can be defined as a 'small effect size', if it is at 0.50, it can be defined as a 'medium size', and at 0.80 it can be defined as a 'large effect' [4].

Results
The DAI-score has a sensitivity and specificity of 86%, if 0.41 would be considered as the discriminating cutoff point between overactive and normal bladder activity during the filling phase. In a group of 21 female patients with LUTS (mean age 51.6; sd 11.4) the test-retest of the DAI (appr. 2 months) showed an ICC_R = 0.85. The ICC_A for 1 observer obtained in data from 2 blinded observers analyzing 111 ACMs was 0.96. The volume of urineloss/hour of padtest obtained in the previous patient group (n=21) showed an ICC_R = 0,87. Table 1 shows the standardized effects of each outcome variable in both studies. It is striking that loss/hour and incontinence episodes/week, respectively, frequency/6 and /24-hours show almost the same St.Eff. for two different treatment modalities representing a minimal effect, while the DAI shows a large effect for FES.

Conclusions
This study can be seen as a contribution to the construct validity of ambulatory urodynamics and the Detrusor Activity Index for research purposes. The results also indicate that for overactive bladder assessment, the DAI is a relatively strong instrument for efficacy measurements compared to conventionally applied outcome variables.

References:
1. Neurourol Urodyn, vol. 19, 2000: p. 113-125 2. BJU, vol. 83, Suppl. 2, 1999: p. 16-21 3. Clin-Drug-Invest, vol. 19, 2000: p. 83-91 4. Foundations of clinical research,Appleton & Lange, Norwich, Connecticut 1993: p. 653