Overactivity index: A noninvasive and objective outcome measure in overactive bladder in children

Reports showed that observing detrusor overactivity (DO) and maximum cystometric capacity (MCC) may guide rational pharmacotherapy. Since urodynamic studies (UDS) are challenging for both patients and the healthcare system, a non-invasive objective prognostic marker is preferable.

To investigate the value of the overactivity index (OI), a non-invasive measure calculated from the frequency-volume chart (FVC), for predicting the presence of symptoms and abnormal UDS in children with non-neurogenic OAB.

This was a prospective interventional study on a consecutive sample of 92 children with urgency treated with anticholinergics and standard urotherapy. Data from history, physical examination, bladder diaries, kidneys and bladder ultrasonography, uroflow, urinalysis, urine culture, and UDS was collected at baseline, and after 3 and 6 months. Binary logistic regression was used to evaluate noninvasive parameters as predictors of Overactive Bladder Symptom Score (OABSS) total score >2 and DO and/or small MCC defined as <65% of expected bladder capacity (EBC) for age. OI was calculated as (1 - (median (all voided volumes in FVC in ml))/(0.65 ∗ EBC in ml)) ∗ 100.

At baseline, 26 patients (36.1%) had DO and small MCC, while 21 patients (29.2%) only had DO. In 18 patients (25.0%) only small MCC was found. Seven patients had normal findings and 20 did not perform a urodynamic study. OI ≥ 23 returned as a single significant predictor of OABSS >2 (OR 7.97, 95% 1.97-32.22, p = 0.004) in multivariate regression (R² = 30.8%; AUC = 0.86). OI correlated with "urgency episodes over two weeks" and MCC/EBC with medium (r = 0.45) and large effect (r = -0.56), respectively, p = 0.001.

A strong correlation of OI and MCC/EBC ratio is useful, as rise in MCC is predictive of a positive outcome. Also, calculating the OI is more practical than performing UDS. This could contribute to the use of OI as a predictive marker for starting (or continuing) anticholinergic treatment (when OI ≥ 23) or for maintaining urotherapy alone (when OI < 23) in children with OAB. The limitations were lack of external validation of OI, a 37-49% drop-off rate for follow-up visits at 3 and 6 months, respectively, and not performing UDS on all participants at every follow-up visit.

OI was found to be a significant predictor of the presence of OAB symptoms and correlated with the number of urgency episodes. It could estimate how much MCC differs from EBC.