The prevalence of overactive bladder symptoms (OAB) increases with age and is approximately 30% in women over 65 years of age. The differences in mode of action between tolterodine and solifenacin might result in diverse clinical effectiveness and urodynamic parameters. Correlation  of clinical efficacy, adverse effects and urodynamic changes before and during the OAB treatment may be helpful in choosing the most suitable pharmacological mode of treatment. Especially when dealing with underactive bladder patients presenting also OAB symptoms. The Flow Index was recently introduced and proposed as a potential marker confirming OAB symptoms. It also might be helpful in assigning patients with mixed symptoms to the proper first line treatment. According to authors, an FI value of less than 0.47 may be used as a simple and objective OAB marker with reasonable sensitivity (69.8%) and specificity (69.9%) as a potential marker confirming OAB symptoms [1]. 
The aim of the study was to determine if uroflowmetric parameters, including Flow Index, may be used as a tool for monitoring of antimuscarinics treatment efficacy in OAB patients.

The Local Ethics Committee approved the study concept and all patients signed informed consent. 38 OAB patients were assigned to the tolterodine 4 mg ER and solifenacin 10 mg once a day treatment in 1:1 ratio. Four women stopped treatment due to adverse events (dry mouth: n=3 and blurred vision: n=1). All women underwent full gynecological evaluation at baseline visit. Uroflowmetry was performed at baseline and at follow-up visit after 12 weeks according to Good Urodynamic Practice. Statistical analysis was performed using Statistica 13PL package (StatSoft, Inc., Tulsa, OK, USA). Data distribution was assessed using Shapiro-Wilk test. For skeved distribution nonparametric tests were used. P value of less than 0.05 was considered as statistically significant.

Both treatments were equally effective in decreasing urgency and nocturia episodes (tolterodine: p=0.003 and p=0.007, respectively; solifenacin: p<0.001 and p=0.01, respectively). Some uroflowmetric parameters of OAB patients changed significantly on treatment with antimuscarinics (Table 1).

Our results are in contrary to the previously reported, in which no statistical differences in uroflowmetric parameters were found in healthy volunteers (bladder compliance, Qmax, Qave, and post void residual volume) [2].  The possible reason for such discrepancy is that antimuscarinics do not influence on healthy bladder as on overactive bladder. In paper published in 2011 the influence of antimuscarinic treatment on urodynamic parameters was also investigated [3]. Some of those results are similar to ours. First, the FI value for that patients is ≤ 0.5, which confirms that the patients with OAB have lower FI values than patients with stress UI [1, 3]. Similarly, increase in post void residual volume was also lower on solifenacin 5 mg treatment compared to tolterodine 4 mg ER (Δ+6.5 ml and Δ+23.5 ml respectively). On the contrary, authors did not find significant differences in maximal and average urethral flow rates on both treatments. Moreover, both solifenacin and tolterodine had similar adverse events profile however, tolterodine had a greater effect in increasing heart rate than solifenacin (p = 0.0004) [3].

Both treatments decreased maximal urethral flow significantly but Flow Index value did not changed significantly on solifenacin 10 mg treatment. Interestingly, on after 12 weeks of treatment post void residual volume was significantly higher on tolterodine compared to solifenacin which may be important in patients with detrusor underactivity presenting OAB symptoms. Our results suggests that both antimuscarinics are effective in OAB treatment but solifenacin does not influence negatively on bladder emptying even after 12 weeks.
In order to draw more significant conclusion our results must be confirmed on larger group of patients with longer period of follow-up, but the  results are encouraging.

Futyma K, Nowakowski L, Bogusiewicz M, Zietek A, Wieczorek AP, Rechberger T. Use of uroflow parameters in diagnosing an overactive bladder-Back to the drawing board. Neurourol Urodyn. 2017; 36:198-202.
Mehnert U, Reitz A, Youssef SA, Schurch B. Proof of principle: The effect of antimuscarinics on bladder filling sensations in healthy subjects--a placebo controlled double blind investigation using 4 and 8 mg tolterodine extended release. Neurourol Urodyn. 2010; 29:464-9.
Hsiao SM, Chang TC, Wu WY, Chen CH, Yu HJ, Lin HH. Comparisons of urodynamic effects, therapeutic efficacy and safety of solifenacin versus tolterodine for female overactive bladder syndrome. J Obstet Gynaecol Res. 2011; 37:1084-91.