**1. Introduction**

Overactive bladder (OAB) is a syndrome defined by the International Continence Society and is characterized by urinary urgency, with or without urgency urinary incontinence, and usually accompanied by frequency and nocturia [1,2]. Large population-based surveys revealed that the prevalence of OAB increases with age, and is slightly higher for elderly males over females [3,4]. Many studies have described the negative influence of OAB on health-related quality of life, including anxiety, depression, sleep disorder, social withdrawal, and sexual life impairment [5–10]. Likewise, urinary incontinence is known to negatively affect the quality of life in the elderly population [11] and is also responsible for low self-esteem and depression [12]. To treat the bothersome storage symptoms of

**Citation:** Ou, Y.-C.; Kao, Y.-L.; Ho, Y.-H.; Wu, K.-Y.; Kuo, H.-C. Intravesical Injection of Botulinum Toxin Type A in Patients with Refractory Overactive Bladder—Results between Young and Elderly Populations, and Factors Associated with Unfavorable Outcomes. *Toxins* **2023**, *15*, 95. https://doi.org/10.3390/ toxins15020095

Received: 14 December 2022 Revised: 16 January 2023 Accepted: 16 January 2023 Published: 19 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

OAB, antimuscarinic agents have been developed to inhibit spontaneous detrusor smooth muscle contractions and reduce afferent signals during bladder filling [13]. The therapeutic effect of antimuscarinic agents has been proven; however, insufficient symptom relief and concomitant adverse events cause poor medication persistence and adherence [14]. Solifenacin and fesoterodine have been shown to have limited impact on cognitive function and few central nervous system adverse events for patients ≥65 years after short-term exposure [15–17]. Even though, uncertainty regarding cognitive decline after long-term cumulative anticholinergic exposure still limits their use in the elderly population [18,19]. On the other hand, β3-adrenoceptor agonists facilitate relaxation of the detrusor muscle during bladder filling. Both mirabegron and vibegron have been confirmed to be effective and well-tolerated in the elderly population [20–25]. However, most participants in the clinical trials were relatively healthy and did not have uncontrolled cardiovascular diseases. The long-term application of these pharmacological agents in the elderly population remains questionable.

The efficacy of intravesical injection of botulinum toxin type A (BoNT-A) has been established for patients with OAB who have an insufficient response to first-line pharmacological agents [26,27]. However, most of these published data did not focus on the elderly population, and only a handful of studies included a population with a mean age of ≥65 years old [28–35]. In addition, 75 years of age has been proposed as a new cutoff value to redefine the elderly because of the global extension of life expectancy [36]. Exploring the therapeutic outcomes and adverse events associated with BoNT-A injections in this vulnerable population is urgently necessary [37]. The commonly reported adverse events after BoNT-A injection include large post-void residual urine volume (PVR), urinary retention, and urinary tract infection (UTI) [38]. However, factors that can help identify patients at risk of these unfavorable outcomes are still limited, especially in the elderly population. Therefore, the primary aim of our study is to retrospectively evaluate the therapeutic efficacy of intravesical BoNT-A injection for refractory OAB, and the secondary aim is to investigate the factors associated with unfavorable outcomes in an elderly population aged ≥75 years.
