**1. Introduction**

Bladder sensation arises from the urothelial cell and detrusor muscle stretching by signal transduction from peripheral receptors to the cerebral cortex. Many factors may contribute and effect the sensory transduction pathways, resulting in increased bladder sensation with or without associated symptoms such as urgency, urgency incontinence or bladder pain. The factors which will increase bladder sensation include aging, the urinary bladder, bladder outlet conditions, or systemic diseases. Currently, there is no definite definition nor specific medication targeting bladder oversensitivity. Bladder oversensitivity is usually existent with other lower urinary tract disorders (LUTD) such as bacterial cystitis, ketamine cystitis, interstitial cystitis (IC), bladder outlet obstruction (BOO), overactive bladder syndrome (OAB), idiopathic detrusor overactivity (DO), neurogenic DO (NDO); or systemic diseases such as diabetes mellitus, end-stage renal disease, or congestive heart failure [1]. Antimuscarinics or beta-3 adrenoceptor agonists have therapeutic effects on OAB and DO and also have some effect on reducing bladder oversensitivity. [2,3] Treatment of bladder oversensitivity should begin with treating the underlying causative diseases such as BOO, acute or chronic bladder inflammatory diseases, diabetes mellitus, or other systemic diseases. Previous studies of botulinum toxin A (BoNT-A) treatments for OAB, DO and IC have shown significant improvement of bladder oversensation and increased bladder capacity, in addition to the main target of urinary urgency or bladder pain. If medical treatment fails, intravesical BoNT-A might play a second- or third-line therapeutic alternative for bladder oversensitivity. This article reviews the pathophysiology, therapeutic mechanisms and treatment effects of BoNT-A on the improvement of the increased bladder sensation in bladder disorders, specifically focused on OAB, DO, IC and bladder oversensitivity. This review study searched relevant articles identified by a literature search using MEDLINE/PubMed. Key words included OAB, bladder oversensitivity, DO, IC, bladder sensation, botulinum toxin A, BOTOX, and BTX-A. References of retrieved articles were also hand searched to find additional articles related to the topic of this review. All trials examining the use of BoNT-A injections into the urinary bladder for the treatment of OAB, DO, IC and bladder oversensitivity, and the studies reporting the treatment outcome such as urodynamic parameters, urinary incontinence improvement, and adverse events were included in this review.

#### **2. Lower Urinary Tract Disorder and Bladder Oversensitivity**

Lower urinary tract symptoms (LUTS) are highly prevalent and greatly impact the health-related quality of life and cause social economic burden, especially in elderly men and women [4–6]. The etiology of LUTS could be bladder outlet dysfunction or bladder dysfunction, including bladder oversensitivity and DO, or a combination of both bladder and bladder outlet dysfunctions. [7] Recent studies revealed the prevalence of OAB is high in Europe and the United States, as well as in Asian countries. [8,9] As most men with clinical OAB do not have urinary incontinence, they are frequently mis-diagnosed with bladder outlet obstruction (BOO) [10].

The bladder epithelium, known as urothelium, provides a passive barrier to prevent absorption of urine and its contents. Recent evidence suggests the urothelium might be a responsive organ with sensory and transducer functions. [11] The urothelial cells and suburothelial afferent nerves exhibit a number of common properties, including sensory receptors and ion channels. The local or systemic conditions which alter the afferent nerves or urothelial cells might contribute to the abnormal sensory of the urinary bladder. [11] Patients with several lower urinary tract disorders such as BOO, IC, ketamine cystitis, spinal cord injured (SCI), neurogenic bladder dysfunction, or systemic diseases such as diabetes mellitus, end-stage renal disease, or congestive heart failure may also have symptoms of OAB or bladder oversensitivity [12].

### **3. Pathophysiology of Bladder Oversensitivity**

The pathophysiology of OAB and bladder oversensitivity is multifactorial. Occult neurogenic bladder, undetected BOO, provoked DO due to urethral incompetence, aging or diseases, chronic bladder ischemia, chronic bladder inflammation, central nervous system (CNS) sensitization, and autonomic dysfunction are possible etiologies of refractory OAB. [1] The bladder urothelium and afferent nerves express transient receptor potential vanilloid receptor 1 (TRPV1), the purinergic receptor P2X3, the sensory neuropeptides substance P, and calcitonin gene-related peptide (CGRP). [13–15] These receptors are believed to be involved in the afferent pathways that control bladder sensation and urinary volume reflexes [16].

Bladder sensation can be transmitted by the myelinated A-δnerves and unmyelinated C-fibers. In mammalian bladders after SCI, the unmyelinated afferent C-fibers are found to become predominant and mediate the voiding reflex. [17] Intravesical vanilloid treatment using capsaicin or resiniferatoxin had been found effective in SCI patients with NDO or idiopathic DO through acting on TRPV1 [18,19]. After intravesical instillation of resiniferatoxin, urinary frequency significantly decreased and maximal cystometric capacity increased, and patients with NDO became dry days after treatment. [20] In

addition, many C-fibers in the bladder urothelium contain sensory neuropeptides which can modulate the micturition reflex and cause DO [21]. Since TRPV1 receptors are found on the afferent nerves and co-localized with P2X3 receptors and other sensory nerves expressing substance P and CGRP, desensitization of the TRPV1 by vanilloid treatment can also decrease bladder oversensitivity and DO [22–24].

The urothelium functions as a sensory organ which receives information from the urinary bladder content and responds to mechanical, thermal or chemical stimuli by releasing adenosine triphosphate (ATP), nitric oxide and acetylcholine (Ach). Changes in the external environment or direct insult on the urothelium produced by different bladder disorders may convey these signals to nerves, detrusor muscles, and transmit to the CNS, resulting in bladder oversensitivity or detrusor contractions. [25] The suburothelial sensory nerves of the urinary bladder are abundant with vesicles containing Ach and ATP, suggesting the bladder lamina propria also play an important role in the transmission of bladder filling and fullness in response to bladder stretch by activating P2X3 receptors [26–28]. When the urinary bladder is infected or under traumatic conditions, the production of ATP, CGRP and substance P acts on afferent nerves in an autocrine fashion to increase afferent nerve activity [29]. Recent studies also found a suburothelial nexus of myofibroblasts or interstitial cells which might be involved in the micturition reflexes [30]. These cells are closely linked by gap junctions and have a response to ATP in a mode similar to the activated ATP-gated P2Y receptors [31,32]. The Ach and ATP released from urothelial cells on bladder filling were noted in patients with aging, which also implies the main pathological mechanism of OAB and bladder oversensitivity in older people [33]. In addition, increase of stretch-activated ATP release was found in the cultured human urothelial cells from patients with IC and SCI, suggesting the involvement of these neurotransmitters in DO and bladder oversensitivity [34].

Bladder inflammation is commonly found in patients with OAB, IC, and systemic diseases and lower urinary tract disorders, resulting in bladder storage symptoms such as urgency and frequency [12]. In previous studies of IC, chronic inflammation leads to increase of urothelial cell apoptosis, lower adhesive protein E-cadherin and lower tight junction protein zonula occludens-1 expression [35]. The chronic inflammation in the IC bladders also inhibits the basal cell proliferation, causing defective apical cell maturation and impaired barrier function [36]. Further study of apoptotic markers such as Bad, Bax, and caspase 3 all increased in the IC bladder tissues, and the inflammatory signals such as p38 mitogen-activated protein kinase and tumor necrosis factor alpha were upregulated [37]. Chronic inflammation is also found in a large proportion of patients with OAB [38]. Chronic neural plasticity due to chronic inflammation and activation of sensory receptors may change the sensory afferent activity via influencing antinociceptive activity, resulting in bladder oversensitivity or DO. The urinary nerve growth factor (NGF) levels in patients with OAB, IC, or other types of bladder oversensitivity have been found to elevate, suggesting these different lower urinary tract disorders might share common pathways in increasing bladder sensation [39–41].

Based on the above evidence, the pathophysiology of bladder oversensitivity or IC syndrome might be sequentially developed by: (1) urothelial injury caused by bacterial cystitis, foreign body, instrumentation, or surgical trauma; (2) suburothelial inflammation developed after urothelial trauma, toxin or autoimmune response; (3) acute inflammatory cell infiltrations in the bladder wall after the acute injury; (4) chronic inflammatory reaction and scar formation in the bladder wall; (5) increased inflammatory reaction leading to neuroplasticity in the dorsal horn ganglia and corresponding sacral cord, resulting in lowering bladder sensation threshold and oversensitivity [42]. The insult of the bladder wall initiates an acute inflammatory process. The sensory impulse from the bladder wall also ascends to the corresponding cortical gyrus and produces frequency symptoms. Therefore, patients might have an early inflammatory reaction and characteristic IC symptoms, such as urgency, frequency and bladder pain. If the urothelial insult ceases, the urothelial cell regeneration will rebuild the defense mechanism and solves the inflammatory reaction to relieve bladder symptoms after treatment. However, if the urothelial insult continues, the inflammatory reaction might exacerbate not only at the bladder wall but also at the spinal cord or cortical gyrus, causing permanent inflammation printing [43]. Similar pathogenesis might also occur in OAB, in which bladder condition may progress from early stage (bladder oversensitivity) to late stage (urgency urinary incontinence) bladder conditions [44].
