Sensitivity of urethral flow-evoked voiding reflexes decline with age in the rat: insights into age-related underactive bladder

Underactive Bladder and Detrusor Underactivity: New Advances and Prospectives

Underactive bladder (UAB) is a prevalent but under-researched lower urinary tract symptom that typically occurs alongside detrusor underactivity (DU). Unlike UAB, DU is a urodynamic diagnosis which the International Continence Society (ICS) defines as "a contraction of reduced strength and/or duration, resulting in prolonged bladder emptying and/or a failure to achieve complete bladder emptying within a normal time span". Despite the widespread prevalence of UAB/DU, there are significant gaps in our understanding of its pathophysiological mechanisms, diagnosis, and treatment compared with overactive bladder (OAB) and detrusor overactivity (DO). These gaps are such that clinicians regard UAB/DU as an incurable condition. In recent years, the understanding of UAB has increased. The definition of UAB has been clarified, and the diagnostic criteria for DU have been considered more comprehensively. Meanwhile, a number of non-invasive diagnostic methods have also been reported. Clinical trials involving novel drugs, electrical stimulation, and stem cell therapy have shown promising results. Therefore, this review summarizes recent reports on UAB and DU and highlights the latest advances in their diagnosis and treatment.

Age is associated with reduced urethral pressure and afferent activity in rat

Age-related changes in the lower urinary tract (LUT) can affect the coordination of reflexes and increase the incidence of bladder disorders in elderly. This study examines the age-related loss of urethral signaling capability by measuring the afferent activity directly. We find that less urethral pressure develops in response to fluid flow in old rats compared to young rats and that pressure and flow evoke less urethral afferent activation. These findings are consistent with our previous study demonstrating that the urethra-to-bladder reflex, which is required for efficient voiding, becomes weaker with age. We measured the pudendal afferent response in young (4-7 months) and old (18-24 months) rats to fluid flow in the urethra across a range of flow rates. We used paraffin embedding and hematoxylin and eosin staining to quantify age-related changes in the sensory branch of the pudendal nerve. Urethral afferent signaling in response to the same urethral flow rates was weaker in older animals. That is, the sensitivity of urethra afferents to flow decreased with age, and higher flow rates were required in older animals to recruit urethra afferents. There was also a reduction in the myelin thickness of pudendal afferents in old rats, which is a possible contributing factor to the sensory activity. Furthermore, the same flow rates evoked less pressure in the urethras of old animals, indicating there is an age-related change of the urethral tissue that reduces the pressure stimulus to which these afferents respond. These results help characterize the underlying changes in LUT system with age.

High percentage of neurologic deficits in the electrophysiology study of the lower urinary tract in patients with detrusor underactivity and chronic urinary retention

OBJECTIVES

Both detrusor underactivity (DU) and bladder outlet obstruction are the common causes of chronic urinary retention. Some novel treatment approaches focus on modulating micturition reflex and external urethral sphincter (EUS) function. This study used electrophysiologic (EP) studies to investigate the micturition reflex and EUS conditions of chronic urinary retention patients.

METHODS

Sixty patients with urodynamic DU and chronic urinary retention were studied using (1) bulbocavernous reflex (BCR) by electric stimulation, (2) electromyography (EMG) of the EUS, and (3) nerve conduction velocity (NCV) studies of the pudendal nerve. The EP findings were analyzed in DU patients with different etiologies.

RESULTS

The BCR was positive in 41.7% of patients. In EMG studies, denervation, reinnervation, and reduced recruitment of the EUS were observed in 21.7%, 71.7%, and 88.3% patients, respectively. Decreased amplitude of pudendal nerve conduction in NCV studies was noted in 73.3% of patients. Patients with sacral neuropathy had a lower BCR positive rate (p = 0.001), a nonsignificant but higher denervation rate (p = 0.059) in EMG studies, and a higher rate of decreased amplitude in NCV (p = 0.011) than those without sacral neuropathy. Excluding patients with sacral neuropathy or diabetes mellitus, a high percentage of neurologic deficits was still detected in EP studies.

CONCLUSIONS

Chronic urinary retention patients with urodynamic DU not only have bladder dysfunction, but also potential neuropathy in the sacral reflexes, pudendal nerve, or urethral sphincter innervation. The neurologic deficits explored in EP studies may affect the decision-making around the therapy to restore the voiding function in DU.