External urethral sphincter (EUS)

The external urethral sphincter (EUS) is a striated muscle complex that serves as the primary voluntary mechanism for maintaining urinary continence [StatPearls, Wikipedia]. Located distal to the bladder neck, it surrounds the membranous urethra in males and the mid-to-distal urethra in females [StatPearls, Kenhub]. Innervation is provided by the pudendal nerve, which originates from Onuf's nucleus in the sacral spinal cord (S2-S4) [PMC, Kenhub]. The EUS maintains a tonic contraction to prevent urine leakage and can be voluntarily contracted to stop the flow of urine or relaxed to facilitate voiding [Wikipedia, Kenhub]. It is distinct from the internal urethral sphincter, which is composed of smooth muscle and is under autonomic control [StatPearls, Kenhub]. Dysfunction of the EUS is a major factor in several lower urinary tract disorders, including stress urinary incontinence (SUI) and detrusor-sphincter dyssynergia (DSD) [PMC, ResearchGate]. In SUI, the sphincter is often weakened due to aging, childbirth, or surgery, leading to leakage during physical exertion [ResearchGate, PMC]. Conversely, in DSD, the EUS fails to relax during bladder contraction, causing high-pressure urinary retention [PMC, PubMed]. Pharmacological management includes the use of duloxetine to enhance sphincter tone via central serotonergic and noradrenergic pathways [PMC] and botulinum toxin injections to induce local muscle relaxation in cases of overactivity [Medscape]. Emerging therapies also explore 5-HT2C receptor agonists like TAK-233 to improve sphincter function [Synapse, PMC].

Other names

Sphincter urethrae membranaceaeStriated urethral sphincterRhabdosphincterExternal sphincter muscle of urethra

Serotonin-norepinephrine reuptake inhibition; Inhibition of acetylcholine release; GABA-B receptor agonism; 5-HT2C receptor agonism