Evaluate and manage urinary incontinence, including urodynamics, mid-urethral slings, pharmacotherapy and voiding dysfunction

In one line

Female urinary incontinence is a symptom, not a diagnosis: the entire management pathway is decided by which storage failure you are treating — stress (sphincter/support failure), urgency (detrusor overactivity), mixed, or the overflow that masquerades as either — and the consultant's job is to classify correctly, treat conservatively first, and reserve surgery and third-line therapy for the woman in whom mechanism and failed first-line therapy genuinely justify them.

The classification, definitions and continence physiology are assumed here from urinary incontinence at Intermediate; this develops the consultant layer — when urodynamics earns its place, how to choose between the mid-urethral sling and its alternatives in a post-mesh-pause world, how to sequence overactive-bladder therapy to third line, and how to recognise the voiding dysfunction that the inattentive registrar treats as the opposite disease.

Mechanism & pathophysiology

Continence is the product of two pressures and one reflex. Urethral closure pressure must exceed bladder (detrusor) pressure at rest and during effort; storage depends on a quiescent detrusor that does not contract until socially permitted. The switch between storage and voiding is a spinobulbospinal reflex: bladder-wall afferents ascend to the pontine micturition centre, which is held under tonic inhibition by the prefrontal cortex during storage and, when voiding is permitted, coordinates detrusor contraction with synchronous urethral-sphincter relaxation. Storage is sympathetic (hypogastric, β3-mediated detrusor relaxation and α-mediated bladder-neck tone) and somatic (pudendal, voluntary external sphincter); voiding is parasympathetic (pelvic, M2/M3 detrusor contraction). This wiring is not academic — it is why a sacral (S3) neuromodulator can restore inhibitory tone in refractory overactivity, why a β3-agonist augments storage without touching voiding, and why a spinal or pontine lesion produces detrusor–sphincter dyssynergia rather than simple incontinence. Stress and urgency incontinence are failures of these two systems, and they fail by different mechanisms that the treatment must match.

Stress urinary incontinence (SUI) is a failure of the urethral closure mechanism under a rise in intra-abdominal pressure. Two distinct lesions produce it, and distinguishing them changes the operation. The first is urethral hypermobility — loss of the suburethral hammock (the anterior vaginal wall and its attachments to the arcus tendineus and levator ani, the DeLancey support) so that the bladder neck and proximal urethra descend and rotate out of the abdominal pressure zone on straining; the urethra is no longer compressed against a stable backboard. The mid-urethral sling works precisely here, restoring a backstop at the mid-urethra against which the urethra kinks under stress. The second lesion is intrinsic sphincter deficiency (ISD) — the urethra itself fails to coapt, classically after pelvic radiation, multiple prior continence operations, ageing or denervation. ISD is the "low-pressure urethra" (a maximum urethral closure pressure below roughly 20 cmH₂O, or a Valsalva leak-point pressure under about 60 cmH₂O); it leaks with minimal provocation, often a fixed non-mobile urethra, and it is the SUI subtype most likely to fail a standard sling and to need a bulking agent, an autologous sling tensioned more snugly, or an artificial sphincter. The two coexist on a spectrum, but a fixed, leaking, previously-operated urethra is a different surgical problem from a hypermobile one.

Urgency urinary incontinence (UUI) is the storage symptom of the overactive bladder (OAB) syndrome — urgency, usually with frequency and nocturia, with or without incontinence and in the absence of infection or other pathology. Its urodynamic correlate, where one is sought, is detrusor overactivity: involuntary detrusor contractions during the filling phase. The pathophysiology is a loss of the normal cortical and afferent control of the micturition reflex — a combination of detrusor myogenic instability (spontaneous, propagating contractions in an electrically coupled detrusor), afferent hypersensitivity (urothelial signalling through purinergic and muscarinic pathways), and impaired central inhibition. This matters for drug choice: antimuscarinics target the M2/M3 receptors mediating detrusor contraction, while the β3-agonist promotes detrusor relaxation during storage through a different receptor entirely — which is why the two can be combined and why the β3-agonist avoids the antimuscarinic side-effect profile.

Mixed urinary incontinence is the coexistence of both, and the consultant skill is identifying the predominant symptom, because treating the wrong component first wastes time and an anti-incontinence operation done for a woman whose real problem is detrusor overactivity can worsen her urgency.

Voiding dysfunction and overflow is the trap that sits underneath all of this. Chronic incomplete emptying — from detrusor underactivity (ageing, diabetic cystopathy, neurological disease, pelvic surgery denervation), bladder outlet obstruction (a previous over-tight sling, severe prolapse kinking the urethra), or a combination — produces a chronically distended bladder that leaks by overflow, mimicking stress or urgency incontinence. The mechanism is the opposite of what it looks like: the bladder is too full and underactive, not overactive, and an antimuscarinic or a sling will make her worse. This is the single most important pathophysiological discrimination in the topic, and the reason a post-void residual is mandatory before any treatment.

Assessment

The assessment is built to answer three questions in order: which type, is there a red flag, and is the bladder emptying.

• History characterises the symptom. Leakage on coughing, lifting or exercise points to stress; leakage preceded by a sudden compelling urge points to urgency; both, with a dominant component, points to mixed. Continuous leakage (day and night, no provocation) is not ordinary incontinence — it signals a fistula or an ectopic ureter and demands a different work-up. Quantify severity (pads, fluid intake, caffeine, constipation, obstetric and surgical history, prior continence surgery, oestrogen status, neurological symptoms, and medication — diuretics, ACE-inhibitor cough, sedatives).
• The bladder diary (3 days, frequency-volume chart) is the highest-yield single investigation and is underused: it separates true incontinence from polyuria and nocturnal polyuria, quantifies functional bladder capacity, reveals fluid and caffeine excess, and demonstrates the urgency pattern objectively. It costs nothing and reshapes management more often than urodynamics.
• Examination assesses the pelvic floor, oestrogenisation and coexisting prolapse (incontinence and prolapse cluster, and a large prolapse can mask occult SUI that is unmasked once the prolapse is reduced). The cough stress test with a comfortably full bladder — demonstrable leakage synchronous with the cough — is the bedside confirmation of stress incontinence; immediate leakage confirms SUI, delayed leakage after the cough suggests a cough-provoked detrusor contraction.
• Urinalysis is mandatory in every woman to exclude infection (treat and reassess before labelling OAB), glycosuria and haematuria — the last because persistent unexplained haematuria with irritative symptoms mandates cystoscopy and upper-tract imaging to exclude malignancy, not an antimuscarinic.
• Post-void residual by catheter or bladder scan is non-negotiable before treatment: a high residual reframes the whole problem as voiding dysfunction/overflow and forbids both antimuscarinics and a sling until it is explained.
• The pad test quantifies leakage objectively (a standardised 1-hour or 24-hour weighed pad test) and is useful when symptoms and signs disagree or to document baseline severity, but it does not discriminate type and is not routinely required.

Urodynamics — reading the study. Multichannel cystometry derives the true detrusor pressure by subtracting abdominal (rectal/vaginal) pressure from intravesical pressure (p_det = p_ves − p_abd), so a pressure rise that appears on both channels is a straining artefact, not a detrusor contraction — the commonest misreading. During filling, phasic rises in p_det are detrusor overactivity; leakage synchronous with a cough and no detrusor rise is urodynamic stress incontinence; the abdominal/Valsalva leak-point pressure estimates sphincter competence (a low value flags ISD). The voiding (pressure-flow) phase distinguishes the two causes of a poor flow that look identical on uroflowmetry alone: a high detrusor pressure with a low flow is obstruction (the over-tight sling, the obstructing prolapse), whereas a low detrusor pressure with a low flow is detrusor underactivity — opposite problems needing opposite management, and the reason a flat trace plus a high residual must never be assumed to be "just" weak voiding. The indication and the limits. The consultant position is that it is not required before first-line conservative or pharmacological treatment, and not required in the uncomplicated index woman with clearly demonstrable, pure stress incontinence proceeding to a standard sling — the VALUE trial showed no outcome benefit from adding pre-operative urodynamics in exactly that group. It earns its place when the picture is not straightforward: predominant mixed symptoms before surgery, suspected voiding dysfunction or a raised residual, neurological disease, recurrent incontinence after a failed previous operation, symptoms that do not match the examination, or where a detrusor-overactivity component would change the surgical consent. Its limits are real — it is invasive, non-physiological (an artificial bladder fill in a laboratory does not always reproduce the woman's daily symptom), and a normal study does not exclude a symptom she genuinely has. Use it to answer a specific question that will change the plan, not as a reflex before every operation.