Micturating Cystourethrogram

The micturating cystourethrogram (MCUG, also voiding cystourethrogram or VCUG) is a fluoroscopic study of the bladder and urethra.

A urethral or suprapubic catheter is passed under sterile conditions. The bladder is filled with water soluble contrast (warmed, diluted to about 15 to 30% iodine concentration) to about the expected capacity. Images are taken during filling, at full capacity, and throughout voiding, with the catheter removed or a small catheter left in situ.

In children, expected bladder capacity is (age in years + 2) times 30 mL up to 12 years. Oblique images show reflux and the posterior urethra (in boys, the urethra is imaged in an oblique position during voiding).

The study answers four questions: is there vesicoureteric reflux and of what grade, what is the bladder shape and emptying, what is the urethral anatomy, and is there an outflow obstruction? The effective dose is low, about 0.2 to 1 mSv in children with pulsed fluoroscopy and last-image-hold, and the gonadal dose is the main concern.

Radionuclide cystography has a lower dose but less anatomical detail, and contrast enhanced voiding urosonography uses ultrasound contrast without radiation.

Vesicoureteric reflux is present in about 30 to 40% of children with a urinary tract infection, in 1% of healthy children, and is the commonest cause of renal scarring and hypertension in children. Posterior urethral valves occur in about 1 in 5000 to 8000 boys, and are the commonest cause of lower urinary tract obstruction in boys. The MCUG remains the reference test for boys with suspected valves and for the grading of reflux.

Indications. Febrile or recurrent urinary tract infection in infants and young children with abnormal renal ultrasound, a dilated system, or scarring. Antenatal or postnatal hydronephrosis with suspected obstruction or reflux. Suspected posterior urethral valves (a male infant with a poor stream, a palpable bladder, and a thick walled bladder on ultrasound).

Neurogenic bladder, spina bifida, and recurrent infection. Voiding dysfunction, urinary retention, incontinence, and suspected urethral abnormalities (diverticulum, stricture, fistula). Assessment of ectopic ureter or urethral duplication. Anorectal malformation with suspected urinary fistula.

Adults: urethral stricture, vesicovaginal fistula, stress incontinence assessment and urethral diverticulum. Not after a single, uncomplicated urinary tract infection in a child with normal renal ultrasound, since benefit is not shown by the findings.

Contraindications and precautions. Active urinary tract infection (treat first, and study 2 to 6 weeks after treatment, or with antibiotic cover). Urethral injury (use retrograde urethrography before catheterisation). Severe contrast allergy (the dose absorbed is small but reactions are possible). Pregnancy: avoid.

Careful explanation is needed for the child and carers, because catheterisation and voiding in front of the table cause distress; play preparation, distraction, and a calm room increase success, and sedation is not usually required. Prophylactic antibiotics are given in many centres at the time of the study.

Preparation and technique. No fasting is needed. The child is placed supine, and a catheter (6 to 8 French in children, 12 to 16 in adults) is passed with sterile technique. The bladder is drained and filled by gravity from a drip height of about 100 cm of water, with intermittent screening for reflux.

At capacity (the child urinates around the catheter or resists filling, or the infant voids), the catheter is removed and voiding films are taken in the oblique position in boys, and anteroposterior in girls. A cyclic study (two or three fill and void cycles) increases the detection of reflux in infants.

Normal appearances. The bladder is smooth and round or slightly oval when filled, with thin walls, and a capacity matching age. The trigone and ureteric orifices are not seen. No contrast enters the ureters at any stage of filling or voiding. The bladder empties completely, with a post-void residue under 10% of capacity.

In boys, the urethra shows a narrow membranous part, a slightly dilated posterior urethra lined by a smooth verumontanum, and a smooth bulbar urethra. In girls, the urethra is short (3 to 4 cm), wide and funnel shaped, with a smooth outline. The bladder neck opens early and widely during voiding, and the urethra fills completely.

Interpretation.

• Vesicoureteric reflux, graded as grade I (ureter only), grade II (ureter, pelvis and calyces without dilatation), grade III (mild to moderate dilatation of the ureter and pelvis, with mild blunting of the fornices).
• Higher grades: grade IV (moderate dilatation with tortuous ureter, and loss of papillary impressions but calyces still visible), grade V (severe dilatation, tortuous ureter, and loss of calyceal and papillary details, with intrarenal reflux).
• Reflux that occurs during filling (low pressure, "passive") is more severe than reflux during voiding (high pressure, "active"). Grades I to III resolve spontaneously in 80% within 5 years, and grades IV to V less often.
• Posterior urethral valves: a dilated, elongated posterior urethra proximal to the valve, the valves seen as a linear filling defect or a thin membrane (obstruction at the level of the verumontanum), and a hypertrophied bladder neck.
• Associated findings: a thick walled, trabeculated bladder with diverticula, and often reflux (one third), with a narrow distal urethra jet. The "keyhole" sign is seen on ultrasound.
• Bladder abnormalities: neurogenic bladder (a "Christmas tree" or "pine cone" shaped, trabeculated bladder with a high capacity and diverticula), bladder diverticula, a thick walled, small capacity bladder (obstruction, cystitis), bladder exstrophy, a persistent urachus, and ureterocele (a filling defect at the bladder base).
• Urethral abnormalities: anterior urethral valves and diverticula, urethral strictures (an irregular narrowing with a proximal dilated segment), urethral duplication, hypospadias, ectopic ureter inserting into the urethra, and cloacal malformations.
• Voiding dysfunction: spinning top urethra (dilated proximal urethra with a lack of bladder neck relaxation) in girls, and dysfunctional voiding (intermittent contractions of the external sphincter).
• Fistulae: vesicovaginal, vesicorectal, urethrorectal (an anorectal malformation shows contrast in the rectum or vagina), and urethrovaginal.
• Female adults: urethral diverticulum (a contrast filled outpouching posterior to the urethra), stress incontinence (bladder neck descent and funnelling), and a cystocele.

Critical values and findings needing immediate action.

• Posterior urethral valves in a boy with a poor stream, renal failure or sepsis: bladder catheterisation, correction of fluid and electrolytes, antibiotics, and paediatric urology referral for valve ablation.
• High grade reflux (grades IV to V) with scarring and recurrent infection: antibiotic prophylaxis and paediatric urology or nephrology review.
• Contrast extravasation or a bladder rupture: stop filling, drain the bladder and arrange surgical review.
• Urosepsis after the procedure (fever, rigors, hypotension): blood and urine cultures, intravenous antibiotics, and resuscitation.
• Urinary retention with an obstructed system and rising creatinine: catheter drainage and urgent specialist review.

Limitations and artefacts.

• Reflux is intermittent, and a negative study does not exclude it; cyclic filling, a larger fill volume, and a repeat study increase the yield.
• Anxious children may not void on the table, or the contrast may be diluted; a failure to void leaves reflux and posterior urethral valves undetected.
• Catheterisation causes a urinary tract infection in about 1 to 3% (less with prophylactic antibiotics), trauma to the urethra, bladder spasm, and distress; psychological harm is reduced by preparation and distraction.
• Reflux grade is subjective, and the finding depends on filling pressure and rate.
• Contrast study does not show renal scarring or function; the dimercaptosuccinic acid scan is the test for scarring and the ultrasound shows renal size and hydronephrosis.
• Radiation dose to the gonads is not trivial, so repeat studies should be limited, and radionuclide cystography or contrast enhanced voiding urosonography should be used for follow up in girls.
• Overdistension causes transient reflux and false abnormalities; a catheter may hide a urethral valve or stricture, and the catheter should be removed for the voiding phase.
• Interpretation of a normal variant, such as a bladder neck "pseudo obstruction" and the "bladder ears" in infants (inguinal herniation of the bladder), must be careful.

Special groups.

• Infants: the commonest group, with a high rate of reflux, particularly in boys at birth (often resolving by 2 years).
• Boys with antenatal hydronephrosis: imaging within the first days to weeks after birth to exclude valves.
• Girls: reflux and urethral abnormalities; radionuclide cystography is preferred for follow up studies.
• Children with spina bifida: urodynamics and cystography for bladder pressure and reflux.
• Adults: urethral stricture assessment combines MCUG with retrograde urethrography.
• Pregnancy: avoid.
• Regions: urinary schistosomiasis, tuberculosis, and delayed presentation of valves with renal failure are common, and a cystogram with a simple catheter and a fluoroscopy unit may be the only means of diagnosis.

Follow up tests. Renal and bladder ultrasound, dimercaptosuccinic acid scan for scarring, mercaptoacetyltriglycine renography for obstruction and function, radionuclide cystography for follow up of reflux, contrast enhanced voiding urosonography, urodynamic studies, urethrocystoscopy.

Also urine culture and microscopy, serum creatinine and electrolytes, blood pressure, urine protein to creatinine ratio, and referral to paediatric urology or nephrology.