What amount of urine on a bladder scan is concerning in a full-term newborn with urinary retention?

Bladder Scan Volumes in Full-Term Newborns with Urinary Retention

In full-term newborns with urinary retention, a bladder scan volume greater than 30 ml is concerning and warrants intervention. 1

Normal Bladder Physiology in Newborns

• Water contributes to approximately 75% of body weight in term infants, compared to 90% in premature infants and 50% in adults 1
• Newborns have smaller renal glomerular surface area available for filtration compared to older infants and adults, affecting urine production rates 1
• Maximum urinary concentration capacity in term infants reaches only up to 700 mosm/l, compared to 1200 mosm/l in adults, limiting their ability to concentrate urine 1
• Water turnover is high in neonates and decreases with increasing age and the concomitant decrease of metabolic rate and growth velocity 2

Assessment of Urinary Retention in Newborns

• When performing clean intermittent catheterization in newborns, volumes less than 30 ml per catheterization for the majority of catheterizations for 3 consecutive days indicate adequate bladder emptying 1
• In neonates, initial imaging should be delayed at least 48-72 hours after birth due to relatively low urine production in the immediate postnatal period 2
• Exceptions requiring earlier imaging include newborns with severe bilateral hydronephrosis, bladder abnormalities, oligohydramnios, or situations where follow-up studies may be difficult to obtain 2

Diagnostic Approach to Urinary Retention

• Renal and bladder ultrasonography (RBUS) is recommended for evaluation of febrile infants with UTIs, which may be associated with urinary retention 2
• It is important that the study be a complete renal and bladder ultrasonogram, not just a limited renal ultrasonogram 2
• The patient should be well-hydrated for the examination and the bladder should be evaluated while distended 2

Management Algorithm for Urinary Retention in Newborns

1. Initial assessment: Evaluate for bladder distension and perform bladder scan

   • If volume >30 ml, consider intervention 1

2. Rule out obstructive causes:

   • Posterior urethral valves (PUV) - most common cause of neonatal bladder outlet obstruction, occurring in 0.2-1% of cases of mild antenatal hydronephrosis 2
   • With high suspicion for PUV (bladder wall thickening, dilated posterior urethra on ultrasound), the bladder should be catheterized immediately 2

3. Consider non-obstructive causes:

   • Urinary tract infection (UTI) - prevalence in term neonates varies from 0.1% to 1%, with male predominance in first 2 months 2
   • Neurological issues affecting bladder function 2

4. Intervention:

   • For acute retention, prompt and complete bladder decompression by catheterization is indicated 3
   • Suprapubic catheters may improve patient comfort and decrease bacteriuria in the short term 3

Important Considerations

• Concomitant bacteremia with UTI is common in neonates and was observed in 4% to 36.4% of cases 2
• Neonates with UTI have a high incidence of urinary anomalies; the most common is vesicoureteral reflux (VUR) 2
• In male newborns with UTI, voiding cystourethrography (VCUG) may detect VUR even if ultrasound is normal 2
• A single episode of UTI in premature infants without congenital anomalies of the kidney and urinary tract (CAKUT) may not constitute a risk factor for recurrence of infections or kidney injury 4

Pitfalls to Avoid

• Do not delay intervention for significant urinary retention (>30 ml) as this can lead to bladder decompensation and kidney damage 1
• Do not rely solely on ultrasound to rule out vesicoureteral reflux, as it has low sensitivity for detecting VUR and renal scarring 2
• Avoid unnecessary catheterization for minimal residual volumes (<30 ml) as this can increase risk of infection 1
• Do not miss posterior urethral valves in male infants with urinary retention, as this requires immediate urological intervention 2