What type of hyponatremia occurs in urinary retention secondary to bladder outlet obstruction?

Dilutional Hyponatremia Occurs in Urinary Retention Secondary to Bladder Outlet Obstruction

Urinary retention secondary to bladder outlet obstruction causes dilutional hyponatremia with characteristics similar to SIADH (Syndrome of Inappropriate Antidiuretic Hormone secretion). This type of hyponatremia presents with elevated urinary sodium levels and inappropriately concentrated urine despite low serum sodium 1, 2.

Pathophysiology

• Urinary retention due to bladder outlet obstruction leads to increased pressure in the urinary tract, triggering non-osmotic release of ADH 2
• The increased ADH causes water retention, leading to dilutional hyponatremia with laboratory findings mimicking SIADH 1, 2
• This condition presents with euvolemic hyponatremia characterized by:
  • Low serum sodium (<135 mEq/L) 1
  • Low serum osmolality (<275 mosm/kg) 1
  • Inappropriately elevated urinary osmolality (>300 mosm/kg) 1
  • Elevated urinary sodium (>20-40 mEq/L) 1

Diagnosis

• Measure serum and urine osmolality, urinary sodium, and assess volume status 1
• Look for signs of urinary retention:
  • Distended bladder on physical examination 3
  • Inability to urinate during handling or abdominal palpation 3
  • Elevated post-void residual (PVR) volume 3
• Perform pressure flow studies (PFS) to evaluate for bladder outlet obstruction (BOO) 3
• Distinguish from other causes of hyponatremia with elevated urinary sodium:
  • SIADH (euvolemic) 1
  • Cerebral salt wasting (hypovolemic) 1
  • Advanced renal failure (hypervolemic) 1

Management

• The definitive treatment is relief of urinary obstruction through catheterization 2
• After catheter placement, monitor for rapid autocorrection of sodium levels 2
• Consider hypotonic fluid administration if rapid autocorrection occurs to prevent osmotic demyelination syndrome 2
• Avoid hypertonic saline or normal saline as these can exacerbate rapid autocorrection 2
• Limit sodium correction to no more than 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 4

Important Considerations

• This type of hyponatremia is unique because sodium autocorrection begins after urinary catheter placement 2
• Careful monitoring is essential to prevent complications from overly rapid correction 2
• Patients with advanced liver disease, alcoholism, malnutrition, or prior encephalopathy require more cautious correction (4-6 mmol/L per day) 4
• For patients with BPH causing urinary retention, consider alpha-adrenergic blockers (alfuzosin, doxazosin, tamsulosin, terazosin) as medical therapy 3
• Surgical intervention may be necessary for persistent bladder outlet obstruction 3

Common Pitfalls

• Misdiagnosing the type of hyponatremia can lead to inappropriate treatment 1
• Treating with hypertonic saline or normal saline can worsen rapid autocorrection after catheterization 2
• Failing to monitor sodium levels closely after relieving obstruction 2
• Not recognizing urinary retention as the underlying cause of hyponatremia 2