Posterior Pituitary Disorders

Question 1

Diabetes Insipidus (Central)

Answer 1

• Definition:
  
  • inability of the kidney to concentrate urine → production of large amounts of dilute urine caused by no production of AVP → water leaves collecting duct→ urine output increases
• Etiology:
  
  • idiopathic, destruction of posterior pituitary, head trauma, CNS tumor, infx
• S/sxs:
  
  • increased urinary frequency, polyuria, nocturia +/- enuresis, polydipsia
• PE:
  
  • normal, may show signs of dehydration
• Dx:
  
  • Urine output: increased (>40mL/kg/24hours)
  • Thirst: increased
  • Serum AVP: low (<1pg/mL)
  • Urine Osmolality: low (<300 mOsm/L)
  • Serum Sodium: high
  • Serum osmolality: high
  • Water deprivation Test: patient produces large amounts of dilute urine (even though they aren’t drinking water)
  • MRI:
    
    • evaluate posterior pituitary for “bright spot”(if absent → likely central DI) & pituitary stalk for trauma, hypophysitis, mass
• Tx:
  
  • assess pt’s ability to keep up with hydration
  • endocrine referral: for discussion of water deprivation testing and use of DDAVP

Question 2

Diabetes Insipidus (Nephrogenic)

Answer 2

• Definition:
  
  • inability of kidney to concentrate urine → production of large amounts of dilute urine due to partial or complete renal insensitivity to AVP at the level of the kidney
• Etiology:
  
  • congenital, pregnancy, meds (lithium, demeclocycline)
• S/sxs:
  
  • urinary frequency
  • polyuria, nocturia +/- enuresis, polydipsia
  • severe: neurologic sxs of hypernatremia: confusion, lethargy, seizures, coma
• PE:
  
  • normal (may show signs of dehydration: poor skin turgor, dry mucous membranes, drinking lots of water)
• Dx:
  
  • Urine Output: increased (>40 mL/kg/24 hours)
  • Thirst: increased
  • Serum AVP: high (>1pg/mL)
  • Urine Osmolality: low (<300 mosmol/L)
  • Serum Sodium: high
  • Serum Osmolality: high
  • **MRI has no role in dx
• Tx:
  
  • Thiazide diuretic (hydrochlorothiazide) &/or amiloride with low sodium diet
  • identify cause: d/c offending med if possible
  • Nephro consult
  • Pregnant: consult with OB before meds

Question 3

SiADH

Answer 3

• Definition:
  
  • excessive AVP production that impairs urinary water excretion & predisposes the pt to euvolemic hyponatremia if water intakes is not reduced
• Pathophys:
  
  • impaired urine dilution often caused by defect in osmotic suppression of AVP
• Etiology:
  
  • malignancy, brain trauma, stroke, subarachnoid hemorrhage, PNA, lung CA, meds
• S/sxs:
  
  • Severe: hyponatremia: confusion, lethargy, disorientation, seizures, coma
• PE:
  
  • normal
  • weight gain: increased in total body water by up to 10%
• Dx:
  
  • Urine Output: normal
  • Thirst: normal
  • Serum AVP: not measured
  • Urine Osmolality: high (>300 mOsmol/L)
  • Urine Sodium: high (>40 mEq/L)
  • Serum sodium: low (125-130)
  • Serum Osmolality: low
• Tx:
  
  • identify cause: d/x med if possible, look for malignancy & tx, look for lung (small cell lung cancer) or brain pathology & tx
  • free water restriction (<2 L/day)
  • endocrine or nephrology consult if no resolution: may need hypertonic (3%) NS infusion or tolvaptan/conivaptan (V2 agonists)