Renal · Year 2 · from Renal
Case 1: Central Diabetes Insipidus Post-Pituitary Surgery
Patient Presentation
A 34-year-old female undergoes transsphenoidal resection of a pituitary macroadenoma. On post-operative day 2, she develops sudden onset of severe thirst and produces large volumes of dilute urine.
History of Present Illness
- Transsphenoidal surgery 48 hours ago for non-functioning pituitary adenoma
- Producing 400-500 mL of urine per hour
- Intense thirst, drinking 4-5 liters of water since last night
- No headache or visual changes
- Serum sodium was 141 mEq/L pre-operatively
Physical Examination
- Blood pressure: 118/72 mmHg
- Heart rate: 88 bpm
- Mucous membranes dry
- Neurologic exam intact
- No signs of volume depletion (normal skin turgor)
Workup
Laboratory Studies:
- Serum sodium: 152 mEq/L (elevated)
- Serum osmolality: 312 mOsm/kg (elevated)
- Urine osmolality: 85 mOsm/kg (inappropriately dilute)
- Urine specific gravity: 1.002
- Urine output: 8.5 liters in 24 hours
Water Deprivation Test (abbreviated):
- After 4 hours of water deprivation: Urine osmolality 95 mOsm/kg (no concentration)
- After desmopressin (DDAVP) administration: Urine osmolality 650 mOsm/kg (appropriate response)
Diagnosis
Central Diabetes Insipidus (Post-surgical)
Discussion
This case demonstrates the role of ADH in urine concentration:
- ADH Mechanism: The lecture explains that ADH binds to V2 receptors on collecting duct principal cells, activating cAMP signaling and triggering AQP2 vesicle insertion into the apical membrane. Without ADH, the collecting duct remains water-impermeable.
- Central vs Nephrogenic DI: In central DI, ADH is not produced. The dilute urine delivered from the thick ascending limb (the "diluting segment") passes through the collecting duct unchanged because water cannot exit. Response to exogenous DDAVP confirms central etiology.
- Medullary Gradient: The hyperosmotic medullary gradient (up to 1200 mOsm/kg) remains intact in central DI but cannot be utilized without ADH-mediated water permeability.
Treatment
- DDAVP (desmopressin) 1 mcg IV initially, then intranasal or oral maintenance
- Free water replacement to correct hypernatremia (target correction <10-12 mEq/L/24 hours)
- Monitor for triphasic response (initial DI, then SIADH from dying neurons releasing ADH, then permanent DI)
- Endocrinology follow-up for long-term management
Clinical Pearl
Post-pituitary surgery diabetes insipidus often follows a "triphasic" pattern: initial DI (days 1-5), transient SIADH from unregulated ADH release (days 5-10), then permanent DI if the posterior pituitary is damaged.