Partial Central Diabetes Insipidus (Answer: B)
The most likely diagnosis is partial central diabetes insipidus, as the patient demonstrates some residual ADH production (evidenced by partial urine concentration during water deprivation) and further improvement with exogenous desmopressin, confirming responsiveness to ADH replacement. 1
Diagnostic Reasoning
The clinical scenario describes a classic diagnostic pattern that distinguishes between different forms of diabetes insipidus:
Partial urine concentration after water deprivation indicates the kidneys retain some ability to concentrate urine, suggesting either residual endogenous ADH production (partial central DI) or partial kidney responsiveness to ADH 2
Further improvement with desmopressin administration is the critical differentiating feature—this response confirms the kidneys are responsive to ADH and that the primary defect lies in insufficient ADH production, not kidney resistance 1, 3
Why Other Diagnoses Are Excluded
Complete Central DI (Option A) - Excluded
- Patients with complete central DI show no urine concentration during water deprivation because they produce no endogenous ADH 2
- The fact that this patient achieves partial concentration rules out complete deficiency 1
Nephrogenic DI (Option C) - Excluded
- Nephrogenic DI shows little to no response to desmopressin because the kidneys are resistant to ADH 4, 5
- The patient's improvement with desmopressin definitively excludes this diagnosis 1, 6
- In nephrogenic DI, urine osmolality typically remains around 100 mOsm/kg even after desmopressin administration 5
SIADH (Option D) - Excluded
- SIADH presents with hyponatremia, low serum osmolality, and inappropriately high urine osmolality—the opposite clinical picture from diabetes insipidus 7
- Patients with SIADH have concentrated urine despite low serum osmolality, not dilute urine 8
Primary Polydipsia (Option E) - Excluded
- Primary polydipsia patients can concentrate urine normally during water deprivation once they stop excessive fluid intake 1
- Desmopressin administration provides no additional benefit in primary polydipsia because these patients already have normal ADH production and kidney responsiveness 9
- The improvement with desmopressin rules out this diagnosis 1
Clinical Significance of the Response Pattern
The water deprivation test demonstrates that some endogenous ADH is present (partial concentration occurs), but production is insufficient 2
The desmopressin response proves the collecting ducts retain normal V2 receptor function and can respond to ADH when adequate amounts are provided 8, 10
This two-step response pattern—partial concentration followed by further improvement—is pathognomonic for partial central diabetes insipidus 1, 10
Management Implications
Desmopressin is the treatment of choice for partial central DI, with typical dosing of 0.2-0.4 mg orally or 2-4 mcg subcutaneously/intravenously 8
Serum sodium must be monitored within 7 days and at 1 month after initiating desmopressin, then periodically, as hyponatremia is the main complication 1, 8
Patients should limit fluid intake from 1 hour before until 8 hours after desmopressin administration to prevent water intoxication 8
MRI of the sella with dedicated pituitary sequences is recommended to identify structural causes, as approximately 50% of central DI cases have identifiable etiologies including tumors, infiltrative diseases, or inflammatory processes 1