A 28-year-old woman with a history of recent head trauma presents with sudden onset of polyuria (10 L/day), polydipsia, and hypernatremia. Serum osmolality is 315 mOsm/kg and urine osmolality is 180 mOsm/kg. Which investigation would be most appropriate to confirm the suspected diagnosis of central diabetes insipidus?

Explanation

## Diagnostic Confirmation of Central Diabetes Insipidus ### Clinical Presentation - Recent head trauma (suggests pituitary/hypothalamic injury) - Acute polyuria and polydipsia - Hypernatremia and elevated serum osmolality - Inappropriately dilute urine (osmolality 180 mOsm/kg despite high serum osmolality) **Key Point:** This clinical picture is highly suggestive of **central (neurogenic) diabetes insipidus** secondary to head trauma. ### Diagnostic Algorithm for DI ```mermaid flowchart TD A[Polyuria + Hypernatremia]:::outcome --> B[Measure serum and urine osmolality]:::action B --> C{Urine osmolality low<br/>despite high serum osmolality?}:::decision C -->|Yes| D[Perform water deprivation test]:::action C -->|No| E[Other diagnosis]:::outcome D --> F{Urine osmolality<br/>increases >600?}:::decision F -->|No| G[DI confirmed]:::outcome G --> H[Administer DDAVP]:::action H --> I{Urine osmolality<br/>increases significantly?}:::decision I -->|Yes| J[Central DI]:::outcome I -->|No| K[Nephrogenic DI]:::outcome ``` ### Why Water Deprivation + DDAVP Test? | Step | Purpose | Expected Finding in Central DI | |------|---------|--------------------------------| | **Water deprivation test** | Confirm DI by demonstrating inability to concentrate urine despite osmotic stimulus | Urine osmolality remains <300 mOsm/kg; serum osmolality rises | | **DDAVP administration** | Differentiate central from nephrogenic DI | Urine osmolality rises >600 mOsm/kg (kidneys respond to exogenous ADH) | **High-Yield:** The **water deprivation test followed by DDAVP** is the **functional diagnostic standard** for confirming central DI. It is: - Cost-effective - Widely available - Highly specific - Demonstrates both the defect (inability to concentrate urine) and the mechanism (response to exogenous ADH) **Mnemonic:** **WD-DDAVP** = Water Deprivation followed by DDAVP is the diagnostic sequence for DI differentiation. **Clinical Pearl:** In central DI from acute head trauma, the response to DDAVP is often dramatic and rapid, confirming the diagnosis and guiding treatment (desmopressin replacement).