Diagnosis: Central Diabetes Insipidus
This patient has central diabetes insipidus, characterized by inappropriately low ADH (<0.8 pg/mL) in the setting of high-normal serum osmolality (295 mOsm/kg) and inappropriately dilute urine (osmolality 220 mOsm/kg), and the treatment is desmopressin (DDAVP) starting at 0.1-0.2 mg orally at bedtime. 1
Diagnostic Interpretation
The laboratory findings definitively point to central diabetes insipidus:
- Serum sodium is normal (143 mEq/L) but the serum osmolality is at the upper limit of normal (295 mOsm/kg), which should trigger maximal ADH secretion 1
- ADH is undetectable (<0.8 pg/mL) when it should be elevated given the serum osmolality, indicating central ADH deficiency 1
- Urine osmolality is inappropriately low (220 mOsm/kg) relative to serum osmolality—the kidneys are producing dilute urine when they should be concentrating it 1
- Urine output is elevated (1300 mL documented) with dilute urine, consistent with diabetes insipidus 1
- Low urine sodium (26 mEq/L random, 34 mEq/24hr) indicates the kidneys are appropriately conserving sodium but cannot concentrate urine due to lack of ADH 2
This is not hypernatremia (sodium is 143 mEq/L, which is normal), but rather a patient with central diabetes insipidus who has maintained normal sodium through adequate fluid intake. Without treatment, this patient is at high risk for developing true hypernatremia if fluid intake becomes inadequate. 3, 4
Management Algorithm
Initial Treatment with Desmopressin
Start desmopressin 0.1-0.2 mg orally at bedtime, then titrate upward to 0.2-0.6 mg daily in divided doses based on urine output and serum sodium. 1 The goal is to reduce urine output to the normal range (1-2 L/day) and maintain serum sodium between 135-145 mEq/L. 1
Dose Titration Strategy
- Increase desmopressin incrementally by 0.1 mg every 3-7 days until urine output normalizes and serum sodium stabilizes 1
- Most patients require 0.2-0.6 mg daily in 1-2 divided doses 1
- Twice-daily dosing may be needed for 24-hour coverage given the 10-12 hour duration of action 1, 5
Critical Monitoring Parameters
Monitor the following parameters closely during treatment initiation:
- Serum sodium and osmolality at least once within the first week, then periodically, to prevent water intoxication and hyponatremia 1
- Daily urine output and specific gravity initially, then as clinically indicated, to assess treatment effectiveness 1
- Body weight daily initially to detect fluid retention 1
- Continue monitoring with any intercurrent illness or unexplained symptoms to prevent hyponatremia 1
Critical Pitfalls to Avoid
The most dangerous error is forcing fluid restriction or excessive fluid intake. Patients should drink to thirst, as desmopressin will prevent excessive urination. 1 Forced fluid restriction can cause dangerous hyponatremia once desmopressin is started. 1
Watch for water intoxication, especially during intercurrent illnesses. Obtain serum sodium immediately if the patient develops unexplained headache, abdominal discomfort, nausea, or neurologic symptoms, which may indicate hyponatremia from excessive desmopressin effect. 1, 5
Do not confuse this with SIADH or cerebral salt wasting. The undetectable ADH level definitively rules out SIADH (which is characterized by excessive ADH). 2, 3 The normal serum sodium and low urine sodium also argue against cerebral salt wasting, which typically presents with hyponatremia and high urine sodium. 2
Why Other Diagnoses Don't Fit
This is not SIADH because ADH is undetectable rather than elevated, and urine osmolality is dilute rather than concentrated. 2, 3
This is not cerebral salt wasting because serum sodium is normal (not low) and 24-hour urine sodium is low (34 mEq/24hr, below the reference range of 40-220), indicating sodium conservation rather than wasting. 2
This is not heart failure requiring diuresis—the patient has no volume overload, and the provided heart failure guidelines 2, 6, 7 are not applicable to this clinical scenario of diabetes insipidus.