Question

A 33 years old lady presents with polydipsia and polyuria. Her symptoms started soon after a road traffic accident 6 months ago. The blood pressure is 120/80 mm Hg with no postural drop. The daily urinary output is 6-8 liters. Investigations showed Na 130 mEq/l, blood glucose 65 mg/dL, plasma osmolality 268 mosmol/l and urine osmolality 45 mosmol/l. The most likely diagnosis is:

• A. Central diabetes insipidus
• B. Nephrogenic diabetes insipidus
• C. Resolving acute tubular necrosis
• D. Psychogenic polydipsia

Hint:

Polyuria starting after a head injury, with persistently dilute urine, points to failure of ADH release.

Answer 1

The Correct Option is A

This vignette is testing the classic link between head trauma and diabetes insipidus, along with the ability to read plasma and urine osmolality together. Let's check each option.

1. Central diabetes insipidus: Head injury 6 months earlier is a well known trigger for damage to the hypothalamic-pituitary axis that makes ADH. Without ADH, the kidney cannot concentrate urine, giving the very low urine osmolality of 45 mosmol/L seen here, huge urine volumes, and heavy compensatory water drinking that can push plasma sodium and osmolality down toward the lower normal range, as seen in this patient.
2. Nephrogenic diabetes insipidus: Here the kidney does not respond to ADH even though it is present, usually from causes like lithium therapy, longstanding hypercalcemia or chronic kidney disease. None of these triggers are mentioned, and the timing right after trauma fits a central cause far better than a kidney-based one.
3. Resolving acute tubular necrosis: This is a polyuric recovery phase after a bout of acute kidney injury, usually caused by shock or a toxic insult, and it settles within days to a couple of weeks. Six months of unchanged symptoms and no history of an acute kidney injury event rule this out.
4. Psychogenic polydipsia: This is compulsive excess water drinking without an organic trigger, usually seen in psychiatric patients, and there is no such history here. The clear onset after a head injury points to an organic cause instead.

The combination of a preceding head injury, huge urine output, and a very dilute urine that fails to concentrate despite a low-normal plasma osmolality fits central diabetes insipidus caused by post-traumatic damage to ADH release.

Let's summarize:

• Head trauma is a classic cause of central diabetes insipidus through hypothalamic-pituitary injury.
• A very dilute urine despite large fluid losses, with no drug or renal cause, favors a central rather than a nephrogenic or psychiatric explanation.

So the diagnosis is central diabetes insipidus, option (1).