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

Step 1: Understanding the Question:
A 33 year old woman develops severe thirst and passing large volumes of urine (6 to 8 liters a day) starting soon after a road traffic accident 6 months earlier. Her blood pressure is normal with no postural drop, her sodium is slightly low at 130 mEq/L, blood glucose is normal at 65 mg/dL (ruling out diabetes mellitus), plasma osmolality is 268 mosmol/L and urine osmolality is only 45 mosmol/L. We need the most likely diagnosis.

Step 2: Key Concept:
Antidiuretic hormone (ADH, also called vasopressin) is made in the hypothalamus and released from the posterior pituitary, and it acts on the kidney's collecting ducts to make urine concentrated. Head trauma can shear the pituitary stalk or damage the hypothalamus, and this is one of the most common causes of central diabetes insipidus, where ADH release fails and the kidneys keep passing large volumes of dilute urine no matter how much water the body needs.

Step 3: Detailed Explanation:
A urine osmolality of only 45 mosmol/L means the kidney is producing very dilute urine, which shows a clear failure to concentrate urine appropriately. The temporal link to a road traffic accident 6 months earlier is the key clue, since head injury is a classic and well recognized cause of central diabetes insipidus through direct damage to the hypothalamic-pituitary axis that makes and releases ADH. Because ADH is absent, the kidneys cannot hold on to water, so the patient loses large volumes of dilute urine and drinks heavily to keep up, which over time can pull the plasma sodium and osmolality down toward the lower end of normal as seen here. Nephrogenic diabetes insipidus is possible when the kidney fails to respond to ADH, but it usually follows a clear cause such as lithium use, chronic kidney disease or long standing electrolyte problems, none of which are mentioned here, so the post-traumatic history fits central diabetes insipidus much better. Resolving acute tubular necrosis typically follows an episode of shock or a nephrotoxic insult with oliguria before the polyuric recovery phase, and it would be expected to settle within days to a few weeks, not persist unchanged for 6 months, and there is no history here of an acute kidney injury event. Psychogenic polydipsia is a compulsive water drinking disorder without an organic trigger, and the clear timing with a head injury argues against a purely psychiatric cause. Normal blood pressure without a postural drop also argues against a volume depleted state such as resolving ATN.

Step 4: Final Answer:
The most likely diagnosis is central diabetes insipidus following the head injury, option (1). Note that in practice, the diagnosis would be confirmed with a water deprivation test showing a rise in urine osmolality after giving desmopressin.