A patient presents with a seizure. On evaluation, urine osmolality is 1000 mOsm/kg and serum osmolality are 270 mOsm/kg. Which electrolyte abnormality can be expected in this patient?

Correct Answer: A. Hyponatremia

The discriminating finding here is the osmolal gap: urine osmolality (1000 mOsm/kg) is significantly higher than serum osmolality (270 mOsm/kg). This inverted relationship indicates the kidneys are concentrating urine despite low serum osmolality—the hallmark of SIADH (Syndrome of Inappropriate Antidiuretic Hormone).

In SIADH, excessive ADH causes inappropriate water reabsorption in the collecting duct, diluting the serum. The kidneys respond by producing concentrated urine (high urine osmolality) to conserve water, but this worsens serum dilution. The net result is hyponatremia (low serum sodium due to dilution, not sodium loss). Seizures occur when serum sodium drops acutely below 120 mEq/L, causing cerebral edema and neuronal dysfunction. SIADH is a common cause of seizures in hospitalized patients and can be triggered by CNS disorders, malignancy, pulmonary disease, or medications. The key pathophysiology is water retention → plasma dilution → hyponatremia, not potassium or sodium depletion.

Why the other options are wrong

B. Hypokalaemia — This is wrong because SIADH is a disorder of water and sodium balance, not potassium metabolism. Serum potassium is typically normal or only mildly affected in SIADH unless there is concurrent diuretic use or renal disease. The osmolal gap pattern (high urine osmolality with low serum osmolality) is pathognomonic for water retention, not potassium wasting. NBE may trap students who confuse SIADH with diuretic-induced hyponatremia, which can cause hypokalaemia. C. Hyperkalaemia — This is wrong because SIADH causes water retention and dilution, not potassium accumulation. Serum potassium is not directly affected by ADH dysregulation. Hyperkalaemia would suggest renal failure or aldosterone deficiency, neither of which is indicated by the osmolal pattern. This is a distractor that confuses electrolyte abnormalities; the osmolal gap clearly points to water, not potassium, as the problem. D. Hypernatremia — This is wrong because hypernatremia indicates water depletion or sodium excess, which would produce low urine osmolality (kidneys trying to conserve water). The patient has the opposite: high urine osmolality despite low serum osmolality, indicating water retention. Hypernatremia causes seizures via cerebral dehydration, whereas this patient's seizure is from hyponatremic cerebral edema. NBE may use this to trap students who see 'seizure' and assume hypernatremia without analyzing the osmolal gap.

High-Yield Facts

Mnemonics

SIADH osmolal pattern High Urine, Low Serum = Water Retention. If urine osmolality is higher than serum osmolality, kidneys are concentrating urine inappropriately → hyponatremia. Remember: the osmolal gap is inverted in SIADH. SIADH causes in India (SIADH) Small cell lung cancer, Infections (TB, pneumonia), Antipsychotics/SSRIs, Drugs (carbamazepine), Head trauma. TB is the most common cause in Indian patients.

NBE Trap

NBE pairs seizures with electrolyte abnormalities to trap students into assuming hypernatremia (which also causes seizures) without calculating the osmolal gap. The inverted osmolal pattern is the discriminator that must be recognized.

Clinical Pearl

In Indian hospitals, TB-induced SIADH is the most common cause of hyponatremic seizures in young adults. Always calculate the osmolal gap (urine vs. serum) before attributing seizures to hypernatremia—the pattern tells you whether the problem is water or sodium.

_Reference: Harrison Ch. 295 (Hyponatremia); KD Tripathi Ch. 33 (ADH and Water Balance)_