Seizure in an Elderly Man on a Thiazide
72-year-old man with hypertension, on hydrochlorothiazide 25 mg daily for the past 6 weeks, is brought to the emergency department after a witnessed generalized tonic-clonic seizure. He had 2 days of vomiting and poor oral intake and has been confused since morning. On examination he is drowsy but arousable, afebrile, pulse 88/min, BP 138/84 mmHg, no postural drop, mucous membranes moist, no edema, chest clear, and no focal neurological deficit. Laboratories: serum sodium 112 mEq/L, potassium 3.4 mEq/L, chloride 82 mEq/L, bicarbonate 22 mEq/L, glucose 96 mg/dL, urea 18 mg/dL, creatinine 1.0 mg/dL, serum osmolality 238 mOsm/kg, urine sodium 48 mEq/L, urine osmolality 420 mOsm/kg. Non-contrast CT of the head is normal. The seizure stops after intravenous lorazepam. He takes no other medications and has no history of alcohol use or known pituitary or adrenal disease.
1. How would you classify this hyponatremia, and what is the likely cause? Explain the serum and urine studies rather than simply naming a diagnosis.
Read the worked answer
The serum osmolality of 238 mOsm/kg confirms true hypotonic hyponatremia. Pseudohyponatremia is unlikely because glucose is normal and there is no history suggesting severe hyperlipidemia or paraproteinemia. The urine osmolality of 420 mOsm/kg is inappropriately concentrated for this degree of hyponatremia, and the urine sodium of 48 mEq/L shows that the kidney is not conserving sodium maximally. The patient is clinically euvolemic: no postural drop, no tachycardia, moist mucosae, and no edema. The likely cause is thiazide-induced sodium wasting plus vomiting and reduced intake, producing ADH-driven water retention and a dilutional, euvolemic picture that resembles SIADH. Thiazides impair free-water excretion and can cause hyponatremia even when volume depletion is not obvious. This is not hypovolemic hyponatremia, where urine sodium is usually low, and it is not hypervolemic because there is no edema or ascites. The exact subtype matters less than recognizing that severe symptomatic hyponatremia with seizure is a neuroemergency requiring controlled hypertonic saline, not unrestricted isotonic fluid.
2. Outline your immediate sodium management, including targets, monitoring, and the main safety concern.
Read the worked answer
Because of the seizure, give 3% hypertonic saline 100 mL IV over 20 minutes. This typically raises sodium by about 2-3 mEq/L and is often enough to abort severe neurological symptoms. Repeat the bolus up to two more times if symptoms persist, checking sodium after each bolus. The first-hour goal is a 4-6 mEq/L rise to control cerebral edema; once symptoms improve, stop active correction and aim for no more than 8-10 mEq/L in 24 hours and 18 mEq/L in 48 hours. Check serum sodium every 2 hours during active correction. Do not use 0.9% saline as the main fluid in this SIADH-like state, because after the initial phase it can worsen hyponatremia. Stop the thiazide, treat vomiting, and correct potassium. If sodium rises faster than planned, give DDAVP 1-2 µg IV and 5% dextrose with close monitoring. The main safety concern is osmotic demyelination from overrapid correction, particularly in an older, malnourished patient. Monitor urine output, neurological status, and electrolytes.
3. Twelve hours later his sodium is 124 mEq/L and he is alert. What are the next steps, and what complication would you monitor for?
Read the worked answer
A rise from 112 to 124 mEq/L in 12 hours is about 12 mEq/L, which exceeds the recommended 8-10 mEq/L per 24 hours if the trend continues. First recheck the sample to exclude dilution or sampling error. If confirmed, actively prevent further rise: stop hypertonic saline, give DDAVP to lock the urine output, and use 5% dextrose at a rate guided by urine output to bring the sodium back toward the safe target. At 12 hours the total rise should ideally be under about 10 mEq/L, so the sodium should be lowered slightly rather than allowed to climb further. Monitor for osmotic demyelination, which can appear 2-6 days later with dysarthria, dysphagia, quadriparesis, ataxia, or locked-in syndrome. MRI is supportive but treatment should not wait for imaging. Also watch for rebound cerebral edema if the sodium is lowered too aggressively. Continue treating the cause, avoid restarting thiazide, and document a fixed monitoring protocol with 2-hourly sodium until stable.