An 80‑year‑old woman on telmisartan, cilnidipine, and bisoprolol develops hyponatremia (serum sodium 120 mmol/L) after a few weeks of chlorthalidone (Dytor); how should this be managed?

Management of Chlorthalidone-Induced Hyponatremia (Sodium 120 mmol/L)

Immediately discontinue chlorthalidone and initiate fluid restriction to 1000-1500 mL/day, as this 80-year-old woman has developed severe thiazide-induced hyponatremia requiring urgent intervention. 1, 2

Immediate Actions

Stop the offending agent: Chlorthalidone must be discontinued immediately, as thiazide diuretics are a well-established cause of severe hyponatremia, particularly in elderly women who represent the highest-risk population 3, 4. The elderly are at heightened risk for thiazide-associated hyponatremia, with risk factors including advanced age, female sex, and the combination with other medications that impair free water clearance 1, 3.

Assess symptom severity: Determine whether the patient has severe symptoms (seizures, altered mental status, coma) or is asymptomatic/mildly symptomatic 2. For severe symptomatic hyponatremia, administer 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours or until symptoms resolve, but never exceed 8 mmol/L total correction in 24 hours 2. For asymptomatic or mildly symptomatic patients at this sodium level (120 mmol/L), fluid restriction and supportive care are appropriate 2.

Critical Correction Rate Guidelines

The maximum sodium correction must not exceed 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome. 1, 2 Given this patient's advanced age (80 years), the correction should be even more conservative at 4-6 mmol/L per day 1. Overly rapid correction of chronic hyponatremia—which thiazide-induced hyponatremia typically represents—can cause devastating neurological complications including central pontine myelinolysis, characterized by dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis appearing 2-7 days after overcorrection 2, 5.

Primary Treatment Strategy

Fluid restriction to 1000-1500 mL/day is the cornerstone of management for this hypervolemic or euvolemic hyponatremia 2. This patient likely has a combination of impaired urinary dilution from thiazide effects, possible increased fluid intake, and sodium/potassium depletion 3.

Electrolyte repletion is essential: Aggressively correct potassium and magnesium deficiencies, as thiazides cause renal cation losses that contribute to the hyponatremia 3. Cation repletion helps restore normal sodium handling and supports the correction process 3.

Pathophysiology Considerations

The mechanism of thiazide-induced hyponatremia involves multiple factors: impaired urinary dilution through inhibition of the sodium-chloride cotransporter in the distal tubule, renal sodium and potassium losses, possible stimulation of ADH release, reduced distal delivery of filtrate, and potentially a dipsogenic (thirst-inducing) effect 3, 6. This patient's combination of telmisartan (an ARB) with chlorthalidone may have compounded the risk, as ARBs can contribute to sodium retention abnormalities 1.

Monitoring Protocol

Check serum sodium every 4-6 hours initially during active correction to ensure the rate stays within safe limits 2. Monitor for signs of overcorrection and watch for neurological deterioration that might signal osmotic demyelination syndrome 2, 5. Continue monitoring electrolytes (potassium, magnesium) and renal function throughout the correction period 1.

Special Considerations for This Patient

Elderly women on thiazides represent the highest-risk group for severe hyponatremia 3, 4. The combination of advanced age, female sex, and multiple antihypertensive medications (telmisartan, cilnidipine, bisoprolol) creates a perfect storm for this complication 3, 4. Thiazide-induced hyponatremia can develop acutely within days or insidiously after months to years of stable therapy 3.

Alternative antihypertensive management: Once the acute hyponatremia resolves, blood pressure control should be maintained with non-thiazide agents 2. The patient is already on telmisartan (ARB), cilnidipine (calcium channel blocker), and bisoprolol (beta-blocker), which can be optimized for blood pressure control without the hyponatremia risk associated with thiazides 1.

Common Pitfalls to Avoid

• Never use hypertonic saline unless severe neurological symptoms are present (seizures, coma, altered mental status requiring immediate intervention) 2
• Never correct faster than 8 mmol/L in 24 hours—this patient's age and chronicity demand even slower correction at 4-6 mmol/L per day 1, 2
• Do not restart thiazide diuretics—this patient has demonstrated clear susceptibility to thiazide-induced hyponatremia and should avoid this drug class permanently 3
• Do not overlook potassium and magnesium repletion—cation deficits perpetuate the hyponatremia and must be corrected 3

Expected Outcome

With discontinuation of chlorthalidone, fluid restriction, and electrolyte repletion, most patients show improvement within 24-72 hours 3, 6. The syndrome typically resolves completely once the offending agent is removed and supportive measures are implemented 6. However, vigilance for osmotic demyelination syndrome must continue for 2-7 days after correction begins 2.