A 35-year-old man presents with severe hyponatraemia (serum Na 116 mmol/L) and is confused. Brain MRI done 3 days after correction shows symmetric T2 hyperintensities in the pons and extrapontine structures. This complication is known as osmotic demyelination syndrome (ODS). The MOST important preventable cause was:

• A Too slow correction of hyponatraemia
• B Use of hypertonic saline regardless of rate
• C Over-rapid correction of chronic hyponatraemia beyond 10–12 mmol/L in 24 hours ✓
• D Correction of acute hyponatraemia within 24 hours

Explanation

Osmotic demyelination syndrome (previously 'central pontine myelinolysis' when limited to the pons) is caused by over-rapid correction of chronic hyponatraemia. Brain cells adapt to chronic hyponatraemia by extruding osmolytes; rapid correction causes osmotic stress and astrocyte apoptosis, leading to demyelination. The safe limit is ≤10–12 mmol/L in 24 hours and ≤18 mmol/L in 48 hours. Risk factors include alcoholism, malnutrition, hypokalaemia, and liver transplantation. Acute hyponatraemia (<48 h duration) can be corrected more rapidly as brain adaptation has not occurred.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.