Osmotic Demyelination Syndrome
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Direct answer
Osmotic demyelination syndrome is a demyelinating injury, most typically central pontine myelinolysis, that follows overly rapid correction of chronic hyponatraemia. It usually appears two to six days after correction, with dysarthria, dysphagia and quadriparesis that can progress to locked-in state. Sodium should rise by no more than about 8 mmol per litre in 24 hours in high-risk patients, and about 10 in others.
What you must remember
- Central pontine myelinolysis: demyelination of the central pons, classically after rapid sodium correction in chronic hyponatraemia, is the best-known form.
- Correction limit: no more than about 8 mmol per litre in 24 hours in high-risk patients and about 10 mmol per litre in others, per European guidance.
- High-risk features: alcohol use disorder, malnutrition, hypokalaemia, liver disease and a serum sodium below about 105 mmol per litre raise the risk.
- Timing: symptoms typically appear two to six days after overcorrection, so the patient may look improved before deteriorating.
- Clinical pattern: dysarthria, dysphagia and quadriparesis (often spastic with pseudobulbar signs), progressing in severe cases to locked-in state with preserved awareness and vertical eye movements.
- MRI: T2 hyperintensity in the central pons with sparing of the periphery is typical, while extrapontine myelinolysis affects the basal ganglia, thalamus and cortex.
- Rescue: if overcorrection is detected, desmopressin with 5 per cent dextrose can lower sodium back in a controlled way, under close monitoring.
A pathway for planning a safe correction
Osmotic demyelination is prevented at the bedside rather than treated afterwards. The plan begins with classifying the hyponatraemia and identifying risk before any hypertonic saline is started.
- Classify the hyponatraemia: Establish whether it is acute, usually under 48 hours, or chronic, since chronic hyponatraemia carries the greater risk of demyelination on rapid correction.
- Identify high-risk features: Look for alcohol use, malnutrition, hypokalaemia, liver disease and a serum sodium below about 105 mmol per litre, each of which lowers the threshold for caution.
- Set a correction target: Agree a maximum rise of about 8 mmol per litre in 24 hours for high-risk patients and about 10 in others, and document it.
- Monitor sodium frequently: Check serum sodium every two to four hours during active correction, adjusting the infusion rate to stay within the ceiling.
- Account for potassium: Potassium replacement can raise serum sodium quickly, so its effect must be counted towards the overall correction limit.
- Plan rescue and follow-up: If the rise exceeds the target, give desmopressin with dextrose to lower sodium again, and monitor neurological status for several days.
Where the questions mislead
Most traps here involve treating the sodium number as the goal rather than the rate of change.
- Trap: Treating severe hyponatraemia with aggressive hypertonic saline to normal within hours Correct: the ceiling on correction is strict, and sudden restoration of sodium is the classic cause of osmotic demyelination.
- Trap: Assuming a patient who improved clinically after correction is safe Correct: symptoms typically appear two to six days later, so neurological monitoring continues well beyond the first day.
- Trap: Expecting osmotic demyelination to be confined to the pons Correct: extrapontine myelinolysis in the basal ganglia, thalamus, midbrain and cortex is common in severe cases, particularly in alcohol use and malnutrition.
- Trap: Treating potassium replacement as neutral for sodium Correct: potassium correction can raise serum sodium rapidly, so it counts toward the overall rise.
Frequently asked questions
What is the safe rate of sodium correction in chronic hyponatraemia?
Most guidance limits the rise in serum sodium to about 8 mmol per litre in 24 hours for high-risk patients and about 10 mmol per litre in 24 hours for others with chronic hyponatraemia. Sodium should be checked frequently during correction. Exceeding these limits is the main preventable cause of osmotic demyelination syndrome.
Which patients are at highest risk of osmotic demyelination syndrome?
Patients with alcohol use disorder, malnutrition, hypokalaemia, advanced liver disease and severe hyponatraemia are at highest risk of osmotic demyelination syndrome. The syndrome is most strongly linked to chronic hyponatraemia that is corrected too rapidly. Such patients need a stricter correction target from the outset.
How long after rapid sodium correction does osmotic demyelination typically appear?
Symptoms of osmotic demyelination syndrome typically appear two to six days after overcorrection of serum sodium. The delay means a patient may seem to improve before deteriorating with dysarthria, dysphagia or quadriparesis. Neurological monitoring should therefore continue well beyond the first day.
Which MRI finding is typical of central pontine myelinolysis?
Central pontine myelinolysis shows T2 and FLAIR hyperintensity in the central pons, with the peripheral pontine region typically spared. This pattern is characteristic when the clinical features fit. MRI can be normal early, so a normal early scan does not exclude the diagnosis.
How is overcorrection of sodium rescued?
Overcorrection is managed by lowering serum sodium in a controlled way, commonly with desmopressin and 5 per cent dextrose, under close monitoring. The goal is to bring sodium back within the target range. Uncontrolled lowering can also cause harm, so the rate is monitored closely.