Understanding "8 mmol" in Sodium Correction
The "8 mmol" refers to the maximum safe increase in serum sodium concentration over a 24-hour period, not a target level of 130 mmol/L. This is the critical safety limit to prevent osmotic demyelination syndrome (ODS), a devastating neurological complication that can occur when chronic hyponatremia is corrected too rapidly 1, 2, 3.
What This Number Actually Means
The 8 mmol/L limit represents the change in sodium concentration, not an absolute value. For example:
- If a patient's sodium is 120 mmol/L, you should not increase it beyond 128 mmol/L in the first 24 hours 1, 4
- If starting at 115 mmol/L, the maximum target would be 123 mmol/L after 24 hours 1
- This translates to approximately 0.33 mmol/L per hour 4
Why This Limit Exists
Rapid correction exceeding 8 mmol/L in 24 hours increases the risk of osmotic demyelination syndrome by nearly 4-fold (RR 3.91). 2 This syndrome causes irreversible brain damage with symptoms including dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis, typically appearing 2-7 days after overcorrection 1.
The brain adapts to chronic hyponatremia by losing osmolytes over 48 hours. When sodium is corrected too rapidly, water shifts out of brain cells faster than osmolytes can be restored, causing cellular dehydration and demyelination 5, 3.
Different Limits for Different Patients
High-risk patients require even slower correction of 4-6 mmol/L per day:
- Advanced liver disease patients 1, 4
- Alcoholism or malnutrition 1, 5
- Severe hyponatremia (<120 mmol/L) 1
- Prior encephalopathy 1
Standard-risk patients can tolerate 4-8 mmol/L per day, but should never exceed 10-12 mmol/L in any 24-hour period 1, 4.
Severe Symptomatic Cases
For patients with seizures, coma, or altered mental status, the initial goal is 6 mmol/L over 6 hours or until symptoms resolve, but total correction must still not exceed 8 mmol/L in 24 hours 1, 4, 6. This is achieved with 100 mL boluses of 3% hypertonic saline over 10 minutes, repeated up to 3 times 1, 4.
Monitoring Requirements
Serum sodium must be checked every 2 hours during initial correction for severe symptoms, and every 4-6 hours for mild symptoms 1, 4. This frequent monitoring is essential because inadvertent overcorrection is common 5.
If Overcorrection Occurs
Immediate intervention is required if sodium increases by more than 8 mmol/L in 24 hours:
- Discontinue all sodium-containing fluids immediately 1, 4
- Switch to D5W (5% dextrose in water) 1, 4
- Administer desmopressin to halt water diuresis 1, 5
- Consider therapeutic relowering of sodium 5
Common Pitfall
The most critical error is confusing the 8 mmol/L correction limit with a target sodium level. The 8 mmol/L is a safety ceiling for the rate of change, not a goal to achieve. Even if a patient's sodium is 122 mmol/L, you should not aim to reach 130 mmol/L in 24 hours—this would represent an 8 mmol/L increase and is the absolute maximum, not a target 1, 4.