How to correct hyponatremia in a patient on mannitol?

Sodium Correction in Patients on Mannitol

In patients receiving mannitol who develop hyponatremia, immediately check serum osmolality to distinguish between true hyponatremia (hypoosmolar) and pseudohyponatremia (iso- or hyperosmolar), as this fundamentally determines whether correction is needed and safe. 1, 2, 3

Critical Initial Assessment

The first step is measuring serum osmolality, as mannitol creates a unique clinical scenario where sodium levels can be misleadingly low despite normal or elevated total osmolality 3, 4:

• If serum osmolality is elevated (>295 mOsm/L): This represents pseudohyponatremia from mannitol accumulation, and the patient may be asymptomatic despite sodium levels as low as 99 mEq/L 3
• If serum osmolality is normal or low (<280 mOsm/L): This represents true hyponatremia requiring correction 5, 4

Management Based on Osmolality Status

Hypertonic Hyponatremia (Elevated Osmolality with Low Sodium)

Stop mannitol immediately if serum osmolality exceeds 320 mOsm/L, as this indicates dangerous mannitol accumulation and risk of renal failure 1, 2, 6:

• Do NOT give hypotonic fluids - this will worsen cerebral edema 1, 2
• Administer isotonic (0.9% NaCl) or hypertonic maintenance fluids to maintain intravascular volume while allowing mannitol diuresis to clear the excess osmotic load 1, 2
• Consider hemodialysis if renal function deteriorates, consciousness declines, or metabolic acidosis develops despite stopping mannitol 4
• Switch to hypertonic saline (3%) if continued ICP control is needed, as it has comparable efficacy to mannitol at equiosmolar doses (~250 mOsm) 1, 7

True Hyponatremia (Normal or Low Osmolality)

This scenario is less common but represents genuine sodium depletion from mannitol's osmotic diuresis 2, 8:

For severe symptoms (seizures, altered mental status, coma):

• Administer 3% hypertonic saline to correct 6 mEq/L over 6 hours or until severe symptoms resolve 9
• Total correction must not exceed 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome 9
• Check sodium every 2 hours during active correction 9

For mild symptoms or asymptomatic hyponatremia:

• Fluid restriction to 1 L/day combined with normal saline maintenance 9
• Oral sodium chloride supplementation (100 mEq TID) if no response to fluid restriction 9
• Check sodium every 4-6 hours 9

Critical Monitoring Parameters During Mannitol Therapy

To prevent sodium complications, establish this monitoring protocol 1, 2, 8:

• Serum osmolality every 6 hours - hold mannitol if >320 mOsm/L 1, 2
• Sodium and potassium every 6 hours during active mannitol therapy 1, 2, 8
• Fluid balance tracking - mannitol causes profound osmotic diuresis requiring volume replacement 2, 6
• Foley catheter placement before mannitol administration to accurately monitor urine output 6, 7

Common Pitfalls and How to Avoid Them

Pitfall #1: Treating pseudohyponatremia with hypotonic fluids 3, 4

• Always check osmolality before correcting low sodium in mannitol patients
• Hypotonic fluids will worsen cerebral edema when osmolality is elevated

Pitfall #2: Continuing mannitol when osmolality exceeds 320 mOsm/L 1, 2, 6

• This causes irreversible renal failure and worsening hyponatremia
• Switch to hypertonic saline for continued ICP control if needed

Pitfall #3: Overcorrecting chronic hyponatremia too rapidly 9

• Even in acute symptomatic cases, never exceed 8 mEq/L correction in 24 hours
• Central pontine myelinolysis risk increases with rapid overcorrection

Pitfall #4: Ignoring hypokalemia during mannitol therapy 8

• Hypokalemia occurs in 22% on day 1 and increases to 52% by day 7 of mannitol use
• Monitor and replace potassium aggressively throughout mannitol therapy

Practical Algorithm for Sodium Management

1. Patient on mannitol develops low sodium → Check serum osmolality immediately 1, 3

2. If osmolality >320 mOsm/L:

   • Stop mannitol 1, 2
   • Give isotonic/hypertonic fluids only 1, 2
   • Switch to hypertonic saline for ICP control 1, 7
   • Consider dialysis if renal failure develops 4

3. If osmolality 295-320 mOsm/L with symptoms:

   • Reduce mannitol dose or extend intervals 1
   • Give isotonic fluids for volume replacement 1, 2
   • Monitor osmolality every 6 hours 1, 2

4. If osmolality <295 mOsm/L (true hyponatremia):

   • Severe symptoms → 3% saline, correct 6 mEq/L over 6 hours 9
   • Mild/no symptoms → fluid restriction + oral sodium 9
   • Maximum 8 mEq/L correction in 24 hours 9