Hyponatremia and Seizure in a Postoperative Neurosurgical Patient
A 52-year-old woman is 6 days post transsphenoidal resection of a pituitary macroadenoma. She develops confusion and a witnessed generalized tonic-clonic seizure. Serum sodium is 116 mmol/L (baseline 139). She is euvolemic on exam, urine output is high, urine osmolality is 520 mOsm/kg, and serum osmolality is 248 mOsm/kg. Urine sodium is 62 mmol/L. TSH, free T4, and cortisol are normal. She is currently on maintenance fluids containing 0.45% saline at 100 mL/h.
1. Differentiate the likely cause of her hyponatremia and explain why the fluid prescription may be contributing.
Read the worked answer
The picture is euvolemic hypotonic hyponatremia with concentrated urine (520 mOsm/kg) and high urine sodium (62) in the setting of normal thyroid and adrenal function. After transsphenoidal surgery, the two main considerations are SIADH and cerebral salt wasting (CSW).
- CSW is typically associated with hypovolemia and high urine output; SIADH is euvolemic.
- This patient appears euvolemic, favoring SIADH, which is common after pituitary surgery due to inappropriate ADH secretion.
The maintenance 0.45% saline is harmful in SIADH: hypotonic fluid is retained and worsens hyponatremia. The correct approach is fluid restriction and, in severe symptomatic cases, hypertonic saline.
2. She is actively seizing. Describe your immediate management, including the sodium correction target and the rationale for the rate.
Read the worked answer
Immediate management:
- Airway/breathing - protect airway, give oxygen; benzodiazepine for ongoing seizure activity.
- 3% hypertonic saline bolus - typically 100–150 mL over 10–20 minutes (or weight-based ~2 mL/kg) to raise serum sodium by ~4–6 mmol/L acutely and stop the seizure. Repeat as needed until symptoms abate.
- Stop the hypotonic maintenance fluid.
- Monitor sodium every 1–2 hours during active correction.
Correction rate rationale:
- Acute symptomatic hyponatremia warrants rapid partial correction (~4–6 mmol/L) to relieve cerebral edema.
- Thereafter, limit correction to ≤8 mmol/L in 24 hours (some use ≤10) to avoid osmotic demyelination syndrome.
- Because she is high-risk (likely chronic hyponatremia over days, malnutrition/postoperative state), use the lower end of the limit and consider desmopressin to clamp the correction if it overshoots.