Water Intoxication Due to Oxytocin

Water intoxication due to oxytocin is hyponatraemia (serum sodium below 130 mmol/L) caused by the antidiuretic effect of oxytocin combined with large volumes of hypotonic fluid, usually dextrose 5 percent in water, given over many hours. It occurs when oxytocin is given at high dose for long periods, and in some women with large volumes of oral fluid. It matters because it causes headache, vomiting, confusion, seizures, coma and death, and it can mimic eclampsia.

• Risk factors: oxytocin at high doses for more than 6 to 8 hours, hypotonic IV fluids, large oral fluid intake, prolonged labour, preexisting low sodium.
• Oxytocin is structurally related to vasopressin and reduces free water excretion.
• Prevention is more important than treatment.

Women have headache, nausea, vomiting, restlessness, drowsiness, muscle twitching and confusion, progressing to seizures and coma. Urine output falls and blood pressure is usually normal.

• Symptoms usually appear when sodium falls below 125 mmol/L.
• Weight gain and oedema.
• Differential diagnosis: eclampsia, epilepsy, hypoglycaemia, cerebral malaria, meningitis, intracranial haemorrhage, amniotic fluid embolism, local anaesthetic toxicity.
• Complications: seizures, cerebral oedema, coma, respiratory arrest, osmotic demyelination from rapid correction, neonatal hyponatraemia and seizures, maternal death.

The diagnosis is made by a low serum sodium in a woman on oxytocin with symptoms.

• Serum sodium (below 130 mmol/L; severe below 120 mmol/L), plasma and urine osmolality, urea, creatinine, potassium, glucose.
• Review of fluid balance: total volume and type of fluids, oxytocin dose and duration, urine output.
• Blood pressure, urine protein and reflexes to exclude eclampsia; malaria test and glucose.
• Cardiotocography; newborn sodium if the mother was affected at birth.

The principle is to stop oxytocin and hypotonic fluid, raise the sodium carefully with hypertonic saline when symptoms are severe, and control seizures.

• Supportive: stop oxytocin and all hypotonic fluids; restrict fluid to 500 to 800 mL per 24 hours; hourly urine output; sodium 2-hourly; left lateral position; protect the airway and give oxygen 10 L/minute; continuous cardiotocography; intensive care for seizures or coma; use only isotonic fluid (Ringer's lactate or sodium chloride 0.9 percent) for oxytocin infusions in future.
• Pharmacological: for seizures or severe symptoms give sodium chloride 3 percent 100 mL IV over 10 minutes, repeated up to 2 more times at 10 minute intervals, aiming for a rise of 4 to 6 mmol/L in the first 6 hours and no more than 10 mmol/L in 24 hours; furosemide 20 mg IV once if there is fluid overload; seizure control with lorazepam 4 mg IV (or diazepam 10 mg IV over 2 minutes), repeated once after 10 minutes if seizures continue; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour if eclampsia cannot be excluded; for mild symptoms with sodium 125 to 130 mmol/L restrict fluid and observe; oxytocin 10 IU IM after birth, and use a concentrated infusion of 20 IU in 500 mL sodium chloride 0.9 percent at 60 mL per hour (about 40 mU/minute) for the third stage only if fluid must be limited.
• Procedures: delivery by caesarean section or assisted vaginal delivery for fetal distress or maternal deterioration; intubation and ventilation for coma or respiratory failure.
• Special groups: kidney disease or heart failure needs smaller fluid volumes with senior supervision; seizures in a woman with hypertension and proteinuria need magnesium sulphate until eclampsia is excluded.
• Red flags: seizure, coma, sodium below 120 mmol/L, rising sodium by more than 10 mmol/L in 24 hours, urine output below 30 mL per hour, fetal bradycardia.
• Follow-up: sodium every 2 to 4 hours until above 130 mmol/L and symptoms settle, then daily; neurological review at 6 weeks; newborn sodium and glucose checks.