Syndrome of inappropriate anti-diuretic hormone (SIADH) secretion in pre-eclampsia

SIADH as a cause of hyponatremia in pre-eclampsia is rare. We present the case of a 29 year old woman with this condition and discuss the obstetric and anaesthetic challenges in her management. Our lady presented in her first pregnancy with dichorionic twins at 35 weeks of gestation with irregular tightenings, proteinuric hypertension and severe pelvic girdle pain which limited her mobility. She had a medical history of chronic fatigue syndrome. She was oedematous and weighed 120kg (booking weight of 70kg) on admission. The biochemistry revealed serum sodium (Na) of 122mmol/l, creatinine of 70umol/l and urinary protein creatinine ratio of 529. She was monitored closely as an in-patient and her serum Na dropped to 116mmol/l in 48 hrs. Her pre-eclampsia worsened needing stabilisation with intravenous labetalol and magnesium sulphate followed by a caesarean section. A diagnosis of SIADH was made with a urine osmolality of 307mosm/kgH2O and a plasma osmolality of 248 mosm/kgH2O. The hyponatremia resolved within 24 hrs of delivery. In pregnancy, osmotic release of anti- diuretic hormone is set at a lower threshold and serum Na is physiologically lower than non-pregnancy levels. The mechanism of SIADH in pre-eclampsia is not clear. However the reduced intravascular volume is thought to stimulate the release of ADH. Acute hyponatremia increases the likelihood of seizures in pre-eclampsia. Fluid restriction and close monitoring in a high dependency setup with delivery in a timely manner leads to resolution of this condition in all reported cases.