The Incidence, Characteristics, Management and Outcomes of Peripartum Hyponatraemia in the United Kingdom: A Prospective Study Using the UK Obstetric Surveillance System (UKOSS).

Objective: To determine the national incidence, characteristics, management and outcomes of peripartum hyponatraemia.

Design: Prospective, observational study using United Kingdom (UK) Obstetric Surveillance System (UKOSS) methodology.

Setting: 192 of the 194 consultant-led obstetric units in UK National Health Service (NHS) hospitals.

Population: Peripartum UK obstetric admissions.

Methods: Units submitted cases monthly to UKOSS (April 2019-September 2020). The case definition was symptomatic hyponatraemia (sodium < 125 mmol/L) in labour or ≤ 48 h postpartum, excluding other causes of symptoms. Pre-eclampsia was excluded as a separate aetiology of hyponatraemia and potential cause of symptoms. As symptom documentation varied, characteristics, management and outcomes were compared between cases with and without reported symptoms.

Main outcome measures: Demographics, symptoms, labour details, management and outcomes.

Results: Eighty cases were submitted. We excluded 23 cases (10 with sodium ≥ 125 mmol/L and 13 with pre-eclampsia). A further 25 had sodium < 125 mmol/L with no symptoms documented. Thirty-two met the UKOSS definition, resulting in an estimate of 3.0 cases of peripartum hyponatraemia with documented symptoms per 100 000 maternities (95% confidence interval 2.0-4.1), using a national estimate of 1 050 915 maternities across the study period. Characteristics and outcomes of cases with and without documented symptoms had some variation; 53% versus 24% had critical care admissions, 41% versus 40% had neonatal unit admissions, and 38% versus 20% had spontaneous vaginal birth.

Conclusions: Peripartum hyponatraemia was associated with maternal morbidity. Neonatal morbidity and operative birth were high, even without documented maternal symptoms. Poor recognition and documentation of symptoms have likely underestimated the incidence.