High lithium concentration at delivery is a potential risk factor for adverse outcomes in breastfed infants: a retrospective cohort study.

BACKGROUND: Neonatal effects of late intrauterine and early postpartum exposure to lithium through mother's own milk are scarcely studied. It is unclear whether described symptoms in breastfed neonates are caused by placental lithium transfer or postnatal exposure to lithium through breastfeeding. We aimed to investigate lithium clearance and neonatal morbidity in breastfed infants with high versus low serum lithium concentrations at birth.

METHODS: This retrospective study focused on breastfed infants to women treated with lithium during and after pregnancy, born between 2006 and 2021 in Stockholm, Sweden. Information on serum lithium concentrations and adverse neonatal outcomes was obtained from medical records. Neonatal symptoms and lithium clearance were compared between a high exposure group (HEG, lithium concentrations ≥ 0.6 meq/l) and a low exposure group (LEG, < 0.6 meq/l).

RESULTS: A total of 25 infant-mother dyads were included. Median lithium serum concentration at birth was 0.90 meq/l in the HEG as compared with 0.40 meq/l in the LEG (p < 0.05). The difference was still significant at follow-up (0.20 meq/l vs 0.06 meq/l, p < 0.05), despite reduction in maternal dose. The rate of neonatal symptoms was 85.7% in HEG and 41.2% in LEG (p = 0.08) at birth and 28.6% vs 11.8% at follow-up (p = 0.55). Furthermore, 28.6% of infants in HEG were admitted to neonatal care, vs 5.9% in LEG (p = 0.19). Two infants in the HEG had therapeutic lithium levels at follow-up. All infants with symptoms at follow-up were either in the HEG or exposed to additional psychotropic medication.

CONCLUSIONS: Neonatal symptoms are common after late intrauterine lithium exposure, however transient, treatable and mostly mild. In this study, a high lithium concentration at birth was a risk factor for an increased lithium level at follow-up. Polypharmacy may constitute an additional risk factor. This study suggests that the late intrauterine exposure to lithium might add to the adverse effects in lithium-exposed, breastfed infants. Consequently we recommend breastfed infants with therapeutic lithium concentrations at birth to be followed up promptly to avoid lithium toxicity.