Third-degree atrioventricular block associated with severe acute hyponatraemia

Severe acute hyponatraemia, defined as a sodium concentration of less than 120 mEq/L, typically manifests with neurological manifestations, resulting in obtundation, coma, seizures, respiratory arrest and death. It very rarely is arrhythmogenic, with a literature review revealing seven cases of hyponatraemia-associated atrioventricular (AV) block of various degrees, of which only three were described as having third-degree AV block. The higher-degree AV blocks typically occurred at sodium levels closer to 115 mEq/L. We present a case of severe acute hypo-osmolar hyponatraemia-induced third-degree AV block in a patient without any other risk factors or aetiologies who initially presented with subdural haematoma and developed refractory bradycardia during his admission. The patient's third-degree AV block completely resolved after correction of his sodium. This case highlights the importance of working up the cause of new-onset third-degree AV block and the consideration of rarer electrolyte derangements such as hyponatraemia as a potential cause.