Chronic magnesium supplementation in a patient with relapsing severe hypomagnesaemia due to a novel <i>TRPM6</i> variant diagnosed in adulthood

A man in his late 50s presented with muscle cramps due to severe hypomagnesaemia. Extensive evaluation established the diagnosis of adult-onset familial hypomagnesaemia with secondary hypocalcaemia (HOMG1, OMIM #602014). As expected, due to impaired gastrointestinal absorption, oral magnesium supplementation proved insufficient to maintain stable serum magnesium concentrations, leading to recurrent emergency department presentations and the need for alternative supplementation routes. Intravenous magnesium two times a week was effective for over 3 years, until device infections and venous occlusions rendered intravenous access unfeasible. Subcutaneous magnesium infusion provided an alternative, although local pain required concomitant anaesthetics, and recurrent allergic reactions to these agents introduced additional challenges. Hypomagnesaemia is a common clinical finding and often secondary to proton pump inhibitor or excessive alcohol use. This case illustrates a rare presentation of severe and relapsing hypomagnesaemia in adult-onset HOMG1, with an overview of the successes and complexities of long-term parenteral magnesium supplementation.