A tale of two sisters - delayed diagnosis of genetic hyperinsulinaemic hypoglycaemia.

Summary: Congenital hyperinsulinism is the leading cause of persistent hypoglycaemia in infants and children; however, it is uncommon to be diagnosed in adulthood. We describe the cases of two sisters who presented with hyperinsulinaemic hypoglycaemia aged 47 and 57 years old, who were subsequently diagnosed with compound heterozygous likely pathogenic variants in the ABCC8 gene, a known cause of monogenic congenital hyperinsulinism. We discuss the typical presenting features, investigation findings, and treatment strategies for patients with this condition. Learning Points: Congenital hyperinsulinism is a rare cause of hyperinsulinaemic hypoglycaemia diagnosed in adulthood. Clinical presentation is similar to an insulinoma, and imaging modalities may assist in differentiation. There are minimal medical therapies currently available for patients non-responsive to diazoxide (such as those with ABCC8 and KCNJ11 variants). Continuous glucose monitoring can be helpful in giving patients autonomy in managing their disease, as well as relieving anxiety and fear associated with hypoglycaemia.