Seizure Mimics as the Presenting Feature of a Functional Metastatic Pancreatic Neuroendocrine Tumour in a Patient With Pre-existing Epilepsy: A Diagnostic Challenge.

Insulinomas and functional pancreatic neuroendocrine tumours (pNETs) are rare but important causes of endogenous hyperinsulinaemic hypoglycaemia whose neuroglycopenic manifestations can closely mimic neurological disorders, leading to considerable diagnostic delay. We present the case of a 74-year-old man with epilepsy, seizure-free for 11 years on carbamazepine, and a background of metastatic well-differentiated grade 3 pNET (Ki-67: 40-50%), who was admitted with a 2-3-week history of nocturnal confusion, cold sweats, nightmares, increasing agitation, and an episode of abnormal limb movements. The presentation was initially attributed to possible seizure recurrence or delirium. Routine investigations, including CT brain, carbamazepine levels, and type 1 antineuronal nuclear autoantibody antibody testing, were unremarkable, and blood glucose on admission was normal. The correct diagnosis was reached only during a witnessed ward episode on day two, when capillary blood glucose was found to be critically low and symptoms resolved immediately with intravenous dextrose. Subsequent biochemical evaluation demonstrated markedly elevated serum insulin (42 µU/L; reference: 2-24 µU/L), C-peptide (2,464 pmol/L; reference: <90 pmol/L), and on formal supervised 72-hour fast, which provoked symptomatic hypoglycaemia after just three hours, insulin was 180 µU/L, proinsulin was 730 pmol/L (reference: <10 pmol/L), and C-peptide was 2,136 pmol/L, with a negative sulphonylurea screen, fulfilling the diagnostic criteria for endogenous hyperinsulinaemia. Somatostatin receptor scintigraphy (single-photon emission computed tomography-computed tomography) identified multifocal hepatic metastases and focal uptake in the pancreatic tail, confirming an insulin-secreting primary. The patient was managed with diazoxide and monthly lanreotide but experienced rapid deterioration and died on palliative care. This case underscores the need to consider a functional pNET as a cause of seizure-like episodes even in patients with established epilepsy, and highlights that capillary blood glucose measurement during any witnessed neurological episode is a simple but potentially life-saving clinical step that should never be omitted.