Objective: To compare the features of paraneoplastic syndrome of inappropriate antidiuretic hormone with those of limbic encephalitis.
Design: Case study.
Setting: Academic medical center.
Patient: A 46-year-old woman with progressive memory impairment, hyponatremia, and seizures.
Interventions: Magnetic resonance imaging of the brain, fluoro-2-deoxyglucose positron emission tomography of the body, and immunohistochemical analysis of a resected tumor.
Results: Though the patient presented with clinical features of classic limbic encephalitis, magnetic resonance imaging, electroencephalogram, and cerebrospinal fluid analysis findings were unremarkable. Her chronic hyponatremia was ultimately found to be due to ectopic secretion of antidiuretic hormone by a neuroendocrine tumor with Merkel cell carcinoma phenotype.
Conclusions: Patients presenting with memory impairment, seizures, and hyponatremia should undergo a thorough workup for occult malignancy. In addition to considering classic immune-mediated paraneoplastic limbic encephalitis, the ectopic secretion of antidiuretic hormone should be included in the differential diagnosis.