Complex Hyponatremia: Syndrome of Inappropriate Antidiuretic Hormone Secretion in a Patient with Lupus and Concurrent Central Nervous System Toxoplasmosis and B Cell Lymphoma

Introduction: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a common cause of euvolemic, hypotonic hyponatremia, resulting from persistent ADH secretion despite the absence of appropriate osmotic or hemodynamic stimuli. Its causes include central nervous system (CNS) disorders, medications, malignancies, pulmonary diseases, and others. Case Description: A 49-year-old woman with a history of systemic lupus erythematosus and hypertension presented with a 2-day history of band-like headache, voice changes, and episodes of dizziness. A non-contrast head CT revealed bilateral brain lesions, and MRI of the brain showed enhancing lesions concerning for either malignancy or infection. Given her immunocompromised state and close contact with cats, cerebral toxoplasmosis was high on the differential. Empiric treatment was initiated. Subsequent serology confirmed Toxoplasma antibody positivity. On day 7 of admission, the patient developed altered mental status and vomiting. Labs revealed hypotonic hyponatremia with a serum sodium of 123 mmol/L. Given her symptoms, she was treated with a single 100 mL bolus of hypertonic saline. Workup showed serum osmolality of 270 mOsm/kg, urine osmolality of 307 mOsm/kg, and urine sodium of 83 mmol/L. SIADH secondary to neurotoxoplasmosis was diagnosed. Management included fluid restriction and salt tablets, with gradual correction of sodium over six days, which remained stable afterward. Further investigation ultimately led to the diagnosis of large B-cell lymphoma, which could explain the hyponatremia and CNS findings. Discussion: Conditions affecting the CNS, as seen in this patient, are recognized causes of SIADH due to disruption of normal ADH production and secretion. While treating the underlying cause is the cornerstone of SIADH management, this is not always feasible, particularly in cases with an indeterminate diagnosis or when definitive therapy is delayed or limited. In such situations, supportive management with fluid restriction and increased solute intake becomes essential. This case highlights that the causes of hyponatremia can be broad, and even rare conditions such as toxoplasmosis and malignancy should be considered, as addressing the underlying etiology remains the definitive approach to treatment.