Differential Diagnosis for Acute Altered Mental Status with Hyponatremia

Single Most Likely Diagnosis

• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): This condition is characterized by the excessive release of antidiuretic hormone (ADH), leading to water retention and hyponatremia. Altered mental status can occur due to the rapid onset of hyponatremia, making SIADH a common and likely cause.

Other Likely Diagnoses

• Hypovolemic Hyponatremia: This occurs due to a loss of sodium and water, but with a greater loss of sodium. Causes can include gastrointestinal losses (e.g., diarrhea), renal losses (e.g., diuretics), or third spacing of fluids. Altered mental status can result from both the hyponatremia and the hypovolemia.
• Heart Failure: Congestive heart failure can lead to hyponatremia due to increased ADH secretion and decreased renal perfusion. Altered mental status may occur due to decreased cerebral perfusion or the effects of hyponatremia.
• Liver Cirrhosis: Cirrhosis can cause hyponatremia through a combination of factors including hypoalbuminemia, increased ADH, and renal vasoconstriction. Hepatic encephalopathy, a complication of cirrhosis, can also cause altered mental status.

Do Not Miss Diagnoses

• Subarachnoid Hemorrhage: Although less common, subarachnoid hemorrhage can cause both acute altered mental status and hyponatremia (often due to SIADH or cerebral salt wasting). Missing this diagnosis could be fatal.
• Meningitis: Bacterial or viral meningitis can present with altered mental status and may cause hyponatremia through SIADH or other mechanisms. Prompt diagnosis and treatment are crucial to prevent serious complications or death.
• Pituitary Apoplexy: This is a medical emergency where a pituitary tumor undergoes sudden hemorrhage or infarction, potentially leading to acute hypopituitarism, which can cause hyponatremia and altered mental status.

Rare Diagnoses

• Adrenal Insufficiency: Both primary (Addison's disease) and secondary adrenal insufficiency can lead to hyponatremia. Altered mental status may occur due to the metabolic disturbances and potential hypotension associated with adrenal crisis.
• Hypothyroidism: Severe, untreated hypothyroidism can cause hyponatremia and altered mental status, although this is less common and typically occurs in the context of myxedema coma.
• Cerebral Salt Wasting: A rare condition associated with cerebral disorders, characterized by excessive renal sodium loss, leading to hyponatremia and volume depletion. It can be difficult to distinguish from SIADH but is crucial for management purposes.