Acute Autonomic Sensory Neuropathy Causing Severe Hyponatremia: A Rare Cause of Syndrome of Inappropriate Antidiuresis

A 25-year-old woman had experienced dizziness and numbness following a recent fever accompanied by upper respiratory symptoms. She was subsequently admitted to our hospital because of impaired consciousness and generalized tonic-clonic seizures. Severe hyponatremia and increased urinary sodium excretion were noted, leading to a diagnosis of syndrome of inappropriate antidiuresis (SIAD), being suspected Guillain-Barré syndrome (GBS) as the cause. However, she did not present with muscle weakness or deep sensory impairment, whereas orthostatic hypotension and the results of 123-I-meta-iodobenzylguanidine scintigraphy suggested the presence of autonomic dysfunction. We diagnosed her with acute autonomic and sensory neuropathy (AASN) rather than GBS. AASN is a neurological disorder closely related to GBS, characterized by an acute onset of sensory and autonomic dysfunction without motor impairment. Because AASN may rapidly progress to a severe form, it is important to recognize that AASN is a contributing cause of SIAD and to manage it appropriately at an early stage.