Acute motor and sensory axonal neuropathy-associated syndrome of inappropriate antidiuretic hormone secretion

A 36-year-old man presented with a six week history of progressive ascending　weakness. Physical examination showed generalized motor weakness, more severe in　the lower extremities (LE), muscle wasting, absent LE reflexes, dysesthesia, and no　cranial nerve involvement. Neurologic workup was consistent with acute motor and sensory　axonal neuropathy (AMSAN), a variant of Guillain-Barre syndrome. Concomitantly　on admission, serum chemistry panel showed a sodium (Na) 115 mmol/L with normal　kidney function. Urine showed Na <20 mmol/L, and specific gravity 1.045. Urine osmolality　was not available initially. He received IV fluid for volume expansion. The Na did　not significantly improve after he became euvolemic. Fluid restriction was then tried with　mild improvement. Endocrine work-up ruled out hypothyroidism and adrenal insufficiency.　Repeat labs showed serum Na 124 mmol/L, urine Na 191 mmol/L and urine Osm 531　mOsm, and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was　diagnosed. Our case report suggests that SIADH should be high on the differential diagnosis　for hyponatremia in patients with AMSAN, especially in the setting of euvolemia.