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

Abstract

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.