Hyponatremia in neuromyelitis optica spectrum disorders: Literature review

Hyponatremia is a potentially serious electrolyte abnormality observed in neuromyelitis optica spectrum disorders (NMOSDs), and its most common cause is syndrome of inappropriate antidiuretic hormone secretion (SIADH). Another potential cause of hyponatremia is cerebral salt-wasting syndrome (CSWS), although CSWS has not previously been reported in NMOSDs. Accurate and early differentiation between SIADH and CSWS is difficult. However, the two conditions have important implications for the selection of therapy. Here, we describe two patients with aquaporin-4 antibody (AQP4-Ab)-positive NMOSDs who developed hyponatremia as a result of CSWS and SIADH, respectively. Additionally, we review all previously reported studies of hyponatremia in patients with NMOSDs and propose several potential pathophysiological mechanisms of hyponatremia. In conclusion, NMOSDs accompanied by hyponatremia are not actually rare, but have previously been given little attention. Furthermore, SIADH should not be the only consideration, before the exclusion of rare but significant CSWS.