Rickettsiosis-associated hyponatremia

Two patients with hyponatremia (130 mEq/l and 122 mEq/l, respectively), and rickettsial disease are described. The causes of hyponatremia were attributed to rickettsial vasculitis and increased capillary permeability in the first patient and to the syndrome of inappropriate anti-diuretic hormone (ADH) secretion in the second patient. The differentiation between the mechanisms was established by measurement of urinary sodium excretion which was low in the first patient (7 mEq/l) and high in the second patient (60 mEq/l), and levels of ADH that were inappropriately high in the second patient (7-9 pg/ml) in the presence of low plasma osmolality. The differentiation between these causes of hyponatremia has important therapeutic implications.