Brain myelinolysis following hypernatremia in rats

Brain myelinolysis could develop after excessive correction (Delta SNa > 20-25 mEq/l/24 hour [h]) of chronic hyponatremia; however, this neurological event is not recognized as a complication of hypernatremia when arising from a normonatremic baseline. Previous animal studies were unable to reproduce these brain lesions in hypernatremia after acute increase of serum sodium to moderately hypernatremic levels. We hypothesize that to produce brain dehydration and myelinolysis from normonatremic baseline requires a more important osmotic gradient than when starting from hyponatremic state. Rapid and sustained hypernatremia (at least >6 to 12 h) was induced in male rats by i.p. administration of NaCl 2 M (3 injections at 6 h intervals). The NaCl doses were determined to define two groups of hypernatremic rats (moderate and severe hypernatremia) for further analysis of the neurological outcome. In group 1 (moderate hypernatremia, n = 26) 8 rats died early (<12 h) after the beginning of the NaCl administration without specific neurologic manifestations. All the surviving rats fared well and were asymptomatic at time of death (day 8). They were submitted for at least 6 to 12 h to a serum sodium gradient of 28 +/- 6 mEq/l. Brain analysis was normal in all of them without brain demyelinating lesions. In group 2 (n = 51), 24 rats also died rapidly (<12 h). The surviving rats developed severe neurologic symptoms as typically encountered in hyponatremic rats with myelinolysis. The majority of them died before day 8. The hypernatremic gradient in this group was significantly higher than rats in group 1 that completely recovered (mean Delta SNa: 39 +/- 8 mEq/l, p < 0.001). In the 7 surviving rats (mean Delta SNa: 33 +/- 3 mEq/l) brain analysis demonstrated severe demyelinating lesions similar to the histologic changes observed in hyponatremia-related myelinolysis. We demonstrated for the first time that high and sustained levels of hypernatremia could induce brain myelinolysis and that the osmotic gradient necessary to produce brain lesions is higher for normonatremic than for hyponatremic rats.