Hyponatremia, osmotic demyelination syndrome, and the importance of the patient’s history
Central pontine myelinolysis (CPM), first described in 1959, is a symmetrical non-inflammatory demyelinating disease with loss of oligodendrocytes that occurs most often following a rapid correction of severe hyponatremia (i.e., <120 mmol/L). It presents as a biphasic disease with initial seizure or encephalopathy, followed by clinical improvement and subsequent rapid deterioration with bulbar dysfunction, oculomotor dysfunction, various degree of paresis, and even locked in syndrome. Its occurrence is rare (≈0.6% of severe hyponatremia), it is diagnosed clinically and confirmed with brain imaging, ideally with magnetic resonance image, and it is reversible in approximately half the patients. Lesions are classically identified in the pons but extra pontine lesions (in basal ganglia, cerebellar white matter, thalamus, and hippocampus) have also been identified. The most commonly accepted molecular mechanism involves brain cell volume regulation with a rapid shift of osmole following brain edema which establishes during the chronic hyponatremic phase. For these reasons, osmotic demyelination syndrome (ODS) is a better term. The most identified risk factor is severe hyponatremia, but other electrolyte abnormalities can contribute, in particular, if the patient is an alcoholic or malnourished. This diagnosis should also be suspected in post-op patients with nausea and headache non-responsive to antiemetic and analgesic drugs. An essential step is an appropriate medical history, a list of medications, physical examination, and basic initial lab tests with the goal of identifying possible easily reversible causes of hyponatremia. Correction of severe hyponatremia with neurological symptoms should be done using rapid boluses of hypertonic saline solution in rapid succession with goals of increasing serum sodium by 5-6 mEq/L in the first two hours, which should be stopped if the level has risen by 10 mEq/L in the first five hours, and with the overall correction goal not to exceed 15-20 mEq/L in 48 hrs. This method has been shown safe in all hospital settings studied. Serial measurements of electrolyte levels and neurological examinations are recommended, as are correction of all electrolyte abnormalities, in
particular magnesium and potassium. Thiamine should be given to all patients with chronic alcohol use who present with hyponatremia and encephalopathy.
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