Evaluation and Management of an Asymptomatic Patient with Serum Sodium <100 mEq/L

Introduction: Severe hyponatremia is defined as a serum sodium concentration <120 mEq/L, with potential for complications from the hyponatremia itself and also overcorrection. It is also important to determine symptom severity. Those classified as severe include seizures, obtundation, coma and respitatory arrest. Among the goals of therapy are to prevent a further decline in serum sodium, prevent brain hernation, relieve symptoms, and avoid overcorrection. We present a case of extreme hyponatremia, whereby these goals were jeopardized due to a persistent incalculably low serum sodium, recorded as <100 mEq/L over the first 12 hours of hospitalization. Case Description: 57 year old male with history of mantle cell lymphoma on R-CHOP alternating with R-DHAP presented with nausea. Initial serum sodium found to be <100 mEq/L and this was recorded value over the next 8 blood draws spanning 12 hour course. Two weeks earlier, serum sodium 134 mEq/L. CT head did not reveal any cerebral edema. An effort to determine the exact sodium value was unsuccessful after discussion with various laboratory personnel. Estimation of possible sodium range included extrapolating from serum bicarbonate and chloride measurements (based on historic anion gap and improbability of negative anion gap). Based on such calculations, serum Na presumably 95-100 mEq/L. The patient remained remarkably awake, alert and oriented, without neurological deficit. Urine osmolality 500-600 mOsm/kg and urine sodium 40-60 mmol/L. He received 50 & 100 cc boluses of 3% saline and by 24 hour mark from admission, serum Na rose to 105 mEq/L. Discussion: Management of hyponatremia is well described, based on both chronicity and presence or absence of symptoms. Highest risk for osmotic demyelination syndrome includes those with serum sodium concentration of less than or equal to 105 mEq/L. Goal of initial therapy is to raise the serum sodium by 4-6 mEq/L in a 24 hour period. In our particular patient, this represented a challenge due to the lack of numerical starting point (<100 mEq/L) which persisted over the first 12 hours. The first measurable value was 103 mEq/L, about 18 hours after arrival. As such, undertook creative ways to deduce the serum sodium range and ensure a safe correction rate with proper strategy. This case provides a framework to guide the management of extreme low/undetectable serum sodium values.