Disordered Water Metabolisms: Hyponatremia

Neurologic manifestations of major electrolyte abnormalities

The brain operates in an extraordinarily intricate environment which demands precise regulation of electrolytes. Tight control over their concentrations and gradients across cellular compartments is essential and when these relationships are disturbed neurologic manifestations may develop. Perturbations of sodium are the electrolyte disturbances that most often lead to neurologic manifestations. Alterations in extracellular fluid sodium concentrations produce water shifts that lead to brain swelling or shrinkage. If marked or rapid they can result in profound changes in brain function which are proportional to the degree of cerebral edema or contraction. Adaptive mechanisms quickly respond to changes in cell size by either increasing or decreasing intracellular osmoles in order to restore size to normal. Unless cerebral edema has been severe or prolonged, correction of sodium disturbances usually restores function to normal. If the rate of correction is too rapid or overcorrection occurs, however, new neurologic manifestations may appear as a result of osmotic demyelination syndrome. Disturbances of magnesium, phosphate and calcium all may contribute to alterations in sensorium. Hypomagnesemia and hypocalcemia can lead to weakness, muscle spasms, and tetany; the weakness from hypophosphatemia and hypomagnesemia can impair respiratory function. Seizures can be seen in cases with very low concentrations of sodium, magnesium, calcium, and phosphate.

Sodium Disturbances Commonly Encountered in the Neurologic Intensive Care Unit

Disorders of sodium and water balance are common in patients with central nervous system (CNS) disease. These disorders frequently complicate the treatment course by precipitating or worsening neurological symptoms. These patients are not only at risk for symptoms secondary to dysnatremia but also at risk from the consequences of treatment. If not treated properly, this electrolyte disturbance can vastly increase morbidity and can even lead to death. Appropriate diagnosis and intervention requires an understanding of the physiologic and pathophysiologic mechanisms involved in sodium and water homeostasis.

Conivaptan for the treatment of hyponatremia

Conivaptan is the first dual vasopressin V1a/V2 receptor antagonist approved by the US FDA for the treatment of euvolemic and hypervolemic hyponatremia in hospitalized patients. Short-term use of intravenous conivaptan has been shown to promote effective free-water duresis and resolution of hyponatremia in several clinical trials. Adverse effects reported with short-term use mostly include infusion site reactions. However, they may also include serious effects including unexpectedly rapid serum Na⁺ correction, hypokalemia and orthostatic hypotension. Despite its proven efficacy in hospitalized patients, the development of oral conivaptan has been discontinued due to its shared hepatic clearance with many commonly used drugs. Thus, data is lacking on the long-term efficacy of conivaptan in patients with chronic hyponatremia. The decision to use conivaptan in addition to conventional therapy for euvolemic or hypervolemic hyponatremic patients must be carefully considered according to the patient history and response to conventional treatment.

Vasopressin-receptor antagonist therapy in patients with hyponatraemia

Hyponatraemia often complicates the treatment of underlying conditions in patients who are seriously ill. Arginine vasopressin receptor antagonists block the action of arginine vasopressin and correct sodium and water imbalance in patients with euvolaemic or hypervolaemic hyponatraemia.

Assessment of the efficacy and safety of intravenous conivaptan in euvolemic and hypervolemic hyponatremia

BACKGROUND

Most cases of hyponatremia--serum sodium concentration ([Na+]) < 135 mEq/l (< 135 mM)--are associated with an elevated plasma arginine vasopressin level. This study investigated the efficacy and tolerability of intravenous conivaptan (YM087), a vasopressin V1A/V2-receptor antagonist, in treating euvolemic and hypervolemic hyponatremia.

METHODS

Eighty-four hospitalized patients with euvolemic or hypervolemic hyponatremia (serum [Na+] 115 to < 130 mEq/l) were randomly assigned to receive intravenous placebo or conivaptan administered as a 30-min, 20-mg loading dose followed by a 96-hour infusion of either 40 or 80 mg/day. The primary efficacy measure was change in serum [Na+], measured by the baseline-adjusted area under the [Na+]-time curve. The secondary measures included time from first dose to a confirmed > or = 4 mEq/l serum [Na+] increase, total time patients had serum [Na+] > or = 4 mEq/l higher than baseline, change in serum [Na+] from baseline to the end of treatment, and number of patients with a confirmed > or = 6 mEq/l increase in serum [Na+] or normal [Na+] (> or = 135 mEq/l).

RESULTS

Both conivaptan doses increased area under the [Na+]-time curve during the 4-day treatment (p < 0.0001 vs. placebo). From baseline to the end of treatment, the least-squares mean +/- standard error serum [Na+] increase associated with placebo was 0.8 +/- 0.8 mEq/l; with conivaptan 40 mg/day, 6.3 +/- 0.7 mEq/l; and with conivaptan 80 mg/day, 9.4 +/- 0.8 mEq/l. Conivaptan significantly improved all secondary efficacy measures (p < 0.001 vs. placebo, both doses). Conivaptan was generally well tolerated, although infusion-site reactions led to the withdrawal of 1 (3%) and 4 (15%) of patients given conivaptan 40 and 80 mg/day, respectively.

CONCLUSION

Among patients with euvolemic or hypervolemic hyponatremia, 4-day intravenous infusion of conivaptan 40 mg/day significantly increased serum [Na+] and was well tolerated.

Hyponatremia in neurologic patients: consequences and approaches to treatment

BACKGROUND

Hyponatremia is a common fluid-electrolyte disturbance, particularly in patients with neurologic disorders, in part because of the major role the central nervous system (CNS) plays in the regulation of sodium and water homeostasis.

REVIEW SUMMARY

The classification of hyponatremia is based on an assessment of serum sodium concentration ([Na+]), serum and urine osmolality, and body volume status. In most cases, hyponatremia is associated with hypotonicity, which causes water to move into the brain. Adaptive responses limit the impact of cerebral edema in chronic hyponatremia, but CNS symptoms and death may occur in response to rapid or large decreases in serum [Na+]. The prompt correction of serum [Na+] is mandatory in symptomatic patients, but overly rapid correction must be avoided to limit the risk of myelinolysis. In neurologic disorders, euvolemic hyponatremia (usually caused by the syndrome of inappropriate secretion of antidiuretic hormone) must be distinguished from hypovolemic states such as cerebral salt wasting because the treatment of the 2 conditions differs. Vasopressin antagonists represent a new approach to the treatment of euvolemic and hypervolemic hyponatremia secondary to arginine vasopressin dysregulation.

CONCLUSION

The optimal treatment of hyponatremia is controversial, but appropriate treatment must be determined according to the osmolality and volume status of the patient. If left untreated, serious CNS complications and adverse outcomes, including an increased risk of death, can occur.