The selective vasopressin V2-receptor antagonist, tolvaptan (Samsca), is used to treat adults with hyponatraemia secondary to inappropriate antidiuretic hormone secretion (SIADH). However, over-rapid correction of serum sodium can cause osmotic demyelination, resulting in spastic quadriparesis, seizures, coma and death.
Patients at increased risk
MHRA guidelines recommend that serum sodium be closely monitored in patients receiving tolvaptan, especially in those with very low levels (<120mmol/l) at baseline and those at increased risk of demyelination syndromes (eg, as a result of hypoxia, alcoholism or malnutrition).
Sodium correction greater than 6mmol/l during the first 6 hours of administration or 8mmol/l during the first 6–12 hours may be too rapid. In such cases, close monitoring of serum sodium and administration of hypotonic fluid is recommended. If the increase in serum sodium is greater than 12mmol/l in 24 hours, or 18mmol/l in 48 hours, tolvaptan treatment should be interrupted or discontinued and followed by administration of hypotonic fluid.
Concomitant treatment with medicines containing high levels of sodium or other treatments for hyponatraemia is not recommended.
Tolvaptan may also interact with and reduce the effect of vasopressin analogues, such as desmopressin, used to prevent or control bleeding.