What’s the relationship here?
In the 1990s, a paper in NEJM described the first relationship of the two. Pseudohyponatremia has been classically described with intravenous gamma globulins. Pseudohyponatremia is a laboratory artifact due to hyperlipidemia or hyperproteinemia. Intravenous infusion of immune globulin increases the protein load and as a result the protein phase of serum is higher. Depending on the type of laboratory test used, some centers might note a low Na. Usually, the serum osmolarity then would be normal clinching the diagnosis.
Another mechanism that has been proposed is a form of hypertonic hyponatremia – the one you would see in mannitol or hyperglycemia. most IVIG preparations contain significant amounts of sucrose or maltose leading to the increased osmolarity and leading to fluid shifts leading to hyponatremia. The magnitude of the Na drop might depend on renal function as well and clearance of sucrose.
Finally, a dilutional variant has been noted as well ( Sosm would be low) and you would have a true hyponatremia due to the volume of IVIG infusion that leads to a form of water intoxication.