FCEM2-1247

A 78 year old male is brought into the emergency department from a retirement home due to increasing disorientation and drowsiness. Blood tests reveal a serum sodium level of 117 mmol/L and the patient is administered Intravenous 3% sodium chloride solution. The patient initially demonstrates some improvement, becoming more awake and less confused, but after approximately 90 minutes, he becomes lethargic and experiences difficulty speaking with noticeable dysarthria.

What is the probable underlying reason?