Clinical Mysteries • Electrolytes
The Sodium That Wasn’t Dehydration
The presentation
An elderly patient with pneumonia became progressively confused while receiving isotonic fluids. Sodium kept falling.
The first explanation was plausible, but the pattern did not fully fit. A deliberate second look focused on physiology, medication/exposure history, serial observations and the clue that the initial diagnosis could not explain.
The turn
Euvolaemia, low serum osmolality and inappropriately concentrated urine reframed the hyponatraemia.
Diagnosis
SIADH
Why this case matters
Treat the mechanism, not merely the sodium number.
Take-home message: When the clinical course contradicts the working diagnosis, stop escalating the same treatment and rebuild the problem from first principles.