"We also asked if he had any other patients showing similar symptoms after taking Gleevec but got no response. "
Well, from the cited article "Electrolyte Disorders Induced by Antineoplastic Drugs", we have:
"In cancer patients, hyponatremia has an overall prevalence up to 47% with mild, moderate and severe degrees accounting for 36, 10, and 1%"
So while severe is rare, overall is not at all rare.