A source in the PCB said Umar had fell unconscious on a flight from Trinidad to Antigua while playing in the Caribbean Premier League for the Barbados side.
"He was immediately taken to a hospital in Antigua and kept there overnight for tests. They have informed us of his condition and he has been asked to return to Pakistan immediately," the source said.
He said that PCB caretaker Chairman Najam Sethi had instructed that as soon as the youngster returns from the West Indies today, he should be examined by specialists including a leading neurosurgeon approved by the board.
Umar was in the Pakistan Twenty20 and ODI squad for the tour to Zimbabwe but has now been withdrawn by the PCB and the selectors have replaced him with wicketkeeper Sarfraz Ahmed.
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"In the past in Pakistan cricket there has been no other case of a player suffering from epilepsy seizures. Hopefully when he undergoes a complete medical examination, he should be fine," another official said.
The youngster whose two other brothers, Kamran and Adnan are wicketkeepers and have played for Pakistan, is seen as a key player for the more demanding series against South Africa and Sri Lanka in the UAE later this year.