MacDonald, 21, was riding Russian Royale for Middleham trainer Micky Hammond in a mares’ handicap hurdle on March 18 when falling three flights from home.
MacDonald, who is attached to Hammond’s yard, was to hospital on the same day and later discharged. However he has been readmitted after being taken ill on Wednesday.
In a statement, the Injured Jockeys Fund said: “Conditional Jockey Aidan MacDonald had a fall at Hexham racecourse on March 18 and went to the RVI (Royal Victoria Infirmary) in Newcastle, from where he received extensive testing and was subsequently discharged.
“Yesterday, after being taken ill, he went in an ambulance to the James Cook Middlesbrough Hospital. He is in a critical condition and receiving active treatment for a suspected carotid artery dissection.
“Aidan’s parents kindly ask for privacy at this difficult time.”
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