Can Atlanto-axial Instability Cause Episodes Of Syncope?

My son is 18 with down syndrome with a typical past including heart surgery. He has had several episodes of syncope in the last couple of years and tonight he had another lasting longer this time, over 1 min. He had worn a cardiac monitor X 7 days without abnormalities last year following a similar episode. He has marginal A A I (atlanto-axial instability) and is marginally highly functional. We push intake levels constantly. He is on thyroid meds with good levels and has recently undergoing Pharma treatments for a possible TBI from car accident last year. There has been no additional sycopal episodes as a result of the accident that we know of. He had been to neurology, psychology, mri , x ray, eeg etc... All coming back within limits or unremarkable.
My question is this; can A A I cause syncope episodes if there is a vertebral slip? If not, what else can we check for? What else could be causing these episodes? Your input is greatly appreciated.