Horizontal Craniocervical Instability
Horizontal instability is when parts of the craniocervical junction slide forward, backwards, sideways on a horizontal plane. The most sensitive imaging is in dynamic (flexion-extension) because the Radiologists can do measurements of the craniocervical junction in these positions.
Most definitely a uMRI in flexion and extension is helpful in investigating horizontal craniocervical instability. Many people do not have access to uMRI, so in this case a flexion -extension xray, CBCT or CT could provide valuable info as would a supine c-spine and Brain MRI.