As the article and my comments say, a NUCCA or Atlas Orthagonal chiropractor is a good start.

Upper neck injuries are often symptom free except for the PCS symptoms. There may be tenderness behind the ears where muscles may be spasming.

Usual PT and other range of motion exams and such are not valid at denying an upper neck injury. Upper neck injuries are very subtle.

Mine was only symptomatic with head aches and breathing interruptions, neither of which seemed to have any connection to my upper neck. It was only after resolving the upper neck issues that those symptoms were reduced or eliminated.

A big part of my improvements came from learning to sleep on my back with my head in a straight posture.

It's a long story about how I learned these issues with my neck. Suffice it to say, I would have told you my neck was fine, too.