Diagnosis is via lateral cervical spine radiographs or CT scan.  Besides the fracture line:

• Look for prevertebral soft tissue swelling
• Assess alignment of the vertebrae as assessed by intact anterior spinal, posterior spinal, and spinolaminar lines (it should be possible to draw a smooth curved line touching all 7 cervical vertebrae at each of these locations without interruption). 

Any soft-tissue swelling or disruption of normal lines, in the setting of trauma, suggests occult fracture or ligamentous injury.

Caveat:

     In children under 8, ligamentous laxity of the upper C-spine can cause anterior displacement of C2 on C3, a normal variant called pseudosubluxation, which can be confused with an acute ligamentous injury.  Up to 4 mm or 40% subluxation of C2 on C3 is normal in the pediatric population (but not in adults).

If in doubt:

• Look for soft tissue swelling (absent in pseudosubluxation)
• Take an extension film (displacement decreases or resolves in extension if due to pseudosubluxation)
• Check Swishuk’s line (the line drawn in the additional image between the anterior aspects of the spinous processes of C1 and C3—in pediatric pseudosubluxation, it will be within 2 mm of the anterior aspect of the C2 spinous process).