Table 1.

Grading of severity of dorsal compressive AA bands and SM

Grade	AA Band Severity	Severity of SM
0	None	None
1	Underlying SAS reduced but not eliminated	<33% of spinal cord
2	Underlying SAS eliminated, questionable spinal cord compression	33–60% of spinal cord
3	Underlying spinal cord compressed/deformed	>60% of spinal cord

AA, atlantoaxial; SM, syringomyelia; SAS, subarachnoid space.