Table 1: The three types of vertebral artery with comparison of affected and nonaffected sides.

Affected side
Type IType IIType III

Non-affected sideType I11 (4)7 (0)17 (17)
Type II6 (3)1 (1)0 (0)
Type III11 (4)2 (0)0 (0)
None1 (0)0 (0)1 (1)

Total29 (11)10 (1)18 (18)

The location of the dissection with reference to tortuous portions.
( ): just proximal to the tortuous portion.