Fig. 2

Classification of the magnitude of DCT with examples for each degree. A – McRae’s line (FM); B – Upper border of atlas (C1); C – Lower border of atlas (C1). The interval A-B has been divided into an upper (grade 1), middle (grade 2) and lower (grade 3) third. They are followed by grade 4 – between the upper and lower borders of C1, and finally grade 5 – Underneath the lower border of C1. When the tonsil reaches exactly line A, it is considered grade 0, that we call Impaction of Cerebellar Tonsils. The arrows indicate another relevant parameter often associated to DCT, the Increase of the Supracerebellar Space