What is the upper cervical cord abnormality? What differential should be entertained?
Focal syrinx is the most likely cause (given the presence of a Chiari 1 malformation), although a cord tumour (e.g. ependymoma) should be considered.
What should therefore be performed prior to treatment?
Dedicated imaging of the cervical cord with contrast.
Diagram and annotated image of how to measure tonsilar descent. A line (dotted blue) is drawn on the midline sagittal plane across the inner margins foramen magnum (ophisthion to basion), and measuring the inferior most part of the tonsils (green arrows). The tonsils normally protrude through foramen magnum by no more than 5 mm.