To evaluate neuroradiological characters affecting postoperative shrinkage of syringes associated with Chiari type 1 malformation, we assessed 56 patients who underwent foramen magnum decompression (FMD) and were followed-up at least one year. In 48 cases (85.4%), the postoperative courses were quite satisfactory. The cerebellar tonsils have moved upward during surgery and the syringes have decreased in size on MRI taken one or three months after surgery (the prompt group). In eight of 56 patients (14.3%), however, the syringes were slow to shrink and remained over one year after surgery (the delayed group). Basilar impression was more common in the delayed group (p=0.026). These patients had a wider basal angle (142.6 degrees for the delayed group compared with 135.4 degrees for the prompt group), narrower clivo-axial angle (CAA; 121.7 and 142.8 degrees, respectively) and severely lordotic cervical spine (8.0mm and 4.5mm, respectively) on plain X ray films. Those with CAA narrower than 130 degrees showed significant delay in shrinking of the syringes. The CAA might be a good indicator to predict the time course of syrinx shrinkage after FMD.
