By summarizing the pathological changes in the intradural region of the craniocervical junction in patients with syringomyelia and the pathophysiological mechanism of cerebrospinal fluid (CSF) circulation obstruction, this article discusses the procedure of foramen magnum and Magendie dredging (FMMD). Clinical data from 50 adult patients with syringomyelia from July 2018 to January 2019 in Xuanwu hospital were collected and retrospectively analyzed. All operations were performed with FMMD, and all intradural factors that may have induced the obstruction of CSF circulation were recorded. Intradural pathology was found in all of our patients. The pathological changes that may have caused obstruction of the CSF circulation include: tonsil occupying the foramen magnum and overlying foramen of Magendie 88% (44/50), intertonsillar arachnoid adhesions 36% (18/50), tonsil to medulla arachnoid adhesions 18% (9/50), medialized tonsils 70% (35/50), vermian branch of posterior inferior cerebellar artery (PICA) 22% (11/50), arachnoid veil 16% (8/50), cisterna magna cyst 4% (2/50), tonsil to dura mater arachnoid adhesions 8% (4/50). The mean duration of follow-up was 13.3 months. The long-term effective rate was 96.0%. Postoperative MRI revealed that the size of the syringomyelia was reduced or completely resolved in 88% of patients. The mean preoperative Japanese Orthopaedic Association (mJOA) score was 12.9 ± 3.1, which improved to 14.7 ± 3.2 (P < 0.05) at the last clinical follow-up. Intradural pathology that causes CSF circulation obstruction exists in many forms. Relieving the obstruction of the foramen magnum and foramen of Magendie is the key to surgical treatment. Copyright © 2020. Published by Elsevier Inc.