A valuable subarachnoid space named the occipito-atlantal cistern.

The cisterna magna has been defined as the space between the inferior margin of the cerebellar vermis to the level of the foramen magnum, while an enlarged dorsal subarachnoid space at the occipito-cervical junction extending from the foramen magnum to the upper border of the axis (C2) is still ignored. Recently, the myodural bridge complex is proved to drive the cerebral spinal fluid flowing via this region, we therefore introduce the "occipito-atlantal cistern (OAC)" to better describe the subarachnoid space and provide a detailed rationale. The present study utilized several methods, including MRI, gross anatomical dissection, P45 sheet plastination, and three-dimensional visualization. OAC was observed to be an enlarge subarachnoid space, extending from the foramen magnum to the level of the C2. In the median sagittal plane, OAC was a funnel shape and its anteroposterior dimensions were 15.92 ± 4.20 mm at the level of the C0, 4.49 ± 1.25 mm at the level of the posterior arch of the C1, and 2.88 ± 0.77 mm at the level of the arch of the C2, respectively. In the median sagittal plane, the spino-dural angle of the OAC was calculated to be 35.10 ± 6.91°, and the area of OAC was calculated to be 232.28 ± 71.02 mm. The present study provides OAC is a subarachnoid space independent from the cisterna magna. Because of its distinctive anatomy, as well as theoretical and clinical significance, OAC deserves its own name.