• Keisuke Hatano, Hiroki Ohashi, Daichi Kawamura, Akira Isoshima, Hiroyasu Nagashima, Satoru Tochigi, So Ohashi, Jun Takei, Akihiko Teshigawara, Satoshi Tani, Yuichi Murayama, and Toshiaki Abe.
• Department of Neurosurgery, The Jikei University School of Medicine, 3-25-8 Nishishinbashi, Minato-ku, Tokyo, 105-8461, Japan. hatakenosuke@gmail.com.
• Acta Neurochir (Wien). 2021 Jun 1; 163 (6): 1593-1601.

BackgroundIt is important to distinguish foramen magnum arachnoiditis (FMA) from Chiari malformation (CM) before surgery because the operative strategies for these diseases differ. In the current study, we compared pretreatment magnetic resonance imaging (MRI) of FMA with CM and investigated the MRI findings useful to differentiate between these diseases.MethodsWe retrospectively reviewed patients with FMA or CM aged ≥ 18 years who underwent surgeries at our institution between 2007 and 2019. The morphologies of the syrinx, neural elements, and posterior cranial fossa were preoperatively evaluated with MRI. We used the receiver operating characteristic (ROC) curve for the fourth ventricle-to-syrinx distance (FVSD).ResultsTen patients with FMAs and 179 with CMs were included. FVSD in the FMA group was significantly shorter than that in the CM group (7.5 mm [IQR, 2.8-10 mm] in FMA vs. 29.9 mm [IQR, 16.3-52.9 mm] in CM, p < 0.0001). The other MRI findings that showed the height, size, and length of the syrinx; size of the foramen magnum; degree of cerebellar tonsillar descent; shape of the cerebellar tonsil; and dorsal subarachnoid space at the foramen magnum differed significantly between the two groups. The ROC curve analysis showed that patients whose FVSD was less than 11 mm could be diagnosed with FMA with a specificity of 90% and sensitivity of 96%.ConclusionsA more cranial syrinx development (FVSD < 11 mm) appears to be the characteristic MRI finding in FMA.

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