Special Features of Chiari Malformation Type 0

Objective: Chiari malformation type 0 (CM-0) is an uncommon condition characterized by syringomyelia and cerebrospinal fluid (CSF) obstruction at the foramen magnum without cerebellar tonsillar descent. This study aims to delineate its clinical, radiological, and surgical aspects to refine diagnostic and therapeutic protocols. Methods: In a prospective cohort of 80 patients operated for Chiari malformations at Bab El Oued University Hospital between 2017 and 2022, 10 adult cases of CM-0 were identified. All exhibited suboccipital headaches, sensory symptoms, cervicobrachial neuralgia, and syringomyelia. Imaging demonstrated no tonsillar herniation, reduced posterior cranial fossa (PCF) volume (mean 197.4 cm³ vs. normal 228–244 cm³), pointed tonsils, and CSF flow blockage. Treatment involved osteo-dural decompression, arachnoid web dissection, and autologous duraplasty. Results: Headaches resolved postoperatively, with motor improvement reflected by JOA score increase (15 → 16.6). Syringomyelia regressed in length (16.4 → 13.4 vertebral levels) and width (Vaquero index: 0.76 → 0.38). CSF dynamics normalized in 9/10 patients; tonsils changed from pointed to rounded. However, residual symptoms included thermoalgesic dissociation (7/10) and muscle atrophy (9/10). Arachnoid adhesions blocking the foramen of Magendie were observed intraoperatively in all cases. Conclusion: CM-0 is marked by a small PCF, atypical tonsillar morphology, and obstructive arachnoid pathology. Outcomes resemble those of Chiari type 1, but success hinges on thorough intradural inspection and arachnoid release. Tonsillar configuration and arachnoid anomalies may be pivotal in syrinx formation, affirming CM-0 as a separate clinical entity requiring individualized management.