Most studies on Chiari malformation (CM) are focused on CM Type-1. A new subtype, CM Type-1.5, lacks sufficient research. This study aims to evaluate the long-term surgical outcomes of posterior fossa decompression followed by duraplasty with arachnoid-preserving (PFDD-AP) technique for primary and recurrent CM Type-1.5.

The medical charts of patients treated surgically for CM at our institute between January 2011 and January 2022 were reviewed retrospectively. Adult patients consecutively treated for CM Type-1.5 were selected as the core sample for the current study. Group A included primary cases (i.e., patients who had not previously been surgically treated), and Group B included recurrent cases. The surgical outcomes were compared by evaluating clinical and radiological findings.

Thirty-four CM Type-1.5 cases out of 202 CM cases met the study criteria. Twenty-three and 11 cases represented Group A and Group B, respectively. The female-to-male ratio was 2/1. In Group B, the preoperative herniated tonsil extension and the obex position were statistically significantly longer, and retroversion and retroflexion angles were statistically considerably smaller ([p = 0.024; Z = − 2.26]; [p = 0.023; Z = − 2.27]; [p = 0.031; Z = − 2.29]; and [p = 0.0002; Z = − 3.72], respectively). For the cases presented with syringomyelia (SM) in both groups (n = 20), total and partial regression were recorded postoperatively in 65% and 15% of cases, respectively. The satisfactory recovery or improvement rate in neurological symptoms was 94.1%.

Total or partial SM regression occurred following PFDD-AP in most adult patients with CM Type-1.5 who presented with SM. The PFDD-AP approach offers better results with fewer complications and recurrence rates.

This work was carried out at the Department of Neurosurgery, Atlas University–Medicine Hospital.

This comparative retrospective study was approved under decision number: (E-22686390-050.99-26421/2023) by the medical ethics committee of Atlas University, Istanbul, Turkey.

*All authors contributed equally to this work and deserve the first name.

Received: 06 February 2025

Accepted: 16 April 2025

Accepted Manuscript online:
17 April 2025

Article published online:
26 June 2025

© 2025. Thieme. All rights reserved.

Georg Thieme Verlag KG
Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany