Posterior Fossa Decompression Followed by Duraplasty with Arachnoid-Preserving Technique for Primary and Recurrent Adult Chiari Malformation Type-1.5: A Comparative Retrospective Study

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 ([<i>p</i> = 0.024; <i>Z</i> = - 2.26]; [<i>p</i> = 0.023; <i>Z</i> = - 2.27]; [<i>p</i> = 0.031; <i>Z</i> = - 2.29]; and [<i>p</i> = 0.0002; <i>Z</i> = - 3.72], respectively). For the cases presented with syringomyelia (SM) in both groups (<i>n</i> = 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.