Algorithm-Based Surgical Selection in Chiari Malformation Type I with Syringomyelia: A 124-Patient Ambispective Cohort Study

Abstract Chiari malformation type I (CMI) with syringomyelia is traditionally treated with posterior fossa decompression; however, variable outcomes suggest heterogeneous pathophysiology. Recent evidence indicates that, in addition to cerebrospinal fluid (CSF) obstruction, craniovertebral junction instability may play a significant role. The absence of universally accepted treatment guidelines necessitates an individualized surgical strategy. This article evaluates clinical and radiological outcomes of a structured, multifactorial treatment algorithm in patients with CMI associated with syringomyelia. An ambispective (retrospective and prospective) observational study was conducted on 124 symptomatic patients operated between January 2014 and December 2024. Patients underwent one of four procedures: foramen magnum decompression (FMD) (n = 48), FMD with syringostomy (n = 18), C1–C2 fixation (n = 40), and C1–C2 fixation with joint spacer placement (n = 18). Preoperative evaluation included magnetic resonance imaging, computed tomography (CT) craniovertebral junction, dynamic CT for atlantodental interval (ADI) assessment, and CSF flow studies. Radiological parameters analyzed were posterior fossa volume, sagittal foramen magnum diameter, ADI change, and CSF flow abnormality. Patients were followed for 12 months and categorized into improved and nonimproved groups. Clinical improvement was observed in 63 of 105 patients (60.0%) with complete follow-up, with higher improvement rates in stabilization procedures. Improvement rates differed according to surgical mechanism: 65.0% following C1–C2 fixation, 50.0% after FMD, 40.0% after fixation with spacer, and 33.3% following syrinx-directed procedures. Patients with smaller posterior fossa volume, smaller foramen magnum diameter, and abnormal CSF flow showed better outcomes after decompression, whereas patients with greater dynamic ADI change benefited from fusion procedures. No single radiological parameter independently predicted outcome. Younger age and shorter symptom duration were significant prognostic factors (p < 0.05). CMI represents a heterogeneous disorder involving both CSF flow obstruction and craniovertebral instability. Mechanism-based individualized surgical selection using a combined radiological algorithm improves treatment decision-making compared to a uniform decompression strategy.