Redefining Chiari Malformation Type III: a systematic review of prognostic stratification based on a three-tier MRI-based anatomical classification.

backgroundChiari Malformation Type III (CMIII) is a rare and severe hindbrain anomaly typically characterized by a low occipital or high cervical encephalocele containing brain tissue. Despite its traditional definition, considerable variability exists in the morphology and extent of the associated bony defect. With advances in magnetic resonance imaging (MRI), a more detailed understanding of CMIII anatomy is now possible. This systematic review aimed to categorize encephalocele patterns based on MRI findings in CMIII and assess their association with clinical outcomes.

methodsA systematic review of multiple databases was conducted to identify reported CMIII cases published between 1992 and 2022 with MRI-confirmed diagnoses and available outcome data. Inclusion criteria required detailed anatomical description, surgical management, and clinical follow-up. Anatomical classification was based on the extent of the osseous defect reported on MRI. Clinical outcomes included postnatal mortality (primary outcome), and neurological status, hydrocephalus, and need for cerebrospinal fluid diversion (secondary outcomes). Statistical analysis used Fisher's exact test.

resultsTwenty-five CMIII cases from 14 studies met inclusion criteria. Based on these cases, a classification into three types of CMIII was proposed: Type 1-isolated encephalocele in the occipital region; Type 2-encephalocele extending from the occiput (C0) to C2; and Type 3-encephalocele extending from C0 to the subaxial cervical spine. Mortality occurred in 20% of cases, being highest in Type 3 (37.5%) and absent in Type 1. Favorable neurological outcomes were most common in Type 1 (55.5%) and least in Type 3 (12.5%), showing a statistically significant association with defect type (p = 0.048). Hydrocephalus occurred in 72% of cases, most frequently in Type 3 (87.5%), though this was not statistically significant (p = 0.32). All surviving Type 3 cases required shunting.

conclusionRostrocaudal extension of the encephalocele defect in CMIII correlates with increasing mortality and worsening neurological outcomes. This MRI-based anatomical classification offers a practical framework for risk stratification, prognostication, and surgical decision-making in this rare and complex malformation.