Ventrolateral Tonsillar Position Defines Novel Chiari 0.5 Classification.

Area of Science:

• Neurology
• Pediatric Neurosurgery
• Medical Imaging

Background:

• Cervicomedullary compression in young children is often associated with Chiari type 1 malformation, historically defined by tonsillar herniation ≥5 mm.
• Chiari symptoms can occur in young children without classic imaging findings, necessitating a revised classification.
• A new classification is needed to address Chiari malformations in very young children presenting with symptoms but lacking typical imaging criteria.

Purpose of the Study:

• To define a novel Chiari malformation entity in young children.
• To identify imaging characteristics and clinical presentations of this new entity.
• To compare outcomes of patients with this novel entity to those with typical Chiari morphology.

Main Methods:

• Retrospective review of 41 pediatric cases (≤5 years) evaluated for posterior fossa decompression.
• Recording of preoperative symptoms, MRI findings, surgical management, and outcomes.
• Definition and identification of Chiari type 0.5 malformation based on ventral herniation (VH) of tonsils around the caudal medulla.

Main Results:

• Twenty of 41 reviewed cases met criteria for VH (Chiari type 0.5 malformation).
• Patients with VH predominantly presented with medullary symptoms compared to those without VH.
• Eleven VH patients underwent successful surgical decompression; 3 of 9 initially managed conservatively later required surgery.

Conclusions:

• Chiari type 0.5 malformation is a novel entity characterized by ventral tonsillar wrapping around the medulla in young children, often without classic Chiari type 1 imaging.
• Patients with Chiari type 0.5 malformation are more prone to medullary symptoms.
• This patient group demonstrates a higher likelihood of requiring and benefiting from surgical decompression.