Outcome of Surgical Management for Syringomyelia Associated with Chiari Malformation Type Ⅰ―Deciding Surgical Procedures Based on Evaluation of Cerebrospinal Fluid Flow at the Foramen Magnum―

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Introduction : We classified Chiari malformation typeⅠ (CM-Ⅰ) according to the pathogenesis of ptosis of the brain stem and cerebellum based on morphometric analyses of the posterior cranial fossa (PCF) and craniovertebral junction. Three independent subtypes were confirmed―CM-ⅠTypes A, B, and C. CM-Ⅰtype B is characterized by a normal PCF volume (PCFV), small volume of the area surrounding the foramen magnum (VAFM), and a small occipital bone size. CM-ⅠType C is characterized by a small PCFV, small VAFM, and occipital bone size. CM-ⅠType A (other pathogenesis) is characterized by conditions such as craniocervical instability and tethered cord syndrome. We examined the pathogenesis of ptosis of the brain stem and cerebellum and reported the preliminary outcomes of each surgical approach.