Abstract
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.
Materials and Methods : Foramen magnum decompression (FMD ; 207 cases) was performed for CM-ⅠType B and CM-Ⅰborderline cases. Expansive suboccipital cranioplasty (ESCP ; 128 cases) was performed for CM-ⅠType C cases. We examined neurological symptoms and determined the Japanese Orthopaedic Association (JOA) scores. Cerebrospinal fluid (CSF) flow dynamics were assessed pre- and post-surgery using cine phase-contrast magnetic resonance imaging (MRI). During surgery, CSF flow dynamics were examined using color Doppler ultrasonography (CDU).
Results : ESCP and FMD showed a high improvement rate for neurological symptoms and a high recovery rate of the JOA score (77.5%). Craniocervical fixation showed a high improvement rate for neurological symptoms (89%) and a high recovery rate of the JOA score (76.9%). Lysis and/or section of the filum terminale (SFT) and ventriculoperitoneal shunt (VPS) placement exhibited a low improvement rate for neurological symptoms (35-40%). The maximum CSF flow velocity (cm/s) was significantly lower preoperatively than in controls and increased postoperatively. There were no significant differences in the percentages of cardiac cycles. During surgery, CDU indicated that the volume of the cisterna magna was 8 ml and the maximum flow velocity was>3 ml/s.
Conclusions : In the management of CM-Ⅰ, an appropriate surgical method that addresses the ptosis of the brain stem and cerebellum should be chosen. ESCP is appropriate for cases with a small PCFV. FMD is suitable for cases with a normal PCFV and small VAFM. However, other approaches should be considered for CM-ⅠType A (other pathogenesis). For CM-Ⅰ, lysis and/or SFT and VPS are not the only available surgical methods, and FMD or ESCP should be considered.
