Defining limits of foramen magnum decompression in Chiari I malformation to prevent complications related to cerebellar migration: An anatomico-radiological study

Background: Foramen magnum decompression (FMD) is the primary modality of treatment for symptomatic Chiari I malformation. However, the surgery is associated with cerebrospinal fluid (CSF) dynamic complications which may include non-improvement of symptoms, progression of syrinx, or cerebellar slump which are related to the size of decompression. Objective: The objective of the study was to define the limits of FMD to prevent CSF dynamic complications, including cerebellar ptosis. Materials and Methods: Anatomical relations of cerebellar tonsils and occipital bone were studied in 42 sagittal cut sections and 105 sagittal magnetic resonance imaging (MRI) sections. The limits of decompression were defined by our proposition of decompressing till the inferior semilunar fissure. In total, 27 patients underwent FMD and 21 were followed up. Results: Occipital bone in cut sections and MRI sections showed four configurations termed as single cusp, bicuspid, straight, and question-mark. Of these, straight and question-mark configurations were more commonly seen. Of the 21 patients who were followed up after FMD, all had good improvement with none developing features of CSF dynamic complications. Conclusion: The FMD surgery for Chiari I malformation should be performed with limits tailored to individual patients. Decompression should stop at the inferior semilunar fissure so as to prevent CSF dynamic complications, including cerebellar ptosis.