The main reason is that the volume of the posterior cranial fossa is small, resulting in insufficient volume in the cranial cavity and finally herniation into the spinal canal. The main symptoms include: 1. headache: severe headache, mainly occurs when exerting or coughing. 2, cerebellar symptoms: mainly manifested as unstable walking, inflexible fingers, walking deviation, etc. 3.Brain stem symptoms: mainly manifested as weakness of one or both limbs. 4.Spinal cord cavity symptoms: If cavity appears, it also mainly manifests as dissociative, segmental sensory impairment. If the above symptoms appear, it is necessary to check and treat them as soon as possible.