Inferior cerebellar tonsil herniation, also known as “Chiari malformation”, is a congenital disorder characterized by inferior cerebellar tonsil herniation, most of which is accompanied by spinal cord cavitation (SM). The interrelationship between the two and the pathogenesis are not clear. Clinical manifestations: 1. Posterior cerebral nerve symptoms: dysphagia, hoarseness, delayed or absent pharyngeal reflex. 2, cervical nerve involvement symptoms: occipital and neck pain, ankylosis, limited movement. Unilateral or bilateral upper limb numbness, weakness, muscle atrophy, etc. Symptoms of medulla oblongata and upper cervical medulla: different degrees of sensory impairment of the limbs, weakness, muscle atrophy, and positive pyramidal fasciculus sign. Cerebellar symptoms: nystagmus, slurred speech, Jejunal movement disorder. 5. Increased intracranial pressure: headache, nausea, vomiting and optic papillary edema. Examination method: MRI is the most effective diagnostic tool for ACM-I due to its high soft tissue resolution, multi-directional, multi-parameter and multi-layer imaging, no bone artifacts and radiation damage, and its ability to clearly display the structure of the hindbrain and craniocervical junction area without injecting any contrast agent. Sagittal (SE) T1WI is the best diagnostic sequence. Diagnostic criteria: In China, the lower edge of cerebellar tonsil is lower than the foramen magnum, while in other countries, the cerebellar tonsil is lower than the foramen magnum, but within 2mm, which is not clinically valuable.Pillay proposed a new diagnostic criterion in 1991, which is that cerebellar tonsil herniated out of the foramen magnum ≥2mm can diagnose the disease. Nowadays, the herniation of the lower cerebellar tonsil below the anterior and posterior lip of the foramen magnum is more than 3mm as the diagnostic criterion.