Arnold-Chiari malformation, also known as Chiari malformation, is a congenital developmental abnormality in which the cerebellar tonsils extend downward, or protrude through the foramen magnum into the cervical spinal canal with the lower part of the medulla oblongata or even the IV ventricle, due to abnormal development of the brain structures in the midline of the posterior cranial recess during the embryonic period. Clinical manifestations】 Cranial nerve and cervical nerve symptoms include hoarseness, difficulty in swallowing, neck pain and restricted movement. Brainstem medulla symptoms: Motor disorders of the limbs, hemiparesis and quadriplegia, sensory disorders of the limbs, and urinary and fecal disorders may occur. Cerebellar symptoms Ataxia, unstable walking and nystagmus may occur. Symptoms of increased intracranial pressure Headache, vomiting, fundus edema, and decreased visual acuity may be seen in hydrocephalus. Spinal cord cavitation symptoms may include sensory separation or muscle atrophy of both upper limbs in the presence of spinal cord cavitation. Diagnosis] Based on the above clinical manifestations and MRI examination, it is not difficult to establish the diagnosis. MRI examination can clearly show the specific location of the subungual herniation, the presence of medulla oblongata and subventricular herniation, the displacement of the brainstem, the presence of spinal cord cavity and hydrocephalus, etc. X-ray plain film examination and CT can understand the bony deformity of the cranial neck. The purpose is to relieve the compression of the cerebellum, brainstem, spinal cord, fourth ventricle and other neural tissues by occipital foramen and cervical spine, unblock the cerebrospinal fluid circulation, and relieve the symptoms of neurological compression and hydrocephalus. In a few patients with severe subcerebellar tonsillar herniation, resection of the cerebellar tonsils can be considered; for those with evidence of adhesions to the median foramen of the IV ventricle, microdissection can be performed; in those with hydrocephalus, shunts can be performed as appropriate. Case] A 41-year-old male with numbness and discomfort in both upper extremities for six months was diagnosed with subcerebellar tonsillar herniation and spinal cord cavity. Microsurgical decompression was performed, and the bilateral cerebellar tonsils were retracted, the median foramen at the latch of the medulla oblongata was opened, and the decompression was satisfactory; the bilateral posterior inferior cerebellar arteries and perforating branches were preserved intact. The artificial spinal membrane was used for anti-adhesion treatment. Postoperatively, the symptoms improved significantly, and MRI showed that the spinal cord cavity was reduced.