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 posterior cranial sulcus during the embryonic period. Pathology and pathogenesis Currently, there are different opinions on the pathogenesis of this disease. It is mostly believed that the lower herniation is due to the overgrowth and extension of brain tissue in the midline structure of the posterior cranial recess during the embryonic period, which, together with the reduction in the volume of the posterior cranial recess, further encourages it to herniate downward through the foramen magnum into the cervical spinal canal. Some of them even descend to the cardinal vertebrae or lower, so as to seriously damage the cerebellum, brainstem and high cervical medulla and cervical nerve, and may cause hydrocephalus. The disease is often combined with other occipital malformations, such as skull base depression, atlanto-occipital fusion, flattened skull base, and incomplete cervical segmentation. Mild subcerebellar tonsillar herniation may be asymptomatic, but trauma, infection, cough and lumbar puncture may induce symptoms or aggravate them. The symptoms caused by subcerebellar tonsillar herniation are generally as follows: cranial and cervical nerve symptoms, such as hoarseness, difficulty in swallowing, pain in the neck and limitation of movement. Brainstem medulla symptoms may include limb movement disorders, hemiparesis and quadriplegia, sensory disorders of the extremities, and urinary and fecal disorders. 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 can be accompanied by sensory separation or muscle atrophy of both upper extremities when spinal cord cavitation is present. The patient shown on the left is a young male with bilateral lower extremity paraplegia after cervicothoracic spinal cord injury during labor, followed by consciousness irritation and abnormal breathing. After surgical decompression, the patient’s consciousness and respiratory condition improved. This case is a case of trauma-induced subcerebellar tonsillar herniation symptoms. As shown in the figure, (i) the subcerebellar tonsillar herniation and compression at the foramen magnum; (ii) the cervical spinal cord injury site. Diagnosis Based on the above clinical manifestations and MRI examination, the diagnosis is not difficult to establish. The 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 main treatment is surgery, which aims to relieve the compression of cerebellum, brainstem, spinal cord, fourth ventricle and other neural tissues by occipital foramen and cervical spine, unblock cerebrospinal fluid circulation, and relieve the symptoms of neurological compression and hydrocephalus. In a small number of patients with severe subcerebellar tonsillar herniation, removal of the cerebellar tonsils can be considered; in cases with evidence of adhesions to the median foramen of the IV ventricle, microdissection can be performed; in cases with hydrocephalus, shunts can be performed as appropriate.