Arachnoid cyst, a type of benign brain cyst. There is an arachnoid-like cyst wall and cerebrospinal fluid-like cyst fluid. Arachnoid cysts are divided into two categories: congenital and secondary. CT manifestations of lateral fissure arachnoid cysts Congenital arachnoid cysts are pouch-like structures formed by cerebrospinal fluid being enclosed within the arachnoid membrane and not communicating with the subarachnoid space. Secondary to arachnoid adhesions, cysts form in the subarachnoid space and contain cerebrospinal fluid. They occur in the lesser cranial recess, but also in the greater occipital pool, the periaqueductal brain pool, and the suprasellar pool. Intracranial arachnoid cysts can be classified into congenital, traumatic and post-infection arachnoid cysts according to their etiology. Congenital arachnoid cysts Congenital arachnoid cysts are a common type of cyst, the cause of which is not fully understood, and there are the following speculations: 1. Starkman et al. suggested that the cause of this disease may be the development of a small piece of arachnoid membrane falling into the subarachnoid space during embryonic development. The cyst was located within the arachnoid membrane, and microscopically, the arachnoid membrane was seen to split into two layers around the cyst, with the outer layer forming the surface part of the cyst and the inner layer forming the base of the cyst, and there was still a subarachnoid cavity between the soft meninges and the base of the cyst. Dajie Jiang found that the surface part of the cyst wall also consisted of two layers of arachnoid membrane, that is, the cyst was all located in the subarachnoid space. 2, Many people believe that during embryonic development, due to the pulsation of the choroid plexus, which acts as a pump for cerebrospinal fluid, it can separate the loose peri-medullary network around the neural tissue and form a subarachnoid cavity, and if the flow of cerebrospinal fluid is abnormal in the early stage, a cyst can be formed in the peri-medullary network. 3. Because the syndrome is often accompanied by other congenital anomalies, such as ectopic choroid plexus within the cyst, local absence of the cerebral falx and absence of the orbital plate, temporal lobe and internal carotid artery, all confirm that the basic cause of the syndrome is due to cerebral dysplasia. There is no unified opinion on the cause of increasing arachnoid cysts, which may be: 1. There are small holes in the wall of the cyst that are connected to the subarachnoid space, and cerebrospinal fluid continuously flows into the cyst from this hole, which acts as a live valve, and the cyst gradually increases due to the pulsation of the arteries at the base of the skull. It is also possible that some factors cause the small hole to be blocked and cause the intracranial pressure to increase. 2. There is an ectopic choroid plexus inside the sac, which secretes too much cerebrospinal fluid and cannot be absorbed. In some cases, the cyst does not communicate with the subarachnoid space, and the protein in the cystic fluid increases, and the difference in osmotic pressure inside and outside the cyst causes the cyst to increase gradually. 4. The cystic cavity increases rapidly due to intracapsular or supracapsular venous hemorrhage. Post-infection arachnoid cysts, post-injury arachnoid cysts, also known as soft meningeal cysts. The clinical presentation is similar to that of intracranial occupying lesions. Some patients may have mild paralysis or seizures. There may also be local symptoms such as ocular protrusion and head disproportion. Treatment Surgery is not required if there are no clinical symptoms. Those with symptoms should have surgery to remove the cystic fluid and excise the surface portion of the cystic wall and open the inner wall to the subarachnoid space. Good results can often be obtained. If surgery fails to relieve the symptoms of increased intracranial pressure, or if the cyst recurs after surgery, cerebrospinal fluid shunt is feasible. If an ectopic choroid plexus is found within the cyst at the time of surgery, it should be removed by electrocoagulation. The treatment of this disease is to remove the cyst, repair the dural defect and repair the skull defect.