Arachnoid cysts are cystic occupying lesions outside the brain or spinal cord parenchyma and are not tumors. They are mostly solitary, but rarely multiple, and are often located in the brain fissure and brain pool, such as temporal lobe cysts, lateral fissure cysts, and occipital pool cysts. It has arachnoid-like cyst wall and cerebrospinal fluid-like cyst fluid, individual protein content is higher than cerebrospinal fluid, or slightly yellowish transparent appearance. Most of the cysts are located on the surface of the brain, individually they are closely related to the subarachnoid space and are actually huge expansions of the cerebral pool, or there is no traffic in the surrounding subarachnoid space. Arachnoid cysts are formed superficially and deeply by a transparent arachnoid membrane that is isolated from the ventricles in the brain. Large volumes may compress both brain tissue and skull, which may produce neurological symptoms and cranial changes. The syndrome is more common in children, more often in males, and more often on the left side than on the right side. It can be divided into congenital, traumatic and post-infection arachnoid cysts according to their etiology. I. Common types of congenital arachnoid cysts 1. Lateral fissure arachnoid cyst: enlarged lateral fissure, sometimes accompanied by anterior temporal lobe and inferior frontal gyrus agenesis. It is common in young people from a few years old to less than 20 years old, with a higher incidence in males than in females, often with headache, seizures (can be limited or generalized epilepsy, psychomotor seizures), temporal bony bulges, a few with ipsilateral proptosis, and in late stages, optic papilledema and contralateral mild hemiparesis. The disease has no symptoms, or mild headache, or is found accidentally during an incidental physical examination in many cases. 2, the brain convex arachnoid cyst: seen in infants or adults, infants often progressive cranial enlargement, asymmetric on both sides, transillumination test can be seen cyst boundary, sometimes there are seizures. 3, longitudinal arachnoid cysts: often no clinical symptoms, about half of them are accompanied by corpus callosum dysplasia. 4.Arachnoid cysts in the saddle area: located in the saddle or in the saddle. The suprasellar ones are rare and can occur at any age. Cysts and the optic cross pool can be connected or not. Small cysts may be asymptomatic, but large ones may destroy the pterygoid saddle and compress the pituitary gland, optic cross and interventricular foramen, producing visual impairment, hypopituitarism and obstructive hydrocephalus. The intra-saddle cyst is asymptomatic, but it can also develop to the saddle through the enlarged saddle diaphragm foramen, forming the empty saddle syndrome. 5.Arachnoid cyst in the tetrahymenal area: the cyst is connected or not connected with the tetrahymenal pool. In the early stage, it may compress the aqueduct to produce obstructive hydrocephalus and increased intracranial pressure. 6.Arachnoid cyst in the pontocerebellar angle: early stage has neurological deafness and hypoacusis, late stage has cerebellar sign and intracranial pressure increase symptoms, individual may have peripheral facial palsy and trigeminal neuralgia. 7.Cerebellar arachnoid cyst: it can be located in cerebellar hemisphere, earthworm or occipital pool. Clinical symptoms of increased intracranial pressure are often present, and some cases have signs of cerebellar damage. For the most common temporal base and lateral aspect, young children may present with temporal skull bulge, thin temporal scales and chronic intracranial pressure increase, mostly with mild headache or no obvious symptoms, only found during incidental physical examination or not until adulthood. The diversity of symptoms is related to the size of the cyst and the site of growth. Small cysts may be asymptomatic. Second, post-infection arachnoid cysts are formed after meningitis due to local adhesions of the arachnoid membrane, and the cysts are filled with cerebrospinal fluid, or fluid components with high protein content. Most of them are multiple. Most often seen in children. They are commonly found in the optic cross pool, basal pool, cerebellar medullary pool, and cricoid pool. Due to the obstruction of cerebrospinal fluid circulation pathway, clinical symptoms of hydrocephalus and increased intracranial pressure may be manifested. Cysts in the optic chiasm may produce visual impairment, while those in other areas may also produce limited symptoms. Children often have an enlarged skull. The diagnosis is based on a history of meningitis and signs of increased intracranial pressure. The diagnosis can be confirmed with a CT scan. However, it is sometimes difficult to differentiate from congenital arachnoid cysts. Multiple cysts should not be operated on, but the main cyst producing clinical symptoms can be removed. Cerebrospinal fluid shunts can be performed in the presence of hydrocephalus and increased intracranial pressure. Arachnoid cyst after injury, also known as soft meningeal cyst The mechanism of its occurrence is a linear fracture of the skull caused by injury, accompanied by a tearing defect of the dura mater, with hemorrhage in the subarachnoid space below it or adhesions around the arachnoid margin, causing local cerebrospinal fluid circulation disorders, resulting in local arachnoid protrusion into the dural fissure and fracture line, gradually forming a cyst under the constant impact of brain pulsation, so that the fracture margin is constantly expanding, called a growing fracture. The cyst may protrude under the scalp and also compress the cerebral cortex below. The cyst is filled with clear fluid and surrounded by scar tissue. If the soft meninges are broken during trauma, the brain tissue may herniate into the fracture and the ipsilateral ventricles may enlarge, even forming a cerebral penetration malformation.