Intracranial arachnoid cysts (IAC) are benign lesions caused by the accumulation of cerebrospinal fluid between the two layers of the arachnoid membrane and in the subarachnoid space of the cerebral sulcus and brain pool, of which 60 to 90% occur in childhood, with the majority occurring on one side and more common in the temporal lobe. IAC in the midline region in children is less common. 17 cases of giant IAC in the midline region in children were treated by cystic ventral shunt in our hospital since January 2005 to December 2011 with good results, which are reported below. DATA AND METHODS General data: A total of 17 cases of giant IAC in the midline, 10 males and 7 females, aged 2 months to 7 years old, with an average of 19.6 months, were admitted and underwent shunt surgery from January 2005 to December 2011. There were 11 supratentorial cases, 5 infratentorial cases, and 1 case with both supratentorial and infratentorial presence. 7 cases were combined with hydrocephalus. Clinical features: 4 cases were found by ultrasound during pregnancy, 11 cases had enlarged head circumference, 1 case had headache and vomiting, and 1 case had poor limb movement; 6 of them were accompanied by growth retardation. The physical examination mostly showed chronic cranial hypertension. Auxiliary examinations: CT or MRI was performed in all cases, and the imaging revealed a huge cystic occupancy in the supratentorial or infratentorial midline area with the same density and signal as cerebrospinal fluid, which could be accompanied by hydrocephalus (Figure 1). Surgical approach: all cases underwent cystic ventral shunt surgery with medium and low pressure valves in early cases and adjustable valves applied mostly in later cases. The dura mater and the cyst wall layer were exposed in the head and then punctured with a puncture needle, and after seeing the cyst fluid outflow the brain needle was placed to confirm the entry inside the cyst, then the shunt was punctured to enter with the direction towards the cyst root, and the depth of placement was about 4 cm, and the fixation clip was mostly not placed at the cranial exit. The valve was placed behind the ear, and the rest of the operation was similar to ventriculoperitoneal shunt. Results There were no surgical deaths or complications, and the postoperative follow-up ranged from 5 to 88 months, with a mean of 39 months. Only one case was reoperated to replace the shunt system 2 years after surgery due to separation of the cephalic end tube from the valve, and no sequelae such as shunt obstruction or infection were seen in the rest. In all cases, the cysts shrank (Figure 2) and the symptoms disappeared. The preoperative developmental laggards caught up or approximated to normal children of the same age after surgery. Discussion IAC accounts for about 1% of intracranial occupying lesions, and the ratio of male to female occurrence is about 2:1. It is most often located on the surface of brain tissue in the main cerebral sulcus, with the supratentorial predominance, half of which are located in the lateral fissure of the middle cranial fossa [1], with relatively few midline sites, and the clinical features have their specificity. There were 17 cases in our group, accounting for only 18% of the cases that underwent IAC surgery during the same period. IAC is mostly considered a congenital malformation and can also be secondary to trauma and inflammation, but the latter is less common. The formation of congenital IAC may be due to alterations in CSF flow during the early stages of subarachnoid formation, or it has been suggested that it is caused by duplication of malformations and splitting during the development of the arachnoid lining [1]. Some cases may be combined with malformations of other brain tissues and venous systems. Superficial cerebral IACs are more often associated with other developmental malformations because of their location and the less important brain tissue surrounding them, and are less likely to produce symptoms early. Supratentorial IACs in the midline are more likely to be combined with agenesis of the corpus callosum, while subratentorial cases are more likely to be combined with cerebellar hypoplasia. In our group, 8 of the 11 cases of supratentorial IAC were combined with agenesis of the corpus callosum or hypoplasia, and 5 cases of subretinal IAC had varying degrees of cerebellar hypoplasia, which is presumed to be a developmental defect formed congenitally. The vast majority of IACs are asymptomatic, are detected after trauma or on unintentional imaging, and can remain stable for many years. However, some cases present with enlarged cysts, such as increased CSF secretion from the cyst wall, high protein content of the cyst fluid producing an osmotic pressure difference or the presence of live valve-like traffic in the cyst wall and subarachnoid space [2].Sandberg et al. analyzed the chemical composition of cyst fluid in children with IAC and found that the chemical composition of the cyst fluid was similar to that of CSF, but some cysts contained higher levels of protein, so it was postulated that it may be that the protein concentration inside and outside the cyst The difference in colloidal osmotic pressure generated by the difference in protein concentration inside and outside the cyst caused the enlargement of the cyst [3]. Enlarged cysts can cause localized brain tissue dysplasia, epilepsy, developmental delay, visual loss, or motor impairment. In addition, cysts may rupture due to trauma resulting in acute intracranial pressure increase producing symptoms such as headache and vomiting. Infants and children often present with non-specific head circumference enlargement [4-5], and 11 of our children, or 65% of our cases, presented with head circumference enlargement as the first presentation. This increase in head circumference produced by IAC reinforces the correlation between the production of IAC and the altered fluid dynamics of CSF. IAC at the midline site is diagnosed due to larger cysts, which present with symptoms or are detected earlier, even during pregnancy. It also shows pushing pressure on the surrounding normal brain tissue, and mostly needs to be treated early to relieve intracranial pressure, improve brain tissue compression, and provide space and time for brain tissue development. Some midline IAC can cause hydrocephalus, which is mainly caused by cysts compressing cerebrospinal fluid circulation pathways, therefore it is mostly obstructive hydrocephalus, and hydrocephalus can mostly improve after cyst reduction. For small IACs that are unintentionally found without symptoms, they can be treated conservatively and followed up with regular imaging examinations.Rao et al. analyzed the cases of children with enlarged IAC and concluded that cases of rapidly enlarging IAC are very rare and recommended that imaging follow-up should be considered at least 1 year after the first detection of IAC and that children with rapidly enlarging head circumference or neurological impairment should always undergo imaging at any time if the child shows signs of rapid head circumference increase or neurological impairment [6]. Surgery is considered for large cysts producing brain tissue compression, hydrocephalus, or neurological symptoms due to the cyst. The indications for surgery for IAC are currently considered to include (1) obvious symptoms of increased intracranial pressure, displacement of localized brain tissue compression or localized cranial augmentation changes (2) clear symptoms of neurological deficits caused by IAC or induced epilepsy; (3) supratentorial cysts of 30 ml or more than 5 cm in diameter, with large cysts that may affect brain development and intelligence; and (4) intracranial hemorrhage or combined hydrocephalus. The diameter of the cysts in this group of cases was close to or more than 10 cm, which occupied a large space in the cranial cavity, affecting the development of brain tissue and possibly producing compression of the surrounding structures, with corresponding consequences or even secondary hydrocephalus. Therefore, all cases in this group underwent early shunt surgery to relieve intracapsular and intracranial pressure and to provide space and time for the growth of normal brain tissue. After timely treatment, the children in this group recovered effectively both in terms of imaging and clinical manifestations such as growth and development. There are controversies about the surgical methods of IAC, including craniotomy or neuroendoscopic windowing, stereotactic aspiration and shunt surgery. Bypass surgery is easy to perform, with low surgical risk and good results, and can relieve intracranial hypertension and restore the impaired CSF fluid dynamics, especially for cases with combined hydrocephalus or cystic window failure, and remains the most widely used treatment method. The shunt valve is mostly chosen as a low-pressure valve to ensure that it can quickly and effectively relieve symptoms and relieve intracranial hypertension, and some cases with high intracapsular pressure may produce brain tissue collapse, subdural effusion or even hemorrhage due to excessive shunting. With the continuous improvement of shunt systems and economic development, more and more cases are opting for the application of adjustable valves, thus enabling adjustment according to pressure changes and more individualized relief of postoperative discomfort and reduction of complications. The head end of the tube should be placed as deep as possible in the middle of the cyst so as to prevent the obstruction of the tube caused by the shrinking of the cyst. However, shunt surgery still has relatively obvious disadvantages, requiring mostly lifelong tubes, possible complications such as shunt dysfunction such as fissure syndrome, infection, and the possible need to change tubes midway through the procedure, with a reported rate of approximately 30% of those undergoing shunt surgery in IAC cases requiring tube changes [7]. In our group of children, due to the small number of cases and short follow-up period, only one case had separation of the tube from the shunt valve, which improved after valve replacement, and no other complications have yet occurred. Cystotomy facilitates direct observation of the cyst wall, allows electrocoagulation of the arachnoid vessels, and enables obtaining tissue specimens for pathological diagnosis. It is particularly indicated for IAC in critical areas, where placement of a shunt may result in damage to surrounding structures, or where the cyst wall may collapse . Craniotomy allows for maximal removal of the cyst wall, windowing of multifocal cysts, and easy communication with adjacent brain pools. However, it is highly invasive and prone to brain tissue collapse and cyst recurrence. It is estimated that about 5 to 25% of cases will have recurrence of headache and other symptoms due to cyst recurrence [8]. In recent years, with the continuous development of neuroendoscopic techniques, neuroendoscopic cyst opening has been more and more widely used in the treatment of IAC, which is able to perform almost all operations of craniotomy in a minimally invasive manner, and again eliminates the risk of carrying a tube, especially for the lateral fissure, suprasellar, tegmental area and posterior cranial fossa [9]. However, the procedure has high requirements for instruments and operations, requires certain hardware conditions and surgical cooperation, is difficult to perform, and cannot be performed in general hospitals. The midline site of IAC is easy to be lost after endoscopic access to the cyst due to the presence of other developmental brain malformations, displacement and variation of normal brain anatomy, and it is difficult to communicate with the normal brain pool or subarachnoid space at the planned location, and the auxiliary navigation system helps in intraoperative localization and operation. Its complications mainly include subdural effusion and hemorrhage [10].