In the clinic, patients or family members often come with CT reports and ask, “What is this arachnoid cyst? How do I need to treat it? Do I need an operation?” An arachnoid cyst is a water-sac-like lesion with a thin, transparent envelope on the outside and a colorless, clear, water-like cystic fluid on the inside. After hearing this, many families say, “Oh, so it’s hydrocephalus, that’s fluid in the brain, fluid in the brain, blisters, right”? Wrong! There is a clinical disease called hydrocephalus, which is completely different from arachnoid cyst, and there is also a disease called subdural fluid, and there is a normal component in the skull called cerebrospinal fluid. “There is also a normal component of the skull called “cerebrospinal fluid,” and these terms all mean something completely different, so they cannot be paraphrased plausibly. The correct term is “arachnoid cyst” and there is no other name for it. If you refer to other doctors with the wrong name, they may misdiagnose and mistreat the condition! Arachnoid cysts are benign lesions that do not turn into tumors and are not invasive. They are dangerous because they can become too large and occupy too much space in the cranial cavity, compressing normal brain tissue and causing degeneration of the surrounding brain tissue, resulting in headaches, dizziness, and in a few severe cases, even epilepsy. What are the causes of arachnoid cysts? Most arachnoid cysts are congenital malformations (but not hereditary diseases, not related to genes); it may be that a small piece of arachnoid membrane develops into a double layer during embryonic development and gradually accumulates a large amount of cerebrospinal fluid in the middle, becoming a cyst; so more than 60% of patients with arachnoid cysts are children; some patients have no symptoms or only a mild headache, which is unintentionally discovered during a certain cranial CT; some patients There are obvious headaches and dizziness, which are aggravated when studying is stressful, rest is poor, or even nausea and vomiting, but there is usually no neurological deficits such as limb paralysis, blindness, or aphasia. A small number of patients may have combined epilepsy, which is due to too severe pressure on the brain tissue around the cyst and neurodegeneration that produces epileptic foci. Very few particularly large cysts not only cause epilepsy, but also increase in head circumference, restricted brain development, dementia, and cerebral palsy. On CT, arachnoid cysts are most commonly found in the two areas of the lateral fissure and the greater occipital pool, and range in size from as small as 2-3 cm to as large as 8-9 cm in diameter. If the arachnoid cysts are small in size and have not changed over the years on review, and the patient has no obvious symptoms, such arachnoid cysts do not require treatment, as long as they are reviewed regularly. However, some arachnoid cysts are very large, or are found to be getting larger and larger during the review process, and the brain tissue is significantly compressed, they should be treated with minimally invasive surgery. There are two possible reasons why an arachnoid cyst may gradually increase in size: the cyst wall secretes fluid that gradually enlarges the cyst, or the cyst has a small one-way flowing valve-like opening through which cerebrospinal fluid flows into the cyst, but not out. This type of cyst is very high pressure and requires microdissection. We can make a small incision of about one inch, cut a coin-sized locking hole in the skull, remove most of the cyst wall under the microscope, relieve the pressure, and open the arachnoid cyst to the normal brain pool to create a bidirectional flow to avoid recurrence. In addition, there are neuroendoscopic surgery and bypass surgery. Neuroendoscopic surgery is a little less invasive, but the recurrence rate is higher and it is not easy to stop bleeding in case of recurrence; bypass surgery is only suitable for patients with recurrence. The efficiency of microdissection is more than 99%, the safety is very high, and an experienced neurosurgeon can completely achieve zero mortality and zero disability. Finally, two points should be especially emphasized: 1. After surgical removal of the cyst, there is still a cavity at the surgical site, which is because the original cyst occupies a large space and the brain tissue is shrunken by long-term pressure, and the regeneration after surgery is very slow, so the cavity is still there if the CT is reviewed in the short term; long-term follow-up will reveal the gradual growth of brain tissue, and the younger the child is, the greater the possibility of complete repair; 2. If there was already epilepsy, usually there is still epilepsy after surgery, and long-term antiepileptic medication is needed; in other words, for large arachnoid cysts, you cannot wait for epilepsy to appear before surgery, it is too late.