You can either be born with arachnoid cysts (these are labelled primary arachnoid cysts) or they can emerge as a result of disease or an injury (secondary). You are more likely to find an arachnoid cyst if you are under the age of 20, and they are more frequent among young men than women.
On occasion, some arachnoid cysts are both symptomless and harmless, but of course if the cyst grows exponentially to a large size it can affect the central nervous system. This can cause headaches and lethargy, and lead you to feel weak and fatigued. More seriously, it can cause neurological damage, which can indeed be life threatening. Indeed, these types of cysts can lead to seizures, impairment of your senses, development problems and a build-up of fluid in the skull. Furthermore, it can affect your hearing and cause muscles spasms and seizures.
If any symptoms show, your cyst will need to be surgically drained, as if you ignore the problem, your vision may well be damaged as well. Therefore, if your arachnoid cyst is small, generally it will not require surgery. Of course, if the cyst expands then surgery will be required. Surgery could involve a couple of methods: needle aspiration can be used to drain the cyst; however this method could see the cyst return again. Alternatively, riskier method is through neurosurgery to drain the cyst; this could lead to scarring, and your whole arachnoid membrane may need to be taken out. Either way, the surgery could well be risky, but it is worth remembering that it is better for the problem to be resolved than to let it fester; that way, your cyst could cause even more significant damage.