Neurosurgery - Arachnoid Cyst

This patient information leaflet sets out to explain the diagnosis, and management of arachnoid cysts.

What is an arachnoid cyst?

The brain and spinal cord are surrounded by three membranes called meninges. One of these is called the arachnoid layer due to its resemblance to a spider’s web. Arachnoid cysts have a wall composed of this arachnoid membrane and are located between the arachnoid layer and the brain. They contain the watery fluid that normally surrounds the brain: this is called cerebrospinal fluid (CSF). This clear liquid protects and nourishes the brain and spinal cord.

Why do arachnoid cysts occur?

They are developmental, meaning that they form before birth. They originate due to a localised pocket of arachnoid membrane forming which does not allow free flow of CSF between the cyst and the rest of the CSF around the brain. As they originate so early in life while the brain is still forming, the brain can adapt to their presence and continues to develop normally around the cyst.

How are arachnoid cysts diagnosed?

They are usually discovered incidentally, in other words, during investigations for unrelated symptoms. They are readily visible on both CT and MRI scans. They tend to be classified by their location when found. If they have initially been diagnosed on CT, an MRI scan will typically be arranged to ensure that this is an arachnoid cyst rather than another cause. They can be confidently diagnosed on scans rather than needing any biopsy or invasive investigations.

What symptoms can they cause?

It is very rare for arachnoid cysts to cause symptoms, particularly so in adults. They will have been present lifelong and almost never grow. For this reason, any new symptoms such as headaches are invariably not related to the cyst.

In the unusual event of a cyst being very large it can result in symptoms such as severe headache, blurring of vision, nausea or vomiting. We emphasise that these are very infrequent problems, and most arachnoid cysts do not cause issues and are not the cause of headaches, even if that was the reason for the scan.

What is the treatment for arachnoid cysts?

Almost all arachnoid cysts can be safely left alone. This is because they do not affect brain function or cause symptoms or enlarge.

We will be able to reassure if your cyst is small and typically no follow-up scans will be required. We will not necessarily need to see you in a clinic either, but if you do have specific questions or concerns, please feel free to get in touch and we will be happy to address these.

Is there anything I should avoid?

You are safe to drive, travel and work and there are no lifestyle changes we would recommend for a small incidental arachnoid cyst.

Contact details

Email admin.skullbase@nca.nhs.uk
Te: 0161 206 2303