Arachnoid cysts are benign (non-cancerous) fluid-filled sacs that develop on the brain or spinal cord. Arachnoid cysts are the most common type of brain cyst.
They are mostly congenital, which means they are present from birth. Occasionally, they can develop following a head injury, infection, or brain tumour.
Arachnoid cysts affect all age groups, but commonly present in childhood, particularly within the first year or so of life. They are more common in males than females.
There are three layers of membranes that cover the brain and spinal cord: the dura mater, the arachnoid, and the pia mater. Arachnoid cysts develop on the arachnoid membrane. Arachnoid cysts mostly grow on the brain and are referred to as intracranial arachnoid cysts. Less frequently, they can also grow around the spinal cord. These are referred to as spinal arachnoid cysts. Brain (intracranial arachnoid cysts).
Arachnoid cysts develop on the arachnoid membrane, which is one of the three layers of membranes (the meninges) that cover and protect the brain and spinal cord. Arachnoid cysts usually grow on the brain, but they can also grow around the spinal cord. The fluid inside an arachnoid cyst is a clear, colourless fluid called cerebrospinal fluid (or ‘CSF’). CSF surrounds the brain and spinal cord, providing nourishment, protection, and removing waste products.
Many arachnoid cysts will never cause symptoms or require treatment. In these cases, regular monitoring to check the size of the cyst may be recommended. If symptoms do occur, they will depend on the size and location of the cyst, and whether the cyst is causing the pressure in the brain to rise, or hydrocephalus (a build-up of CSF in the brain).
Arachnoid cysts can develop in several areas of the brain including (but not limited to):
Arachnoid cysts are often asymptomatic (have no symptoms) and can go undetected for many years.
When symptoms do occur, they depend on the size and location of the cyst. Small cysts may not cause any symptoms.
Some cysts, particularly if large, can press against the brain, spinal cord, or a nerve, depending on their location. They can also block the flow of CSF out of the ventricles, causing a build-up of CSF (hydrocephalus) and raised pressure in and around the brain.
Most arachnoid cysts require no intervention.
If your child has symptoms, your surgeon may recommend surgery. If your child is asymptomatic, your surgeon may recommend monitoring only, also called a ‘watch and wait’ approach. This may involve regular MRI scans, to monitor the growth of the cyst.
Depending on the size and location of your child’s cyst, your surgeon may recommend an endoscopic procedure to drain or open the cyst, allowing the fluid to be reabsorbed normally. Endoscopes are thin, flexible tubes with small cameras attached. Endoscopes allow for a smaller incision, and smaller tools to be used, meaning the procedure is less invasive than a craniotomy.
If the cyst is putting pressure on the brain or spinal cord or is causing hydrocephalus (a build-up of CSF in the brain), treatment may be recommended. The type of treatment offered will depend on a number of factors, including the size and location of the cyst, and the associated symptoms.
