Arachnoid cyst: symptoms, causes, treatment and whether it is serious

The brain and spinal cord are surrounded by three membranes called the meninges. One of them is the arachnoid layer, named for its resemblance to a spider's web. An NHS patient leaflet explains that arachnoid cysts have a wall made of this arachnoid membrane, sit between the arachnoid layer and the brain, and contain cerebrospinal fluid (CSF), the clear liquid that protects and nourishes the brain and spinal cord.

A children's hospital leaflet adds that arachnoid cysts are not tumours, that they can sometimes develop within the ventricles inside the brain, and that similar cysts can occur around the spinal cord. An NHS referral pathway says they are most commonly seen in the middle cranial fossa, in the lower part of the skull at the side of the head.

The NHS leaflet says arachnoid cysts are developmental: they form before birth. They arise when a localised pocket of arachnoid membrane forms that does not allow free flow of CSF between the cyst and the rest of the fluid around the brain. Because this happens so early in life while the brain is still forming, the brain adapts and continues to develop normally around the cyst.

A children's hospital leaflet describes two types. A primary arachnoid cyst is most common in children and is usually present at birth, and the exact cause is unknown. A secondary arachnoid cyst can develop as a result of a head injury or an infection, but these are rare and not as common in children. An NHS referral pathway also describes arachnoid cysts as congenital or acquired.

Most arachnoid cysts cause no symptoms at all. The NHS leaflet says it is very rare for them to cause symptoms, particularly in adults. They have been present lifelong and almost never grow, so any new symptoms such as headaches are invariably not related to the cyst. In the unusual event of a very large cyst, it can cause symptoms such as severe headache, blurring of vision, nausea or vomiting. The leaflet stresses that these are very infrequent, and that most arachnoid cysts are not the cause of headaches, even if headache was the reason for the scan.

The children's hospital leaflet says that if a cyst is large it may put pressure on the brain or spinal cord. Cysts in the head can then cause headaches, seizures, developmental delay, rapid head growth, dizziness, hydrocephalus (a build-up of fluid) and nausea or vomiting. Cysts around the spinal cord can cause back or leg pain, numbness or tingling in the arms or legs, weakness, and bowel or bladder problems. An NHS referral pathway says spinal arachnoid cysts can cause signs of myelopathy (spinal cord problems).

• Usually none.

• In a large cranial cyst: severe headache, blurred vision, nausea and vomiting.

• In children: headache, seizures, developmental delay or rapid head growth.

• In a spinal cyst: back or leg pain, numbness, weakness, bowel or bladder problems.

See a GP if you have a headache that is not getting better or you get headaches regularly. The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you or a child has a headache with vision or eye problems, a headache triggered or made worse by coughing, sneezing, bending down or exercising, or a headache with vomiting. For a child, ask for urgent help if the headache is getting worse or wakes them at night.

Call 999 or go to A&E if someone has a headache and has had a seizure, has numbness or weakness in the body or face, has a sudden extremely painful headache, has had a head injury in the last 3 months, has difficulty speaking, balancing, walking or remembering things, is drowsy or confused, or has loss of vision. Do not drive to A&E; ask someone to drive you or call 999 (NHS). These are warning signs for many conditions and are not specific to an arachnoid cyst.

The NHS leaflet says arachnoid cysts are usually discovered incidentally, during investigations for unrelated symptoms. They are readily visible on both CT and MRI scans, and are usually classified by their location. If a cyst has first been seen on CT, an MRI scan will typically be arranged to be sure it is an arachnoid cyst rather than another cause. They can be confidently diagnosed on scans without a biopsy or other invasive investigation.

An NHS referral pathway calls MRI the gold standard if there is uncertainty on CT imaging. It advises that all patients with a cranial arachnoid cyst, with or without symptoms, should be referred to a neuro-oncology multidisciplinary team, but that if the person has no symptoms from the cyst, it is likely no intervention or follow-up will be required.

The NHS leaflet says almost all arachnoid cysts can be safely left alone, because they do not affect brain function, cause symptoms or enlarge. If your cyst is small, doctors will be able to reassure you and typically no follow-up scans will be required, and you may not need to be seen in clinic.

The NHS referral pathway says surgical intervention is rarely required, and only needed if the person is symptomatic, for example with seizures, weakness, hydrocephalus or cognitive impairment. The children's hospital leaflet says that where a child has symptoms from the cyst, surgery may be needed to drain the fluid, and that the neurosurgeon will recommend one of two operations: cyst fenestration, where many small holes are made in the cyst so CSF drains into the space around the brain or spinal cord, or shunting, where a thin tube is placed in the cyst wall to drain the CSF into the abdomen, where it is absorbed into the bloodstream.

In a small prospective study from a Swedish hydrocephalus research unit, presented as a conference abstract, 112 adults with newly found cysts were evaluated. Of 54 who accepted surgery, 80% experienced improvement in at least one major symptom 3 months afterwards, and headache improved significantly, although clinical improvement did not correlate with how much the cyst shrank. The abstract also noted that headache and dizziness were as common in the 33 patients regarded as asymptomatic as in the operated patients, which is one reason symptoms and scan findings have to be matched carefully.

The NHS leaflet says you are safe to drive, travel and work, and that there are no lifestyle changes it would recommend for a small incidental arachnoid cyst. If you have had surgery or have seizures or other symptoms, specific advice from your specialist applies, including any driving rules.

An arachnoid cyst found on a scan is usually reviewed by a neurosurgical team on the NHS, often with reassurance and no follow-up. A GP can refer you to neurology or neurosurgery if you have symptoms, and urgent referral pathways exist for people who are unwell. Private neurosurgical opinions are available if you want a second view of a scan. Whichever route you take, it is reasonable to ask whether your symptoms plausibly fit the cyst, whether any follow-up is needed, and what would make surgery worth considering.