Arachnoid cysts are sacs filled with cerebrospinal fluid, commonly found at the base of the skull or on the surface of the brain. They mainly affect children, but adults may get them, as well. These cysts are often present at birth and can pose potential risks to brain function and development based on their size and location.
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What Are Arachnoid Cysts?
Arachnoid cysts are the most common type of brain cyst.
An arachnoid cyst can form:
- At the base of the skull.
- On the surface of the brain.
- On the arachnoid membrane.
- On the spinal cord (this is most rare).
Three different membranes make up the protective lining called the meninges that envelope the central nervous system.
These layers, beginning with the most external layer, are called:
- Dura mater.
- Arachnoid mater.
- Pia mater.
Cerebrospinal fluid (CSF) is mainly contained between the arachnoid mater and pia mater layers. An arachnoid cyst forms when the layers of the arachnoid membrane split apart and become filled with cerebrospinal fluid. It is normal to have CSF in this space, and arachnoid cysts are usually simply larger-than-normal collections of this fluid. Often present since birth, they usually pose no consequences.
Not all arachnoid cysts need treatment, and some people never know they have one. Surgery is sometimes needed if an arachnoid cyst causes obvious brain-related symptoms, like pressure on nearby areas. But this is usually rare.
Arachnoid cysts can grow to be large if they continue to collect CSF. In extreme cases, the size of a cyst may cause the person's head to change shape or displace nearby lobes of the brain.
What are the types of brain cysts?
The four most common types of brain cysts are:
- Arachnoid cysts
- Colloid cysts
- Dermoid cysts
- Epidermoid cysts
There is another classification of the cysts called the Galassi classification, which groups cysts based on size and location.
The different types are:
- Type 1 – Small, usually asymptomatic, located at the back of the brain in the middle cranial fossa.
- Type 2 – Located at the top of the brain along the Sylvian fissure, displaces the temporal lobe.
- Type 3 – Large, taking up the entire middle cranial fossa, displacing the temporal, parietal, and frontal lobes of the brain.
How common are arachnoid cysts?
Arachnoid cysts are very uncommon. They often present at birth or within the first decade of life, although they can develop at any point. Arachnoid cysts are also more common among men, occurring at a ratio of 2:1.
It is estimated that arachnoid cysts affect about 2.6% of children and only 0.2% to 1.7% of adults.
What causes arachnoid cysts?
Arachnoid cysts are believed to be caused by an abnormal flow of cerebrospinal fluid becoming trapped between the layers of the meninges in the brain. The arachnoid mater is the middle layer of the meninges, located between the dura mater and the pia mater. It is avascular, meaning it does not contain blood vessels, but instead allows the transmission of CSF through protrusions into the dura and pia mater.
In the case of abnormal CSF flow, there can be a collection of CSF within the arachnoid layer, creating a sac or pocket full of CSF, called a cyst.
Arachnoid cyst risk factors
Any type of cyst or abnormal growth can affect the surrounding parts of the body. Because they are so close to the brain, arachnoid cysts may cause neurologic complications, including headaches, nausea, and vision issues.
Arachnoid cysts are an abnormality in the flow of cerebrospinal fluid. There is no clear risk factor identified that contributes to the development of these cysts.
Complications of arachnoid cysts
If left untreated, arachnoid cysts can be completely asymptomatic and may not require any treatment. However, based on the size and location of the cysts, complications may arise if they grow large enough to put pressure on the surrounding brain structures.
This increase in pressure on the brain can lead to facial palsy, headaches, nausea, and vision problems.
What Are the Signs and Symptoms of Arachnoid Cysts?
The symptoms of arachnoid cysts depend on their size and location. Small cysts may not cause any symptoms, and doctors usually only find them during exams for something else.
But large arachnoid cysts can cause neurologic symptoms, including:
- Balance problems.
- Headaches.
- Hearing and vision problems.
- Hydrocephalus (enlargement of the head due to a buildup of CSF).
- Nausea and vomiting.
- Seizures.
- Stroke-like symptoms, such as weakness, coordination problems, or speech issues.
When should I see a doctor about my arachnoid cyst symptoms?
Arachnoid cysts are often recognized in the first decade of life or upon birth. In many cases, the cyst itself can have no impact on brain or body function; however, if you or your child are experiencing any of the symptoms listed above, contact your doctor as soon as possible.
How Do You Diagnose Arachnoid Cysts?
Because arachnoid cysts are present on the brain and in layers of the meninges, they can only be seen with imaging technology unless the growth is so big that it causes a visible lump.
In any case, your doctor will ask about your symptoms, perform a physical exam, and use specialized imaging technology to get the best understanding of your condition.
Tests to diagnose arachnoid cyst
- MRI – Uses magnets to create an image of the interior of the skull.
- CT cisternogram – A specialized test that uses a dye throughout the body to detect CSF trapped in spaces, outlining the size and location of cysts.
How Do You Treat Arachnoid Cysts?
Only arachnoid cysts that cause symptoms need treatment. Surgery is only done if symptoms have no other causes or don't improve from other treatments.
Surgery for arachnoid cysts
The traditional open surgery to treat arachnoid cysts involves making an opening in the skull so doctors can reach the cyst directly to drain it.
To prevent the cyst from refilling, doctors remove the outer membrane of the cyst or open a small window on the cyst in a technique called capsule fenestration. This opening creates wide communications with the normal surrounding subarachnoid spaces. If no other spaces are next to the cyst, the cyst may need a shunt to divert cyst fluid into the belly to relieve brain pressure and reduce symptoms like headaches or vision problems.
Minimally invasive surgery for arachnoid cysts
Doctors often can access certain arachnoid cysts with one of several state-of-the-art, minimally invasive treatments, including:
- Endoscopic endonasal approach (EEA) — Uses the natural corridor of the nostrils and sinuses to reach deep-seated tumors at the base of the skull or the top of the spine. Benefits include faster recovery time and no scarring.
- Eyebrow (supraorbital) craniotomy — Uses a small incision in the eyebrow to reach tumors located near the front of the brain, close to the optic nerves, and above the pituitary gland.
- Neuroendoscopic resection — Uses an endoscope to treat tumors in the brain, spine, or nervous system. Instead of removing a large piece of the skull like in traditional brain surgery, neuroendoscopy allows doctors to make a smaller opening, which can reduce recovery time and lower the risk of complications.
Surgeons use these methods to access the cyst, drain the cyst, and shunt it, if necessary.
What happens during surgery?
In traditional surgical procedures to remove an arachnoid cyst, your surgeon will determine the best place for entry to access the cyst. The access point may be from the side of the head through the bones of the ear, or through the bones around the eyebrows. Then the surgeon will remove the growth by cutting out the tissue it is attached to.
In an endoscopic endonasal approach, the surgeons will enter through the nasal passages with an instrument that can view the area as well as specialized tools to detach segments of the cyst and apply suction to remove them.
How effective is treatment?
Surgery is usually very effective in eliminating cysts and the complications they can cause. In some cases, however, cysts will regrow.
How long does it take to recover after surgery for arachnoid cysts?
Traditional surgical removal can require several days in the hospital for the procedure, healing, and monitoring recovery. Then a week or more of recovery at home.
Minimally invasive procedures can require little to no time in the hospital. With smaller or no incisions, healing is quicker than with traditional open surgery.
