What is Arachnoid Cysts?
Arachnoid cysts are pockets of cerebrospinal fluid (the liquid that surrounds your brain and spinal cord) trapped in a membrane in our brains. They often form in the front or middle parts of the brain or behind the cerebellum (which controls balance). The exact reason why they form is not clear, but it’s thought that they’re due to a developmental abnormality early in pregnancy. The first known account of an arachnoid cyst dates back to the fifteenth century when it was described as a fluid-filled tumor that could lead to blindness.
These cysts are usually present from birth, and are a result of the abnormal development of the dividers or membranes within the brain while a fetus is growing. In fact, studies have shown that these cysts can start forming as early as 6 to 10 weeks into the pregnancy.
Most of the time, arachnoid cysts are found accidentally during brain scans for other reasons. Sometimes, when the cysts grow big enough or are in sensitive parts of the brain, they can cause symptoms that lead to their discovery. Symptoms can vary widely depending on where the cyst is in the brain and can include vision loss, nausea or vomiting, an unusually large head, paralysis of facial nerves, hearing loss, and vertigo among others.
Computerized tomography (CT) scans can often diagnose an arachnoid cyst, provided it’s in a typical location and shows up as a thin-walled, fluid-filled pocket. If more information is needed about its location and size and its relation to other structures in the brain, magnetic resonance imaging (MRI) scans are used.
In most cases, arachnoid cysts don’t cause any problems and don’t need treatment. However, if they cause symptoms due to their size or location, treatment will depend on these factors. There are various surgical options available if needed and the choice of method will depend a lot on where the cyst is located.
What Causes Arachnoid Cysts?
Scientists aren’t completely certain how arachnoid cysts form. However, the current theory suggests these cysts usually develop due to abnormal processes during the early stage of an embryo’s growth. Other findings, like the presence of inflammation cells, excess connective tissue, or residues related to blood breakdown, suggest that an inflammatory reaction or injury could also play a role in their development. No damage typically exists in the brain tissue near the cyst due to pressure effects.
It’s important to note that specific conditions can be associated with these cysts. Conditions include Aicardi syndrome, Mucopolysaccharidosis, Acrocallosal syndrome, and Marfan syndrome. Specific genetic mutations, Chudley-Mullough syndrome, and Glutaric aciduria type 1, which includes dual arachnoid cysts, are also linked.
There are different ways to categorize, or classify, arachnoid cysts. One way is by where they’re located in the brain. They can be:
* Supratentorial, meaning they’re located in the top part of the brain in multiple potential regions.
* Infratentorial, meaning they’re down near the base of the brain again in multiple different areas.
* Supratentorial or infratentorial, which implies they could be in either of the regions.
Another classification system categorizes these cysts according to size and location:
* Type I cysts are small and typically don’t cause any symptoms. They’re located in the front or middle area of skull’s base.
* Type II cysts are located in an elevated region along a brain crevice known as the Sylvian fissure. They displace the temporal lobe.
* Type III cysts are noticeably larger. They take up the entire middle base of the skull and affect the temporal, parietal, and frontal lobes.
Risk Factors and Frequency for Arachnoid Cysts
Arachnoid cysts can show up in people of all ages, from newborns to older adults, but are most common in the first ten years of life. Males are more likely to get them than females, with a ratio of about 2 to 1. Their occurrence varies among different age groups:
- In children, about 2.6% are found to have these cysts.
- In adults, the prevalence ranges from 0.2% to 1.7%.
Signs and Symptoms of Arachnoid Cysts
Arachnoid cysts are a condition where fluid-filled sacs develop on the arachnoid membrane, a thin layer covering the brain. The frequency of symptoms differs greatly among patients, and the location of these cysts varies significantly. The most dominant locations are the middle cranial fossa and the retrocerebellar positions. Few others are found at the convexity and in the suprasellar region, while a smaller percentage of cysts are found in other areas such as the cerebellopontine angle, intraventricular space, and the brainstem.
Often, arachnoid cysts are identified by chance when patients undergo imaging for other reasons. Many arachnoid cysts cause no symptoms, especially when they are located in the middle cranial fossa and retrocerebellar region. However, cysts in other locations are more likely to cause symptoms like headaches, seizures, dizziness, visual problems, and delays in development. Nonetheless, large cysts are more likely to cause symptoms and may require surgery.
In rare cases, arachnoid cysts might result in severe neurological symptoms. Depending on where the cyst is found, patients may experience a variety of symptoms, such as vision loss, nausea, vomiting, facial paralysis, and even hearing loss. An arachnoid cyst can also affect the optical nerve, causing vision issues, or provoke unusual symptoms like rhythmic head movements. Some patients have even reported cases of depression linked to an arachnoid cyst.
Spinal arachnoid cysts, though rarely seen, usually manifest in the thoracic region of the spine. Symptoms often include back pain, weakness in the legs, and sensory disturbance. However, on rare occasions, patients could experience severe symptoms like paralysis and bladder dysfunction.
Testing for Arachnoid Cysts
All medical ways to diagnose a condition like an arachnoid cyst—a fluid-filled sac found in the brain—it starts with a CT scan, which can detect most of these cysts easily (see Image. Arachnoid Cyst). If doctors need more details, they use an MRI. This type of scan is excellent for showing the cyst’s size, exact location, and neighbouring structures. It also helps rule out other possible conditions.
In rare cases, an arachnoid cyst can be filled with blood. On an MRI, these cysts look a lot like cerebrospinal fluid (or CSF, the clear liquid around your brain and spinal cord) unless they contain blood from an injury. Because arachnoid cysts have thin walls, specific types of MRI can make these walls visible.
On an MRI, you’ll see that the cyst appears similar to CSF on different types of sequences, such as T1, T2 fast spin-echo (FSE), T2 spin-echo (SE), T2-fluid attenuated inversion recovery (FLAIR), and diffusion. There might be some mild differences in intensity between the cyst and normal CSF due to movements of the fluid, but overall, these cysts don’t enhance on an MRI.
If further details are required, more specialized tests, like CT and MRI cisternography, can be used. For a CT cisternography test, a safe contrast dye is injected into your spinal canal, after which CT images of the cyst are taken. How quickly and completely the cyst fills with the dye can provide helpful information for deciding on treatment. Arachnoid cysts that fill up quickly and completely are usually freely communicating with surrounding cerebrospinal fluid and may not require any treatment.
MRI cisternography can be performed with or without contrast dye. Using contrast has traditionally been considered helpful because it shows the connection between the cyst and surrounding CSF space.
Several special MRI techniques can present the cyst wall and nearby structures like nerves and vessels, which aids in reducing complications during surgery. One of these techniques, called 3D SPACE, helps show any links to the fluid space around the brain an spinal cord. In areas with flowing CSF, the 3D SPACE test shows a loss of signal, highlighting the connection between the cyst and the CSF space. Phase-contrast MRI can also be used to evaluate the connection, but due to the risk of false positives, the connection must be confirmed with contrast MR cisternography.
Radiologists must then analyze the effects of the cyst on the brain structure, which can range from a subtle to a substantial influence. The final examination report should reflect any affected nerve or vascular structures if known from the patient’s medical history. Also, radiologists need to alert the ordering clinician, and possibly call a neurosurgeon, in situations where the cyst is in a critical location causing hydrocephalus (a buildup of fluid in the brain) or when large cysts are causing mass effect and brain herniation (when the brain shifts from its normal position).
Treatment Options for Arachnoid Cysts
In most cases, arachnoid cysts don’t require any treatment. However, if patients are experiencing symptoms from these cysts, surgery is usually considered. The decision for surgery usually depends on several factors including the cyst’s location, any effects from its mass, disruptions to the flow of cerebrospinal fluid (such as hydrocephalus), specific neurological impairments, headaches, seizures, along with developmental or cognitive complications.
In instances where treatment is necessary, there are a variety of surgical procedures available. The chosen surgical approach often depends on where the cyst is located and whether it’s affecting nearby neurological structures. A common operation involves surgically removing the wall of the cyst, followed by creating an opening to communicate with the space surrounding the brain and spinal cord, if the cyst is easy to correct.
Another common procedure is called ‘surgical fenestration’ or ‘endoscopic fenestration’. This operation involves creating an opening in the cyst cavity to the closest part of the ventricular system, which is a network of fluid-filled cavities within the brain. This technique is regularly used for arachnoid cysts located in the perisylvian region of the brain, as it’s known to be safe and efficient, and it’s associated with fewer complications compared to other surgical techniques.
In rare cases, a ‘cystoperitoneal shunt’ might be inserted to drain the cyst, although this is usually avoided due to potential complications associated with long-term shunt placement. In unusual cyst locations, other less common treatments may be considered, such as draining the cyst through a natural hole at the base of the skull known as the foramen ovale.
Most of the time, surgery can effectively treat arachnoid cysts, but there are some cases where the cysts might come back. If the cyst is found to cause significant hydrocephalus (a build-up of fluid in the brain) or mass effect (pressure on surrounding brain tissues), emergency treatment may be needed.
What else can Arachnoid Cysts be?
When doctors try to identify an arachnoid cyst, they also consider several other similar conditions. These include things like epidermoid cysts, dermoid cysts, abscesses, and other forms of cysts such as cystic neurocysticercosis. These options are primarily determined through special scans.
Epidermoid cysts look quite like a certain type of brain fluid (known as CSF) on certain types of scans, namely T2 FSE and T2 SE. These cysts, however, do not enhance and show significant changes on another type of scan called diffusion-weighted imaging, which helps doctors understand them better. Epidermoid cysts are often located in the front and towards the side of the brainstem.
A dermoid cyst or a lipoma can be distinguished by a different signal that follows fat on CT scans or MRI. It is particularly useful to use a certain type of MRI process (T1 sequence) to confirm if it’s a dermoid cyst or a lipoma.
Abscesses can look similar to cysts, yet, unlike the similar appearance with the brain fluid (CSF), the inside of an abscess contains inflammatory and cellular debris. In fact, it often shows significant elevation in a type of scan called diffusion-weighted image (DWI). Abscesses also have a rim that enhances, normally it’s thin, but sometimes, it can be thick especially when it is due to certain bacterial and fungal infections.
Cysts from neurocysticercosis (a specific type.), often located in the brain fluid can look very similar to an arachnoid cyst, but unlike the latter, they typically have multiple compartments. In addition to this, in people with neurocysticercosis, you will likely find other symptoms of this condition during imaging. Commonly, in these cases, lesions will be found in the brain marked by calcifications.
What to expect with Arachnoid Cysts
In most situations, treatment isn’t required. When necessary, surgery is usually completely effective, though there have been instances of the condition reoccurring. Interestingly, there are cases where arachnoid cysts have disappeared on their own, especially after a physical injury.
Possible Complications When Diagnosed with Arachnoid Cysts
Arachnoid cysts, though rare, can lead to complications depending on where they are and whether they distort nearby body structures. If an arachnoid cyst ruptures, it can lead to subdural hygromas or subdural hematomas that require treatment. Other potential complications include vision loss, feeling ill or vomiting, an overly large head, eye muscle paralysis, nerve issues, facial spasms, hearing loss, facial palsy, vagus nerve palsy, and dizziness.
Potential Complications of Arachnoid Cysts:
- Subdural hygromas or subdural hematomas due to cyst rupture
- Vision loss
- Nausea or vomiting
- Macrocephaly (overly large head)
- Paralysis of eye muscles
- Trigeminal neuropathy (nerve issues)
- Hemifacial spasm
- Sensory neuronal hearing loss
- Facial palsy
- Vagus nerve palsy
- Vertigo (dizziness)
Preventing Arachnoid Cysts
Patients should understand that most arachnoid cysts are discovered by accident, and in most cases, they don’t require treatment. Only in rare instances, the arachnoid cyst might become an issue that needs to be dealt with through treatment.
