Arachnoid Cysts – Causes, Symptoms, Diagnosis, Treatment

What are Arachnoid Cysts?

Arachnoid cysts are common brain cysts that are basically sacs filled with cerebrospinal fluid (CSF). These fluid-filled sacs are located between either the brain or spinal cord and the arachnoid membrane. Arachnoid cysts are congenital lesions ie, they occur at birth. There are 3 membranes that cover the brain and spinal cord. The arachnoid membrane is one of the membrane layers. Most of the arachnoid cysts occur in the middle cranial fossa of the skull. When an arachnioid cyst develops in the brain, it is called an intracranial cyst. When it develops in the skull bones, the cysts are called intraosseous cysts. Spinal cord arachnoid cysts are rare.

Arachnoid cysts are benign cysts that are filled with CSF. They are not classified as tumors. The exact cause for arachnoid cysts is not clear and a study that looked at the cellular origins of the cysts observed that different types of cells are made up the cysts. This indicates that these cysts form spontaneously during the differentiation of organs in the early stage of fetal development. They are present at birth but may not be detected immediately and may persist for years. If there are symptoms, they appear either soon after birth, during adolescence, or before 20 years. Arachnoid cysts are commonly observed in boys compared with girls. The rate of occurrence in men is 4 times greater than in women. Genetic symptoms, such as Menkes syndrome and Cockaye syndrome may be predisposing agents of arachnoid cysts. Nearly 1 to 3% children are affected with arachnoid cysts.

Classification of Cysts

Arachnoid cysts of the middle cranial fossa are also called intracranial cysts and are classified into communicating and non-communicating arachnoid cysts. The intracranial cysts are further classified into type I, type II, and type III cysts. This classification was developed by E. Galassi and colleagues. Type I cysts are small and freely communicate with the subarachnoid space. Type II cysts are larger, rectangular, and slowly communicate with the subarachnoid space. Type III cysts are the largest and do not communicate with the subarachnoid space. The arachnoid cysts in children are classified as supratentorial, infratentorial, and supra-infratentorial cysts based on their location in the cerebrum, cerebellum, or cerebrum/cerebellum, respectively.

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In the case of spinal arachnoid cysts, the Nabor classification was developed in 1998 to classify spinal arachnoid cysts into type I extradural cysts in the absence of nerve tissue, type II extradural cysts with nerve tissue, and type III intradural cysts. However, this is a broad classification and does not take into account the method of formation of the cysts or the abnormalities that may be present in the cysts or the surrounding environment. Hence, treatment may not be as effective.

A novel classification system by Qi et al in 2014 classifies spinal arachnoid cysts into intramedullary, subdural extramedullary, intrapsinal/extrapsinal, subdural/epidural, and intraspinal/epidural. This classification takes into account other defects that may be observed with MRI and hence, targeted treatment may be performed.

What are the Causes of Arachnoid Cysts?

Arachnoid Cysts are of 2 Types:

Primary arachnoid cysts – These cysts form due to defects in the development of the brain and the spinal cord during fetal development. These cysts are congenital in nature.

Secondary arachnoid cysts – These cysts form due to tumors, complications in brain surgery, meningitis, or a head injury. They are rare in occurrence.

What are the Symptoms of Arachnoid Cysts?

In general, arachnoid cysts do not cause any symptoms and are benign. They may remain for many years without complications. However, those cysts that begin to affect the brain, show certain symptoms that are listed below. The symptoms differ based on the location and the size of the cyst.

• Nausea and vomiting
• Seizures
• Vertigo
• Difficulty in movement and walking
• Headache
• Bobbing of the head
• Visual and hearing defects

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• Lethargy
• Premature development of puberty
• Delayed development
• Hydrocephalus or accumulation of cerebrospinal fluid in the brain
• Spontaneous rupture of the arachnoid cyst
• Enlarged head or macrocephaly

The Symptoms of Spinal Cord Arachnoid Cysts include:

• Tingling sensation in the arms and legs
• Progressive pain in the leg and back
• Uncontrolled defecation
• Abnormal functioning of the urinary tract
• Spasms in the limbs

How to Diagnose Arachnoid Cysts?

Diagnosis of arachnoid cysts is performed with a diffusion-weighted MRI. A spine or brain scan using an MRI helps in distinguishing arachnoid cysts from the other types of cysts. Ultrasound is used in infants and small children to detect arachnoid cysts.

A recent study utilized a combination of MRI, CT scans, fluid-attenuated inversion recovery (FLAIR), and diffusion-weighted sequences to confirm the presence of arachnoid cysts in the bones of the skull and thus, rule out other related ear and spine conditions.

A study in 2012, utilized CT cisternography to classify arachnoid cysts based on the communication between the cysts and the arachnoid space. This diagnostic tool was useful in determining the appropriate surgical treatment for the condition.

Phase-contrast cine magnetic resonance imaging has been shown by a research group to be a useful noninvasive diagnostic alternative to CT cisternography and can aid in identifying the communication between cysts and the arachnoid space.

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How to Treat Arachnoid Cysts?

In general, arachnoid cysts are benign and do not require any treatment. If there are no harmful effects on the surrounding areas or if there are no symptoms, the cysts can be left as they are without any treatment.

For symptomatic arachnoid cysts, surgically placed shunts within the cyst is a common procedure to relieve the symptoms. The patient however, becomes dependent on the shunt.

In cases of hydrocephalus (accumulation of cerebrospinal fluid in the brain), removal of the cysts helps in adequate drainage of cerebrospinal fluid from the ventricles of the brain. Craniotomy is performed to remove the cyst wall.

Minimally-invasive fenestration is a procedure to drain the cyst with the help of a needle.

Another minimally invasive procedure is burr hole drainage of the cyst. However, the chance of recurrence of arachnoid cysts is high with this procedure.

Recently, in Russia, a new surgical technique is proving to be effective in removing spinal arachnoid cysts. Thecaloscopy is an endoscopic procedure where a thin flexible endoscope is used to fenestrate (cut) the cysts in the spinal cord. It is proving to be an efficient, minimally invasive procedure that may also be used to treat other arachnoid cysts in the future.