Arachnoid Cyst

What Is It?

An arachnoid cyst is a fluid-filled cyst that that occurs in the arachnoid membrane around the brain. While completely benign, and not a tumor per se, they can cause symptoms by compression of adjacent nervous system structures.

The arachnoid membrane is a thin, filmy tissue that is one of the three coverings of the brain, the meninges. It normally encloses the subarachnoid space, which is filled with cerebrospinal fluid, a water-like fluid that surrounds the brain and spinal cord. It is thought that during development in some cases the arachnoid does not form properly so that a sack, or cyst, is formed which lets fluid into it but not out. This can lead to the cyst enlarging and the adjacent brain under-developing. Therefore, these cysts are not literally brain tumors, they are really congenital (you are born with it) malformations of the arachnoid.

Arachnoid takes its name because the membrane somewhat resembles a spider web.

The cyst wall is basically just arachnoid membrane and the cyst is filled with fluid which is essentially the same as cerebrospinal fluid.

Some cysts do not appear to change in size while others can grow over time, compression adjacent neurological structures.

An arachnoid cyst can occur in many parts of the central nervous system, including the temporal area, the suprasellar area (above the pituitary gland), the cerebellopontine angle (between the brainstem and cerebellum) and the spinal canal.

What Types of Symptoms Are Typical?

The symptoms associated with an arachnoid cyst varies depending on its location and size. Many cysts are completely asymptomatic and others have non-specific symptoms like headache.

In the cyst is symptomatic, it can cause neurological symptoms related to its location, which varies. Some possible symptoms include seizures, hydrocephalus, developmental delay, behavioral changes, and weakness of part of the body. Many others are possible and vary from patient to patient.

In young children, a large cyst can cause deformation of the shape of the head.

How Is The Diagnosis Typically Made?

Because many cysts do not cause any symptoms, the diagnosis is often made incidentally when the patient has a head CT scan or MRI scan for another reason. In symtpomatic cases, after a neurological evaluation, a CT or MRI scan is also typically done to determine the cause. On these imaging studies the cysts have a characteristic appearance and the fluid filling them looks essentially the same as cerebrospinal fluid. One other pathology which can sometimes be confused for an arachnoid cyst is an epidermoid cyst, which is a benign congenital tumor. However, these tumors can usually be distinguished from an arachnoid cyst with specific MRI techniques.

What Are Some Common Treatments?

The treatment for a cyst varies from patient to patient. Complete surgical removal of the cyst is often impossible because its fine capsule is adherent to many neurological structures. However, some common approaches are listed here:

Asymptomatic cysts are often not treated, simply watched and then treated later if they enlarge or begin causing symptoms. This is up to the discretion of the patient and treating physician.

Simple drainage of the cyst through a needle or a small hole in the skull is generally not effective. In most cases this leads to immediate re-filling of the cyst.

Fenestration is a term which basically means making windows into the cyst. In this surgical procedure, holes are made into the cyst on as many sides as possible. If possible, some of the capsule is removed. This allows a free flow of fluid into and out of the cyst with the goal of preventing the cyst to close off and not let fluid out. This can help to prevent future growth of the cyst.

Some physicians recommend a shunt procedure which places a catheter into the cyst. The other end of the catheter can go into another compartment in the body where the fluid can be re-absorbed easily. Most commonly this is the abdominal cavity. The goal of these procedures is to divert the fluid away from the cyst so that it does not grow or cause pressure on neurological structures.

Each case is different, so each patient should discuss their treatment options and plan with their own physician.

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Important Note: This site is not intended to offer medical advice. Every patient is different, and only your personal physician can help to counsel you about what is best for your situation. What we offer is general reference information about various disorders and treatments for your education.

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