Arachnoid Cyst
A fluid-filled sac near the brain or spinal cord
Quick Facts
- Type: Benign (non-cancerous) cyst
- Contains: Cerebrospinal fluid
- Often: Present from birth, symptom-free
- Usually: Found by chance on a scan
Overview
An arachnoid cyst is a fluid-filled sac that develops between the brain or spinal cord and the arachnoid membrane, one of the three layers covering the central nervous system. The cyst contains cerebrospinal fluid, the same clear fluid that normally surrounds and cushions the brain and spine. Most arachnoid cysts are present from birth (congenital), though some develop later after a head injury, infection, or surgery.
Arachnoid cysts are quite common, and the great majority cause no symptoms at all. Many are discovered by chance when a brain or spine scan is done for another reason. When a cyst is large or located in a sensitive area, it can press on nearby structures and cause symptoms. Arachnoid cysts are benign, meaning they are not cancer, and many require no treatment beyond occasional monitoring.
Symptoms
Most arachnoid cysts cause no symptoms. When symptoms do occur, they depend on the cyst's size and location.
- Headaches
- Nausea and vomiting
- Dizziness or balance problems
- Seizures
- Hearing or vision changes
- Developmental delays or, in infants, an enlarging head
A cyst near the spine can cause back pain, numbness, weakness, or problems with bladder or bowel control. Symptoms may develop gradually, and a sudden worsening, especially after a head injury that causes bleeding into a cyst, can produce a rapid increase in headache and other neurological signs that needs urgent evaluation.
Causes
Arachnoid cysts are divided into two types by how they form:
- Primary (congenital) cysts: The most common type, present from birth, resulting from a difference in how the brain and its membranes formed during early development.
- Secondary (acquired) cysts: Develop later in life as a result of a head injury, bleeding, infection such as meningitis, brain surgery, or other conditions affecting the membranes.
Congenital cysts are not caused by anything a parent did during pregnancy. The reason most cysts cause no problems is that they stay small and stable, while a minority enlarge or press on nearby structures.
Risk Factors
- Male sex, in which congenital cysts are somewhat more common
- A previous head injury, particularly for acquired cysts
- A history of brain or spinal infection such as meningitis
- Prior brain or spine surgery
- Certain genetic conditions in some cases
Diagnosis
Arachnoid cysts are usually found and assessed with imaging, often incidentally. Evaluation may include:
- MRI scan: The most detailed test, which shows the cyst clearly, defines its size and location, and distinguishes it from other types of cyst or tumor
- CT scan: Can detect cysts and is useful in emergencies, such as after a head injury
- Monitoring scans: Repeat imaging over time to check whether a cyst is growing or causing pressure
- Neurological assessment: To determine whether symptoms are related to the cyst
Treatment
Treatment depends entirely on whether the cyst causes symptoms. Many cysts need no treatment at all.
- Watchful waiting: For small, symptom-free cysts, doctors often simply monitor with periodic scans, as most remain stable.
- Surgery: Reserved for cysts that cause symptoms or are growing. Options include making an opening so the fluid can drain into the normal fluid spaces, or placing a shunt to divert fluid elsewhere.
- Treating complications: Managing seizures or, in urgent cases, treating bleeding into a cyst.
The decision to treat is individualized and weighs the cyst's size, location, symptoms, and any change over time. A neurologist or neurosurgeon usually guides care.
When to See a Doctor
If you have been told you have an arachnoid cyst, follow your doctor's advice about monitoring and report any new headaches, seizures, dizziness, weakness, numbness, vision or hearing changes, or balance problems. Seek prompt or emergency medical care for a sudden, severe headache, repeated vomiting, sudden weakness or trouble speaking, or a head injury followed by worsening symptoms, since bleeding into or around a cyst can occasionally cause a rapid increase in pressure on the brain.
Frequently Asked Questions
What is an arachnoid cyst?
It is a benign, fluid-filled sac that forms between the brain or spinal cord and the arachnoid membrane covering them. The fluid inside is cerebrospinal fluid, the same fluid that normally cushions the brain and spine. Most are present from birth and cause no symptoms.
Are arachnoid cysts dangerous?
The great majority are harmless and never cause problems, and many are found by chance on a scan. A small number grow or press on nearby structures, causing headaches, seizures, or other symptoms. Arachnoid cysts are not cancer.
Do arachnoid cysts need to be removed?
Usually not. Small, symptom-free cysts are typically just monitored with occasional scans because most stay stable. Surgery is considered only when a cyst causes symptoms or is growing, to drain the fluid or relieve pressure.
What symptoms can an arachnoid cyst cause?
When a cyst is large or in a sensitive area, it may cause headaches, nausea, dizziness, seizures, or hearing and vision changes. A spinal cyst can cause back pain, numbness, weakness, or bladder and bowel problems. Most cysts, however, cause no symptoms.
When should I worry about an arachnoid cyst?
Seek prompt care for new or worsening headaches, seizures, weakness, numbness, or vision or hearing changes. Get emergency care for a sudden severe headache or a head injury followed by worsening symptoms, since bleeding into a cyst can raise pressure on the brain.
References
- National Institute of Neurological Disorders and Stroke (NINDS). Arachnoid Cysts.
- Cleveland Clinic. Arachnoid Cysts.
- MedlinePlus, U.S. National Library of Medicine. Brain cysts.
- American Association of Neurological Surgeons (AANS). Arachnoid Cysts.