What Is an Arachnoid Cyst?
An arachnoid cyst is a cystic formation associated with the arachnoid membrane, one of the membranes surrounding the brain and spinal cord, and containing fluid similar to cerebrospinal fluid. It can occur in different regions of the brain.
A large proportion of arachnoid cysts are congenital and cause no symptoms. For this reason, they are most often detected incidentally during a brain MRI or CT scan performed for another reason.
Is an Arachnoid Cyst Dangerous?
Not every arachnoid cyst is dangerous.
Cysts that are small, have typical features and cause no compression of the surrounding brain structures generally follow a benign course. Studies have reported that the great majority of incidentally detected arachnoid cysts cause no symptoms and show no significant growth during follow-up.
However, the location of the cyst and its effect on the surrounding structures matter as much as its size. Some cysts may cause compression of the brain tissue or disrupt the circulation of cerebrospinal fluid, leading to hydrocephalus.
For this reason, when an arachnoid cyst is evaluated, the question is not only “how many centimetres is it?” but also where the cyst is located and what it causes.
What Are the Symptoms of an Arachnoid Cyst?
Most arachnoid cysts cause no symptoms. However, depending on the size and location of the cyst, some patients may experience:
- Headache
- Nausea and vomiting
- Balance problems
- Visual disturbances
- Seizures
- Changes in strength or sensation
- Complaints related to hydrocephalus
An important point is this: a common complaint such as headache should not be assumed to be caused by the arachnoid cyst alone. The imaging findings of the cyst and the patient’s complaints should be evaluated together.
How Is an Arachnoid Cyst Diagnosed?
One of the most important methods for evaluating an arachnoid cyst is brain MRI. MRI shows the location and size of the cyst, the compression it causes on the brain tissue and its effect on cerebrospinal fluid circulation.
In some patients, CT or additional imaging may also be needed. The treatment decision is made not on the MRI image alone, but together with the patient’s clinical findings.
Does Every Arachnoid Cyst Require Surgery?
No.
In arachnoid cysts that cause no symptoms, have typical features and cause no significant compression of the surrounding structures, surgery is usually not needed. Depending on the patient’s condition, follow-up may be sufficient.
Surgery is mainly considered in patients with marked or progressive complaints thought to be caused by the cyst, hydrocephalus or significant signs of compression.
Therefore, a large cyst size alone is not a reason for surgery.
How Is Arachnoid Cyst Surgery Performed?
When surgery is needed, the method is determined according to the region where the cyst is located and its anatomical features.
In suitable patients, endoscopic fenestration can be performed. In this procedure, an opening is created between the cyst and the normal cerebrospinal fluid pathways so that the fluid inside the cyst can drain.
In some patients, other surgical methods such as microsurgical fenestration or a shunt may be used. The surgical method is determined individually for each patient.
Frequently Asked Questions
Can an arachnoid cyst turn into cancer?
Arachnoid cysts are typically not tumors, and malignant transformation is not expected.
Must a large arachnoid cyst always be operated on?
No. The size of the cyst alone is not sufficient. Symptoms, compression, location and findings such as hydrocephalus are evaluated together.
Does an arachnoid cyst grow?
In most adult patients, the cyst remains stable for a long time. However, the follow-up approach may vary according to age, the location of the cyst and clinical features.
Does an arachnoid cyst cause headaches?
Some cysts may contribute to headaches. However, because headaches have many different causes, whether the cyst is truly the cause of the complaint should be evaluated separately.
Conclusion
A diagnosis of arachnoid cyst does not always mean surgery. Many cysts are detected incidentally, cause no symptoms and remain unchanged for a long time.
In the treatment decision, not only the presence or size of the cyst but also its location, the compression it causes on the surrounding brain structures, its effect on cerebrospinal fluid circulation and the patient’s complaints should be evaluated together.
Cysts that cause no symptoms and pose no risk can be followed up, whereas surgical treatment may be considered when there is marked compression caused by the cyst, hydrocephalus or progressive neurological findings.
References
- Ahmed AK, Cohen AR. Intracranial arachnoid cysts. Child's Nervous System. 2023;39(10):2771-2778.
- Carbone J, Sadasivan AP. Intracranial arachnoid cysts: Review of natural history and proposed treatment algorithm. Surgical Neurology International. 2021;12:621.
- Hall S, Smedley A, Sparrow O, et al. Natural History of Intracranial Arachnoid Cysts. World Neurosurgery. 2019;126:e1315-e1320.
- Whitney E. A Comprehensive Review of Arachnoid Cysts. Cureus. 2025;17(5):e83894.