Arachnoid cysts are benign, fluid-filled sacs that develop between the brain or spinal cord and the arachnoid membrane — one of the three protective coverings of the central nervous system. They contain cerebrospinal fluid and account for approximately 1% of all intracranial masses. The vast majority are congenital and asymptomatic, discovered incidentally on brain imaging performed for unrelated reasons. They are among the most common incidental findings on brain MRI.
Arachnoid cysts are frequently encountered as incidental findings when MRI is performed for headaches, and the question of whether the cyst is causing the symptoms is one of the most common clinical dilemmas in neuroimaging. Large cysts in the posterior fossa can compress CSF outflow pathways and cause hydrocephalus. Suprasellar arachnoid cysts can compress the pituitary stalk and produce hormonal symptoms similar to a pituitary tumor. On MRI, arachnoid cysts must be distinguished from epidermoid cysts — a distinction made reliably using diffusion-weighted imaging.
Causes
Congenital development
Most arachnoid cysts are congenital, arising from a duplication or splitting of the arachnoid membrane during fetal development — likely between the 8th and 16th weeks of gestation. The enclosed space accumulates CSF and may slowly enlarge over time through one-way fluid movement across the cyst wall driven by osmotic gradients or CSF pulsations. They are more common in males and on the left side without a clear explanation. The most common locations are the middle cranial fossa (temporal region, approximately 50%), followed by the posterior fossa, suprasellar region, and spinal canal.
Acquired arachnoid cysts
A minority of arachnoid cysts are acquired rather than congenital — developing after meningitis, head trauma, intracranial surgery, or subarachnoid hemorrhage from a ruptured aneurysm, when scarring and adhesion of the arachnoid membrane creates a CSF-filled pocket that progressively expands.
Symptoms
The vast majority of arachnoid cysts are entirely asymptomatic throughout a person's lifetime. When symptoms do occur, they depend on cyst location and size:
- Headaches: The most common reported symptom — though establishing causation between an incidental cyst and headaches requires careful clinical judgment, as migraine and tension headaches are far more prevalent in the same population.
- Pressure sensations and dizziness: Nonspecific symptoms that may relate to mass effect in larger cysts.
- Seizures: Can occur when a cyst directly irritates adjacent cortex, particularly in temporal lobe cysts.
- Focal neurological deficits: Corresponding to the cyst's location — visual field defects from posterior fossa cysts, endocrine symptoms from suprasellar cysts compressing the pituitary stalk.
- Hydrocephalus symptoms: Large posterior fossa cysts may obstruct CSF pathways and cause hydrocephalus with headache, nausea, and papilledema.
- Acute rupture or hemorrhage: A rare but important complication — even minor head trauma can cause an arachnoid cyst to rupture or accumulate a subdural hygroma, causing sudden neurological deterioration. This is the primary reason contact sports restrictions are recommended for children with large cysts.
Diagnosis
MRI is the preferred and most informative imaging study, showing a well-defined, smoothly marginated cyst with signal identical to CSF on all sequences — including T1 (dark), T2 (bright), and FLAIR (suppressed, like CSF). This uniform CSF signal throughout distinguishes arachnoid cysts from other cystic lesions. Diffusion-weighted imaging (DWI) is the key discriminating sequence — arachnoid cysts show no restricted diffusion (remain dark on DWI), while epidermoid cysts show characteristic bright restricted diffusion, enabling reliable distinction between these two common cystic lesions. MRI also evaluates cyst size, mass effect on adjacent brain, CSF pathway obstruction, and any evidence of prior hemorrhage within the cyst.
CT scan demonstrates the cyst as a low-density, non-enhancing, smoothly marginated lesion without surrounding edema or contrast enhancement — features that effectively exclude tumor or infection. CT is often the initial study when the cyst is discovered acutely after head trauma. Serial imaging over time — typically annual MRI in children, less frequently in stable adults — monitors for cyst growth, which occurs in a minority of patients.
Classification
Arachnoid cysts are classified by location and, for the most common temporal lobe variety, by the Galassi grading system:
- Galassi Type I: Small cyst confined to the anterior middle cranial fossa — minimal mass effect, rarely symptomatic, typically observed without restriction.
- Galassi Type II: Intermediate cyst extending into the Sylvian fissure — may cause mild mass effect on the temporal lobe. Activity restrictions appropriate for children.
- Galassi Type III: Large cyst occupying most of the middle cranial fossa — may significantly compress the temporal lobe and displace midline structures. Surgical consultation warranted for symptomatic cases.
Treatments
The vast majority of arachnoid cysts do not require treatment. Management is guided by symptoms, cyst size, location, and the patient's age.
Observation with serial MRI:
Asymptomatic cysts of any size are monitored with periodic MRI — typically annual imaging during childhood (when cysts are more likely to grow) and less frequently in stable adults. The goal is to detect any progressive enlargement, new mass effect, or development of symptoms that would warrant intervention. Most arachnoid cysts remain stable throughout life.
Activity modification:
Contact sports and activities with high head injury risk are generally discouraged for children with larger cysts (Galassi Type II–III) due to the small but real risk of cyst rupture or subdural hygroma formation with head trauma. This recommendation is individualized based on cyst size, location, and sport-specific risk.
Surgical treatment:
Surgery is considered when cysts cause clearly attributable symptoms — particularly seizures, hydrocephalus, significant focal neurological deficits, or documented progressive enlargement with mass effect. Surgical options include endoscopic cyst fenestration (creating an opening in the cyst wall to allow communication with normal CSF spaces — the preferred approach for most accessible cysts), cystoperitoneal shunting (placing a drain from the cyst to the peritoneal cavity), or open craniotomy with cyst wall resection. Endoscopic fenestration has become the preferred first-line surgical approach for most cysts due to its minimal invasiveness and avoidance of permanent hardware. Surgical outcomes are generally favorable when the indication is clear.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Brain MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.