Q. What is an arachnoid cyst?
 

An arachnoid cyst is a congenital, CSF-filled lesion arising from splitting or duplication of the arachnoid membrane. It is lined by arachnoid cells, contains fluid nearly identical to CSF, and displaces adjacent brain parenchyma, sometimes remodeling bone. It is not a neoplasm and is distinct from post-traumatic leptomeningeal cysts. Although arachnoid cysts may occur anywhere along the neuraxis, the most common sites are the middle cranial fossa, cerebellopontine angle, suprasellar region, and posterior fossa. Arachnoid cysts also occur in the spine.

 

Q. What is the epidemiology of intracranial arachnoid cysts?
 

Autopsy series report an incidence of approximately 1 per 500 persons. Arachnoid cysts comprise around 1% of all intracranial masses. They occur more commonly in males, with a male-to-female ratio of 4:1, and are more frequently found on the left side. Bilateral arachnoid cysts can occur, particularly in patients with metabolic disorders such as mucopolysaccharidoses.

Q. What are the microscopic and histologic characteristics of arachnoid cysts?
 

Two histological patterns exist:

“Simple” arachnoid cysts show a delicate arachnoid lining with cells capable of minimal CSF secretion. Middle fossa cysts are almost always of this type.

 

Complex cysts contain a mixed lining with potential mucin, ependyma, or other cell types, especially in unusual locations or syndromic patients.

 

Arachnoid cysts are lined by epithelial membrane antigen–positive arachnoid cells and are negative for tumor markers such as CEA.

 

Q. How are middle cranial fossa arachnoid cysts classified (Galassi)?
 

The Galassi classification applies to Sylvian fissure/middle fossa cysts and contains three types:

 

Type I: Small, anterior, biconvex cysts at the temporal tip, minimal mass effect, communicate freely with the subarachnoid space.
 

Type II: Intermediate-sized cysts involving the proximal and intermediate Sylvian fissure; the insula is partly exposed; partial communication with the subarachnoid space is common.
 

Type III: Large cysts occupying the entire middle cranial fossa, producing marked midline shift and bony expansion (elevation of sphenoid wing, outward displacement of squamous temporal bone). These have minimal communication with the subarachnoid space.

 

Q. How are arachnoid cysts classified by location?
 

Almost all arachnoid cysts occur in relation to an arachnoid cistern (intracisternal). The only exception is intrasellar cysts, which are extradural. Based on location, common sites include:
• Sylvian fissure (most common, approximately half of all cysts)
• CPA region
• Supracollicular (quadrigeminal plate)
• Vermian/posterior fossa
• Sellar/suprasellar region
• Interhemispheric fissure
• Cerebral convexity
• Clival
• Spinal arachnoid space

 

Q. How do arachnoid cysts form?
 

They arise during embryogenesis from splitting or non-fusion of arachnoid membranes, creating a pocket that fills with CSF. They are therefore considered intra-arachnoid compartments. Fluid accumulates via CSF pulsation, passive diffusion, or a ball-valve effect. 

Secondary cysts may arise after infection, hemorrhage, trauma, or surgery; these have thickened walls or multiple septations.

 

Q. What mechanisms allow arachnoid cysts to enlarge?
 

 Several mechanisms exist:

• A one-way valve leading to progressive CSF inflow

• Entrapment of CSF within an isolated arachnoid compartment

• Compression of adjacent arachnoid cisterns leading to unidirectional filling

• Intrinsic secretion from the cyst wall (rare)

• Impaired reabsorption or scarring of subarachnoid pathways

 

Q. What is the importance of location in clinical presentation?
 

Location determines which structures are compressed:

• Sylvian fissure: temporal lobe compression can lead to seizures and headaches

• Posterior fossa/vermis: hydrocephalus, ataxia

• Suprasellar: visual deficits, endocrine symptoms, precocious puberty, bobble-head doll syndrome, hydrocephalus

• Quadrigeminal plate: compression of posterior third ventricle can lead to obstructive hydrocephalus

• Convexity/interhemispheric: focal deficits, seizures

• Spinal: myelopathy, radicular pain

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Q. How do arachnoid cysts present clinically?
 

Most are asymptomatic. Symptoms, when present, depend on location and cyst size. Typical presentations include:

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• Intracranial hypertension: headache, nausea, vomiting, lethargy, irritability (especially in infants with large cysts).

• Seizures: particularly with middle fossa cysts due to temporal lobe irritation.

• Sudden deterioration: from intracystic or subdural hemorrhage (especially middle fossa cysts with tearing of bridging veins).

• Focal neurological deficit: due to space-occupying effect.

• Cosmetic skull deformity: outward bulging over the cyst.

• Incidental discovery: on imaging for unrelated issues.

 

Suprasellar cysts have additional typical features:

• Hydrocephalus due to third ventricle/aqueduct compression

• Visual loss

• Endocrine disturbances including precocious puberty (occurs in up to 60%)

• “Bobble-head doll syndrome” (rare but classic), characterized by repetitive anteroposterior head movements

 

Q. What CT features suggest an arachnoid cyst?
 

CT shows a smooth, well-demarcated, non-calcified lesion with density nearly identical to CSF and no enhancement. Chronic cysts remodel adjacent bone in up to two-thirds of supratentorial and up to 80% of infratentorial cysts. The cyst may cause mass effect on adjacent structures, including lateral ventricle distortion or midline shift.

 

Q. What MRI findings are characteristic of an arachnoid cyst?
 

MRI is superior for evaluating cysts. The cyst follows CSF on all sequences:
• Low T1, high T2 signal
• Suppression on FLAIR
• No diffusion restriction (epidermoids restrict)
• No enhancement
• Fine membranes may be visible on CISS/FIESTA sequences
MRI can also show mass effect on the third/fourth ventricles, obstruction of foramina of Monro or aqueduct, and presence of septations.

 

Q. What is the role of CT cisternography?
 

CT cisternography involves the injection of intrathecal water-soluble contrast followed by high-resolution CT to evaluate contrast entry into the cyst. It is useful for:

• Determining communication between the cyst and subarachnoid space

• Differentiating true arachnoid cysts from loculated CSF spaces or diverticula

• Preoperative planning for fenestration (identifying best drainage pathways)

• Evaluation of intraventricular or posterior fossa cysts when MRI is inconclusive

• Confirming patency of a fenestration postoperatively

 

Because some cysts only partially fill with contrast, interpretation must be correlated with MRI.

 

Q. What are the treatment options for arachnoid cysts?

 

Treatment depends on cyst location, size, symptoms, and communication with adjoining cisterns. Options include:

• Endoscopic fenestration into basal cisterns, ventricles, or both.

• Microsurgical fenestration via a craniotomy when endoscopic access is difficult or the cyst is multiloculated.

• Cystoperitoneal shunt as a definitive, low-recurrence option when other methods fail or are anatomically unsafe.

• Suboccipital or retrosigmoid fenestration for posterior fossa cysts.

• Spinal cyst fenestration or excision for intradural spinal lesions.

• Drainage alone (needle aspiration or burr hole) is avoided due to high recurrence.

• Ventricular drainage is contraindicated because it can worsen cyst enlargement.

 

Q. What is the treatment of middle fossa arachnoid cysts?

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Middle fossa (Sylvian fissure) cysts are best treated by microsurgical fenestration into the basal cisterns, especially for large Type III lesions. Small, asymptomatic Type I lesions often require no treatment. Type II cysts may be treated with endoscopic fenestration when safe trajectories exist; however, microsurgical fenestration provides the most reliable and durable communication with the bulk cisterns.
If fenestration fails or anatomy is unfavorable, cystoperitoneal shunting offers excellent long-term control and low recurrence.

 

Q. What is the best treatment for suprasellar arachnoid cysts?

 

The best and most durable treatment for suprasellar arachnoid cysts is endoscopic ventriculocystocisternostomy (VCC).
This creates a wide communication between:

• The cyst

• The third ventricle

• The interpeduncular and prepontine cisterns

 

VCC allows continuous CSF circulation and has the lowest recurrence rate among all treatments.

Endoscopic ventriculocystostomy (VC) alone is less durable because it does not reestablish cisternal flow. Microsurgical subfrontal approaches are reserved for cases where endoscopic access is not possible.

 

Q. What is the best treatment for posterior fossa arachnoid cysts?

 

Posterior fossa cysts compressing brainstem or cerebellum are treated via a midline suboccipital or retrosigmoid fenestration into the cisterna magna or prepontine cistern. Endoscopic treatment is rarely feasible in posterior fossa due to limited endoscopic corridors.

 

Q. What is the best treatment for intraventricular arachnoid cysts?

 

These lesions are treated by neuroendoscopic fenestration into the ventricular system and adjacent cisterns. CT cisternography can help determine communication before surgery. Shunting is reserved for multiloculated or recurrent cysts.

 

Q. How will you surgically treat an arachnoid cyst? (Microsurgical Fenestration)

 

I will begin by reviewing high-resolution MRI, focusing on the cyst’s relationship to arteries, veins, cranial nerves, ventricles, and cisterns. Based on its location, I will select an optimal operative approach. For middle fossa cysts, I will commonly use a pterional craniotomy. For suprasellar cysts, I will plan a burr hole approach for neuroendoscopic ventriculocystocisternostomy. For posterior fossa lesions, I will choose a midline suboccipital or retrosigmoid exposure.

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After positioning and preparing the patient, I will open the dura and gently expose the cyst wall under the microscope or endoscope. I will cautiously dissect arachnoid membranes away from the cyst, protecting cortical vessels and perforators. Once I identify the cisternal or ventricular boundary, I will create a wide fenestration in the cyst wall using microscissors or bipolar forceps, ensuring a robust opening that cannot close spontaneously. If the cyst has internal septations, I will open each one to create a single cavity.

 

For suprasellar cysts, I will identify the floor of the third ventricle, the lamina terminalis, and the interpeduncular cistern, and I will perform a ventriculocystocisternostomy to establish strong dual communication with both ventricle and cistern.

 

I will confirm that CSF flow passes freely through the fenestrations and that no neural or vascular structures are entrapped. If the fenestration is not feasible or if the cyst is multiloculated and deep, I will place a cystoperitoneal shunt using a low-pressure valve to maintain gradual decompression. I will then irrigate, achieve hemostasis, and close all layers meticulously.

 

Postoperatively, I will monitor for neurological improvement, signs of CSF leak, and subdural collections. Follow-up MRI will confirm cyst decompression and patency of fenestrations.

 

Q. What are the perioperative complications of arachnoid cyst surgery?

 

These include:

• Injury to adjacent cortex, cranial nerves, or perforator vessels

• Hemorrhage into the cyst or subdural space

• CSF leak

• Failure to adequately fenestrate cyst walls

• Injury to optic apparatus in suprasellar cases

• Brainstem injury in posterior fossa approaches

 

Q. What are the postoperative complications?

 

Postoperative complications include:

• Subdural hygroma (most common) due to rapid decompression of a large cyst

• Subdural hematoma

• Recurrence due to closure of fenestration

• Persistent hydrocephalus

• Shunt dependency if a shunt was placed

• Seizures in patients with temporal lobe involvement

• Visual or endocrine deterioration in suprasellar cysts if decompression is inadequate

 

Q. How will you counsel the patient or parents before surgery?

 

I will explain that arachnoid cysts are benign lesions that often remain stable, but surgery is indicated when symptoms or progressive enlargement occur. I will emphasize that the goal is to restore normal CSF circulation by fenestrating the cyst into cisterns or ventricles. I will discuss risks such as subdural collections, recurrence, cranial nerve deficits (depending on location), and the possibility of needing a shunt. For suprasellar cysts, I will clarify that endoscopic ventriculocystocisternostomy offers the best long-term results and carries a low recurrence rate. I will address seizure prognosis in temporal cyst cases and reassure parents that many children improve after decompression.

 

Q. What is the prognosis after surgery for arachnoid cysts?

 

Prognosis is excellent. Most symptomatic patients show significant improvement in headaches, hydrocephalus, visual symptoms, or developmental delay. Suprasellar cyst symptoms, especially visual decline and bobble-head doll syndrome, often improve dramatically with timely VCC. Recurrence is uncommon when wide fenestrations are created. Shunted cysts function well but require lifelong surveillance for shunt malfunction. Seizure control depends on underlying cortical pathology; some patients continue to require antiepileptic therapy despite cyst decompression.

 

Reference: Greenberg's Handbook of Neurosurgery, 10th Edition