Arachnoid Cysts
Arachnoid cysts are benign, cerebrospinal fluid (CSF)-filled cystic structures that form by splitting of the arachnoid membrane around the brain or spinal cord. Most are asymptomatic and found incidentally. Only cysts that cause mass effect or symptoms require surgical treatment.
Ultimo aggiornamento: 2026-06-07
Definition
Arachnoid cysts are benign sacs filled with CSF that form by splitting of the arachnoid membrane covering the brain and spinal cord. They account for about one percent of all intracranial masses. Most are congenital (primary); less often they develop after trauma, infection or hemorrhage (secondary). They most commonly lie in the middle cranial fossa (sylvian fissure), followed by the posterior fossa, suprasellar region and convexity.
Classification and Location
Middle fossa cysts are graded by the Galassi classification, from small (type I) to large cysts filling the whole temporal lobe (type III). Location determines symptoms: middle fossa cysts tend to cause seizures and headache; suprasellar cysts visual and hormonal problems; and posterior fossa cysts hydrocephalus and disturbances of balance and coordination. Size alone does not dictate treatment; symptoms and pressure on brain tissue are more decisive.
Symptoms
The large majority of cases are asymptomatic and discovered incidentally on imaging. Symptomatic cysts may cause headache, seizures (especially with middle fossa/temporal cysts), head asymmetry and a bulging fontanelle in infants, nausea and vomiting, balance and coordination disturbance, visual problems and, with large cysts, motor weakness. Symptoms usually arise from cyst growth, mass effect or altered CSF flow.
Diagnosis
Magnetic resonance imaging (MRI) is the reference method; the cyst follows CSF signal on all sequences, does not enhance with contrast and is suppressed on FLAIR. An important differential is epidermoid cyst, which shows diffusion restriction on MRI whereas an arachnoid cyst does not. Computed tomography (CT) can show the cyst at CSF density and thinning of the adjacent bone. EEG may help in patients presenting with seizures. Asymptomatic cysts are adequately managed with periodic MRI surveillance.
Treatment
Asymptomatic cysts are usually followed with periodic MRI; surgery is considered if they grow or cause symptoms. For symptomatic cysts, the least invasive option is endoscopic fenestration, creating an opening in the cyst wall to connect it with the CSF circulation and decompress it. When endoscopy is unsuitable or fails, microsurgical fenestration (via craniotomy) is used. For recurrent cysts or those not amenable to fenestration, a cyst-peritoneal shunt may be used, though it is generally a last resort because of the risk of shunt infection and blockage. When a posterior fossa cyst is accompanied by hydrocephalus, an additional shunt may be needed.
Course
With early diagnosis and appropriate treatment the course is generally favorable, and lasting neurological impairment is rare. After fenestration, some cases show re-accumulation of CSF (recurrence), and revision may be needed if symptoms return; follow-up MRI after surgery is therefore advised. Unnecessary surgery is avoided in asymptomatic cysts. Outcomes vary with cyst location, size and the patient, and none can be guaranteed.
Riferimenti
- Greenberg MS. Greenberg's Handbook of Neurosurgery. 10th ed. Thieme; 2023:260-263.
- Winn HR, ed. Youmans Neurological Surgery. 6th ed. Saunders; 2011:1911-1917.
- Al-Holou WN, Yew AY, Boomsaad ZE, et al. Prevalence and natural history of arachnoid cysts in children. J Neurosurg Pediatr. 2010;5(6):578-585.
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