Arachnoid Cyst

Arachnoid cysts are extra-axial collections of CSF enclosed within a split of the arachnoid membrane. Most are developmental and are thought to arise from an anomalous splitting of the arachnoid during formation of the subarachnoid cisterns; secondary (post-traumatic or post-inflammatory) cysts are less common. The cyst wall is a thin layer of arachnoid cells, and the fluid is indistinguishable from CSF.

Proposed mechanisms of enlargement include active secretion by the cyst wall, osmotic gradients and a ball-valve communication with the subarachnoid space. In practice, the great majority of cysts remain stable for years.

Large MRI cohorts from a single institution found arachnoid cysts in roughly 2.6% of children and 1.4% of adults, most of them asymptomatic and stable on follow-up imaging^[1,2]. The middle cranial fossa is the commonest location, followed by the retrocerebellar and cerebellopontine-angle cisterns, the suprasellar cistern, the quadrigeminal cistern and the convexity.

Middle-fossa cysts are graded by the Galassi classification^[3]:

• Type I: small, spindle-shaped, confined to the anterior temporal pole; usually communicate freely with the subarachnoid space.
• Type II: extend up the Sylvian fissure and displace the temporal lobe; partial communication.
• Type III: very large, fill the middle fossa, with midline shift and bony remodelling; little or no communication.

Most cysts are found on imaging performed for unrelated reasons. Symptoms that can reasonably be attributed to a cyst include signs of raised intracranial pressure, focal deficits from mass effect, macrocephaly or calvarial asymmetry in infants, obstructive hydrocephalus (especially with suprasellar and posterior-fossa cysts) and, for suprasellar cysts, endocrine dysfunction or the classic “bobble-head doll” movement. Headache and seizures alone are common in the general population, so causation should be established carefully before operating.

Middle-fossa cysts predispose to subdural haematoma or hygroma and intracystic haemorrhage after minor head injury, because the bridging veins are stretched across the cyst; this is a recognised acute presentation in children and young adults.

MRI is diagnostic. The cyst follows CSF signal on all sequences, suppresses fully on FLAIR and does not restrict diffusion, which distinguishes it from an epidermoid. There is no enhancement or solid component. CT shows any scalloping or thinning of the overlying bone. Cine phase-contrast MRI or CT cisternography can show whether the cyst communicates with the cisterns, and high-resolution heavily T2-weighted sequences (e.g., CISS/FIESTA) help plan an endoscopic fenestration trajectory.

Asymptomatic cysts are observed; serial imaging is reasonable in young children. Surgery is indicated for hydrocephalus, progressive enlargement with mass effect, focal deficit, or symptoms convincingly linked to the cyst. Haemorrhagic presentations are managed according to the subdural or intracystic collection, with the cyst addressed at the same sitting or later.

The main options are:

• Microsurgical fenestration through a small craniotomy, opening the cyst widely into the basal cisterns (for middle-fossa cysts, into the carotid–oculomotor and interpeduncular cisterns).
• Endoscopic fenestration, which is particularly suited to suprasellar cysts (ventriculocystocisternostomy through a frontal burr hole) and to quadrigeminal and intraventricular cysts.
• Cystoperitoneal shunting, which is reliable at decompressing the cyst but commits the patient to lifelong shunt dependence and its complications; it is now mostly reserved for failed fenestration.

Fenestration controls hydrocephalus and mass effect in most patients, although radiological shrinkage of the cyst is often incomplete and does not need to be complete to be clinically successful. Complications include subdural hygroma, CSF leak, oculomotor-nerve palsy during middle-fossa fenestration, and, after endoscopy, fornix or hypothalamic injury. Recurrence after fenestration usually reflects closure of the stoma and may be treated by repeat fenestration or a shunt.