Arachnoid cyst

Arachnoid cysts are relatively common benign lesions occurring in association with the central nervous system, both within the intracranial compartment (most common) as well as within the spinal canal. They are usually located within the subarachnoid space and contain CSF. 

Epidemiology

Arachnoid cysts account for ~ 1% of all intracranial masses. Although the vast majority are sporadic, they are seen with increased frequency in mucopolysaccharidoses (as are perivascular spaces). 

Clinical presentation

The majority of arachnoid cysts are small and asymptomatic. When symptoms occur they are usually the result of gradual enlargement resulting in mass effect. This results in either direct neurological dysfunction, or distortion of normal CSF pathways resulting in obstructive hydrocephalus ³. 

Pathology

Arachnoid cysts are thought to arise due to congenital splitting of the arachnoid layer with accumulation of CSF within this potential space. The cyst wall is comprised of flattened arachnoid cells forming a thin translucent membrane. There is no solid component and no epithelial lining.

Radiographic features

Most frequently (50-60%) arachnoid cysts are located in the middle cranial fossa, where they invaginate into and widen the sylvian fissure. There is even a classification system for middle cranial fossa arachnoid cysts, although I doubt it is of much use if a good description is provided (see Galassi classification).

They can occur anywhere, including:

• interhemispheric fissure
• cerebral convexity
• posterior fossa
  • cisterna magna (main ddx: mega cisterna magna)
  • cerebellopontine angle (main ddx: epidermoid cyst)
• quadrigeminal cistern
• spinal canal (see spinal arachnoid cysts)
• ventricles (see intraventricular arachnoid cyst)
• suprasellar cistern (see suprasellar arachnoid cyst)

CT

Arachnoid cysts are extremely well circumscribed, with an imperceptible wall, and displace adjacent structures. When large, and over time, they can exert a remodelling effect on the bone.

CT cisternography (introduction of contrast into the subarachnoid space) demonstrates communication of the cyst with the subarachnoid space. As this communication is slow, the cyst often fills later, and contrast may be seen to pool with it, outlining its dependent portion.

MRI

As they are filled with CSF if is not surprising that they follow CSF on all sequences, including FLAIR and DWI. This enables them to be distinguished from epidermoid cysts for example. As their wall is very thin it only occasionally can be seen, and their presence is implied by displacement of surrounding structures. As there is no solid component, no enhancement can be identified. 

Phase contrast imaging can also be employed not only to determine if the cyst communicates with the subarachnoid space, but also to identify the location of this communication.

Treatment and prognosis

Arachnoid cysts are benign and the vast majority remain asymptomatic throughout life. If they are deemed to be causing symptoms then surgery can be contemplated. This can either take the form of a craniotomy (fenestration or excision) or placement of a cystoperitoneal shunt.

Differential diagnosis

General imaging differential considerations include

• enlarged CSF space : e.g.  mega cisterna magna
• epidermoid cyst
• subdural hygroma / chronic subdural haemorrhage
• cystic tumours : often will have a solid / enhancing component and be intra-axial
  • pilocytic astrocytoma
  • haemangioblastoma
• non-neoplastic cysts
  • neurenteric cyst
  • neuroglial cyst
  • porencephalic cyst
• neurocysticercosis: small cyst, usually multiple when in the subarachnoid space

See also

• fetal arachnoid cyst