|Discussion: Arachnoid cysts are benign intra-arachnoidal fluid collections, lined by a membrane of true arachnoid cells. They usually contain clear, colorless fluid that is most likely normal cerebrospinal fluid (CSF); rarely, they contain xanthocromic fluid. Microscopic examination of arachnoid cysts shows that the walls are formed from a splitting of the arachnoid membrane, with an inner and outer leaflet surrounding the cyst cavity.
The exact means of arachnoid cyst genesis remains unclear. Most are congenital, representing developmental anomalies. A small number are acquired, such as those resulting from post-inflammatory adhesions or after trauma, surgery, hemorrhage or infection. These acquired arachnoid cysts have been described variably as acquired, secondary or leptomeningeal.
Arachnoid cysts constitute approximately 1% of all intracranial masses. There is a 2:1 male-to-female ratio. Common locations include the middle cranial fossa (50%-60%), the perisellar and supra sellar region (1,5%-11%), the quadrigeminal cistern (10%), the cerebral convexities (4%-13%), the interhemispheric fissure (3%-7%), the retrocerebellar region (9%-16%) and the cerebellopntine angle (2%-11%).
Arachnoid cysts may be associated with other intracranial abnormalities, but data are not sufficient to indicate whether the association is typical or fortuitous. Of interest are the associations with anomalous venous drainage, hypogenesis of the corpus callosum, malformation of cortical development and cerebellar hamartomas. They may also be associated with Cockayne, Walker-Warburg and Menkes syndromes, as well as with autossomal dominant polycistic kidney disease.
Usually arachnoid cysts are asymptomatic and they are often an incidental finding on imaging. In children, the most common presentation are macrocephaly and intracranial hypertension. There may be associated hydrocephalus. In adults, the most commonly associated clinical features are headache and seizures. Focal neurological signs occur less frequently.
In symptomatic patients, clinical features depend on the location of the arachnoid cyst. Suprasellar arachnoid cysts are most often diagnosed in childhood secondary to hydrocephalus, visual field deficits or endocrine abnormalities. Cerebellopontine angle (CPA) arachnoid cysts may be associated with neuropathies of cranial nerves V, VII and VIII or cerebellar symptoms such as ataxia. Middle cranial fossa cysts most often present with increased intracranial pressure, headaches and seizures.
Arachnoid cysts are non-enhancing well circumscribed extra-axial lesions with the same attenuation on CT or signal intensity on all MR pulse sequences as that of CSF (fig 1, 2, 3 and 4). The walls of the cyst do not demonstrate abnormal contrast enhancement (fig 3), and there is no reactive edema surrounding the fluid collection (figure 1, 2 and 4). They may exert mass effect on adjacent brain (fig 1 and 2) or bone (fig 5).
Hypoplasia of the temporal lobe (fig 1 and 2), as well as bony remodelling and smooth erosion of the inner table of the ipsilateral temporal bone (fig 5) are commonly seen in middle cranial and Sylvian fissure cysts. A lesser frequent finding accompanying arachnoid cysts of the middle cranial fossa is hypertrophy and/or pneumatization of the lesser wing of the sphenoid bone (fig 1).
The marked enlargement of an arachnoid cyst on follow up studies is highly unnusual but occasionally observed. Similarly, spontaneous descompression of arachnoid cysts has been noted.
Rare complications of arachnoid cysts such as intracystic hemorrhage, subdural hematoma or subdural hygroma (fig 6 and 7) may be seen and is specially common with arachnoid cysts of the middle cranial fossa. They may occur either spontaneously or after head injury. Rupture of the outer wall and surrounding fragile veins allows blood to accumulate within the cyst, as well as blood/fluid in the subdural compartment. Hemorrhage into the cyst may be associated with increased or new symptoms.
The major differential diagnosis on routine CT or MR images is with an epidermoid cyst. These lesions are soft, lobulated, infiltrating masses that tend to surround vessels and cranial nerves. By contrast, arachnoid cysts are usually unilocular and smoothly marginated, stretching and displacing adjacent neurovascular structures; bone erosion is commonly associated with arachnoid cysts and rarely seen with epidermoid masses. FLAIR and diffusion-weighted sequences will usually allow differentiation between them. Arachnoid cysts are similar to CSF signal, with very low signal on FLAIR imaging and on ADC, whereas the keratinous semi solid content of an epidermoid cyst is characterized by higher intensity signal on FLAIR studies and on diffusion-weighted images.
Controversy surrounds the treatment of arachnoid cysts. Some clinicians advocate treating only patients with symptomatic cysts, whereas others believe that even asymptomatic cysts should be descompressed to avoid future complications.