| Discussion: |
Arachnoid cysts represent intra-arachnoid cerebrospinal fluid–containing cysts that do not communicate with the ventricular system and usually are not associated with brain maldevelopment. They constitute approximately 1% of intracranial masses, with 50-60% occurring in the middle cranial fossa. Cysts in the middle cranial fossa are found more frequently in males and on the left side. Most arise as developmental anomalies. A small number of arachnoid cysts are associated with neoplasms or occur as complications of adhesions following leptomeningitis, hemorrhage, or surgery.
Arachnoid cysts also occur within the spinal canal, in which arachnoid cysts or arachnoid diverticula may be located subdurally or in the epidural space, respectively. Spinal arachnoid cysts are commonly located dorsal to the cord in the thoracic region. A cyst in this location is usually secondary to a congenital or acquired defect and is situated in an extradural location. Intradural spinal arachnoid cysts are secondary to a congenital deficiency within the arachnoid or are the result of adhesions resulting from previous infection or trauma. Microscopic examination shows that their walls are formed from a splitting of the arachnoid membrane, with an inner and outer leaflet surrounding the cyst cavity.
Arachnoid cysts often are an incidental finding on imaging and, usually, patients are asymptomatic even if the cyst is quite large. The most commonly associated clinical features are headache, calvarial bulging, and seizures, with focal neurologic signs occurring less frequently.
Arachnoid cysts usually occur in association with normal arachnoid cisterns, and such cysts are congenital, arising from arachnoid clefts and arachnoid duplications. Glioependymal cysts are rare; only a few instances of interhemispheric glioependymal cysts are known. Glioependymal cysts may be associated with agenesis of the corpus callosum, heterotopia, and other dysplasias.
True arachnoid cysts are the most common congenital cystic lesions in the brain, accounting for approximately 1% of intra-cranial mass lesions. secondary arachnoid cysts have been reported after trauma and infection, forming when an inﬂammatory process causes arachnoiditis and subsequent encystment of the subarachnoid space.2 In most cases, however, the pathogenesis remains obscure.
About 50% of such lesions occur in the middle cranial fossa, the most common location overall. There is an association with temporal lobe hypoplasia when arachnoid cysts occur in this region, although the exact relationship is controversial.1 About one third occur in the posterior fossa, particularly the retrocerebellar, cerebellopontine, and quad-rigeminal plate cisterns. Posterior fossa cysts should be differentiated from a mega cisterna magna and Dandy-Walker malformation. Approximately 10% of these lesions are found in the suprasellar region, where they can mimic an enlarged third ventricle. An ependymal cyst within the third ventricle could produce an identical image. Occasionally, cysts are found in the perisellar region and over the convexities. A large convexity arachnoid cyst can resemble a chronic subdural hematoma. Rare intraventricular cysts have been reported.3 CT imaging features include a CSF density mass that effaces adjacent sulci and remodels bone. Small cysts may not be seen due to partial volume averaging, especially when located in the middle cranial fossa, and expansion of the sylvian ﬁssure or bony scalloping may indicate the presence of a cyst.
On MRI, the arachnoid cyst appears as an extra-axial mass that follows CSF intensity on all pulse sequences. Occasionally, the signal intensity of an arachnoid cyst can be similar or identical to that of an epidermoid tumor.1 Smooth erosion of the inner table of the skull is seen on both CT and MRI, which is thought to be the result of extremely slow growth and transmitted CSF pulsations.