ARACHNOID CYST ( Suprasellar arachnoid cyst)
INTRODUCTION:
Arachnoid (leptomeningeal) cysts are benign, congenital, intra-arachnoidal space occupying lesions that are filled with clear CSF like fluid covered by arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the arachnoid membrane, one of the three membranes that covers the brain and the spinal cord.
ETIOLOGY:
Most of them are developmental anomalies, due to meningeal maldevelopment. Minor aberrations of CSF flow through the loose primitive perimedullary mesenchyme may result in focal splitting of the developing meninges. Formation of a pocket or diverticulum in the space thus created between the arachnoid and pia results in an arachnoid cyst.
A small number of arachnoid cysts are aquired, such as those in association with neoplasms or those that are due to adhesions following leptomeningitis, hemorrhage or surgery. They constitute approximately 1% of intracranial masses, with 50-60% occuring in the middle cranial fossa. Cysts in the middle cranial fossa are found more frequently found in males and on the left side. Most arise as developmental anomalies.
Arachnoid cysts can also occur within the spinal cord, in which arachnoid cysts or arachnoid diverticula may be located subdurally or in the epidural space. 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 arachnoidal trabecula, especially in the septum posticum, or are the result of adhesions resulting from previous infection or trauma.
PATHOPHYSIOLOGY:
True arachnoid cysts are fluid-filled cavities that lie entirely within the arachnoid membrane. Grossly, these cysts consist of a thin but distinct transparent wall that is separated from the inner dural layer and the underlying pia-arachnoid. Arachnoid cysts range in size from small incidental cysts to large space occupying lesions. Compression of the underlying brain varies from none to severe. Temporal lobe dysgenesis is common with middle fossa arachnoid cysts.
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 cyst wall consists of fibrous connective tissue slightly denser than normal arachnoid tissue, with hyaline change at times. No epithelial lining is present. The outer wall of the cyst adheres loosely to the dura. The cyst wall is devoid of blood vessels, and changes of inflammation or hemorrhage seldom occur.
INCIDENCE:
Aracnoid cysts constitute 1% of inracranial masses, with 50-60% occuring in the middle cranial fossa.
MORTALITY:
Morbidity and mortality depend on the location of the arachnoid cyst and complications, such as acute mass effect by intracystic hemorrhage or the development of a subdural hygroma/hematoma. The exact incidence of mortality and morbidity from an arachnoid cyst is not known.
AGE and GENDER:
About 75% of arachnoid cysts occur in children.
There is 3:1 male: female ratio.
CLINICAL DETAILS:
Arachnoid cysts often are an incidental finding on imaging, and patients usually are asymptomatic even if the cyst is quite large.
Common presenting symptoms are:
1) Headache
2) Calvarial bulging.
3) Seizures.
4) Intracranial hypertension.
5) Craniomegaly.
6) Developmental delay.
7) Visual loss.
8) Precocious puberty.
9) Focal neurological signs occurring less frequently.
Arachnoid cysts are known to rupture in the subdural space or undergo intracystic hemorrhage.
In symptomatic patients, clinical features depend on the location of the arachnoid cyst. Cysts of the middle cranial fossa (50%) may compress the tip of the temporal lobe, displacing it in the occipital direction. Middle cranial fossa cysts have been linked to ipsilateral chronic subdural hematomas. Rarely, they may communicate with the subdural space, forming a slit like extension over the hemispheric surface.
IMAGING FEATURES:
CT SCAN:
NECT scans show a smoothly demarcated noncalcified extraaxial mass that does not enhance following contrast administration.
Arachnoid cysts typically show compression of the subarachnoid space by a cystic structure that may be unilocular or septate and of variable size, although the septa may not always be visible.
The subjacent brain shows minimal mass effect.
On CT scans, arachnoid cysts are characterized by sharp nonenhancing borders and are isodense to CSF.
On a bone window, remodeling of the skull may be evident.
Arachnoid cysts seldom calcify.
MRI FINDINGS:
Arachnoid cysts are sharply demarcated extraaxial masses that may displace or deform adjacent brain. The classical arachnoid cyst has no internal architecture and does not enhance.
Occasionally hemorrhage or high protein content may complicate the MR appearance of an arachnoid cyst.
Diagnostic confusion occasionally may arise between arachnoid cysts and epidermoid cysts. The 2 masses may have similar characteristics on T1-weighted and T2-weighted images, and neither shows enhancement with gadolinium. However, arachnoid cysts follow CSF signals on all sequences—in particular, on the FLAIR sequence—in contradistinction to epidermoid cysts. DWIs allow easier differentiation of the 2 masses.
ULTRASOUND FINDINGS
Cranial ultrasonography is an important diagnostic tool during the first year of life and is limited by the closure of the anterior fontanelle, which normally occurs in full-term infants aged 9-18 months. Although infants with arachnoid cysts rarely are symptomatic, ultrasound provides a noninvasive imaging technique with a high yield in the detection and characterization of cystic masses. Intracranial cysts and ventriculomegaly also can be detected and characterized by transcranial ultrasound through a burr hole.
DIFFERENTIAL DIAGNOSIS:
1) Epidermoid cyst.
2) Dermoid cyst.
3) Supracollicular cysts.
4) CP angle cysts.( Acoustic neuroma and epidermoid cyst )
5) Intracerebral cysts. ( Cerebral abscess, cystic tumors, and parasitic cysts )
6) Basal midline cysts. (Midline cysts associated with agenesis of corpus callosum and other causes of hydrocephalus )
7) Posterior fossa cysts. ( Dorsal cyst associated with holoprosencephaly, Dandy-Walker cyst, and vein of Galen aneurysm )