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Arachnoid Cyst: Fully Endoscopic Resection of Intracranial Symptomatic Arachnoid Cysts

While open craniotomies for excision or fenestration of intracranial arachnoid cysts are considered too aggressive, cysto-peritoneal shunting, although safer and less invasive than open surgery, is associated with long-term shunt dependence and shunt complications that cannot be dismissed.

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At the Skull Base Institute we perform complete removal of the intracranial arachnoid cyst via "anatomical" routes, usually through a small hidden incision within the eyebrow. This approach offers the opportunity to appreciate the natural pathotphysiology of these benign cysts thus providing a more "complete" operation.

Endoscopic removal offers an effective alternative to traditional open craniotomies due to its numerous advantages including better visualization during surgery, less tissue trauma during exposure, shorter hospital stays and minimal postoperative discomfort.

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Arachnoid Cyst Overview
Arachnoid Cyst Symptoms
Arachnoid Cyst Treatment

Overview
Arachnoid cysts are congenital benign fluid collections within the arachnoid membrane-one of the 3 membranous layers that cover the brain and the spinal cord. The cyst is filled with a fluid similar to cerebrospinal fluid (CSF).

Arachnoid cysts may occur either in the brain or along the spinal cord. Most cases present during infancy; however onset of symptoms may be delayed until adolescence when the cyst has enlarged. Intracranial arachnoid cysts are reported to account for about 1% of all intracranial space-occupying lesions. This percentage, however, has probably been underestimated as with the advent of modern imaging techniques intracranial arachnoid cysts have become more frequently detected or diagnosed.

According to their size and location arachnoid cysts may not produce any symptoms and are only discovered incidentally on MRIs of the brain performed for a variety of reasons, or they may enlarge and become symptomatic by exerting pressure on the surrounding brain or by interfering with the dynamics of the CSF circulation.

Arachnoid cysts are more common in males than females. The most frequent localization of congenital intracranial arachnoid cysts is in the middle cranial fossa, constituting more than one half of the total of reported cases. Less commonly, they occur in the sellar and parasellar regions, around the foramen magnum, between the two brain hemispheres, or along the cerebral convexities.

Causes
Primary or congenital arachnoid cysts are maldevelopmental anomalies and should be differentiated from secondary arachnoid cysts that are rare and result from a variety of such etiologies as trauma and infection.

The cyst has an incessant tendency to grow or recur but the growing mechanism is still a mystery and the mechanism by which they expand is not fully understood.

Symptoms
Symptoms of an intracranial arachnoid cyst are related to the cyst's size and location. Small cysts are usually asymptomatic and are discovered only incidentally. Due to their benign nature and slow expansion, congenital intracranial arachnoid cysts may remain asymptomatic, produce only subtle symptoms and signs or result in mild neurological impairment relative to their large size. Large cysts may cause cranial deformation or macrocephaly (enlargement of the head) in new borns, or produce such symptoms as headaches, epileptic seizures, hydrocephalus (excessive accumulation of cerebrospinal fluid), increased intracranial pressure, developmental delay, and behavioral changes. Less commonly they may present with focal signs suggestive of their location such as visual affection or endocrinopathies due to sellar and suprasellar arachnoid cysts. Other symptoms may include hemiparesis (weakness or paralysis on one side of the body) and ataxia (lack of muscle control).

Diagnosis
Routine evaluation with CT or MRI scans is usually satisfactory in identifying the cyst which is seen as a smooth bordered cystic mass composed of a density similar to that of CSF and does not show enhancement with contrast administration. Detection of the cyst is even possible prenatally by ultrasound. Expansion of the nearby bone by remodeling is usually seen, confirming their chronic nature. Increasingly, scans of the brain (done for other reasons, such as assessment of head injury or persistent headache) discover the presence of an arachnoid cyst as an incidental finding.

Treatment
There is a wide variation in the natural course and the clinical manifestations of congenital intracranial arachnoid cysts. Nevertheless, there has also been a considerable controversy regarding the indications, choice and timing of surgical intervention.

While there is a common consensus that small cysts with minimal symptoms should be treated conservatively with regular clinical and radiological follow-up, it is also agreed that large space-occupying cysts that exert a mass effect or those causing neurological impairment require surgical intervention as the potential for hindering normal development and function of the adjacent brain in this group outweighs the risk of operative treatment.

The goals of surgical treatment are to drain the cyst and to prevent the recollection of fluid, allowing for re-expansion of the compressed neighboring neural tissue. However, the optimum surgical modality to achieve these goals has been a subject of debate, mostly due to the inherent risks associated with each surgical modality.

The two main surgical modalities by which intracranial arachnoid cysts have been most commonly treated are formal craniotomy with excision/marsupialization of the cyst into the subarachnoid space and cysto-peritoneal shunting. While cystoperitoneal shunting implies long-term shunt dependence and complications such as shunt obstruction or infection, formal craniotomy, however, involves injurious "transcortical" surgical dissection and a significant degree of brain retraction, which in turn may result in undesirable perioperative morbidity and inability to completely excise the cyst due to a limited exposure. Other operative procedures that have been recommended for treatment of arachnoid cysts include stereotactic cyst aspiration, cysto-cisternostomy and ventriculocystostomy.

Prognosis
Even following successful treatment a portion of the cyst may remain due to the remodeling of the bone and chronic shift of brain contents. Associated symptoms and signs tend to resolve after successful treatment. Aspiration of the cyst only is associated with the highest rate of recurrence as the cysts have a indolent tendency to regrow.


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