Although uncommon, arachnoid cysts periodically arise in the region of the sella turcica, serving as an occasional source of neurologic and endocrinologic dysfunction in both pediatric and adult age groups (123,135-141). With 15% of all arachnoid cysts occurring in the sellar region, this site represents the second most common intracranial location for such lesions. Arachnoid cysts are presumed to be developmental in origin, arising as the consequence of splitting or duplication of the arachnoid membrane, forming an arachnoid-lined sac in which CSF is entrapped.
The natural history of these lesions remains uncertain, with some arachnoid cysts remaining quiescent for years and others demonstrating symptomatic enlargement. The basis for such enlargement is similarly uncertain, with osmotic gradients, endogenous CSF production, and progressive accumulation of CSF through a "ball-valve" mechanism all being invoked as potential growth mechanisms. Although most arachnoid cysts are topologically related to a normal subarachnoid cistern, macroscopic continuity with the subarachnoid space is rarely apparent. Arachnoid cysts of the sellar region are distinguished as being one of two types, intrasellar and suprasellar—each is discussed separately.
Intrasellar arachnoid cysts are the least common of the two, arising from arachnoid remnants within the sella (135). In contrast to arachnoid cysts occurring elsewhere, intrasellar arachnoid cysts tend to occur in older patients, usually in the fourth or fifth decade of life. They behave as expansile intrasellar masses, causing headache, sellar enlargement, hypopituitarism, moderate hyper-prolactinemia, and upward displacement of the diaphragma sellae, resulting in visual loss. An important anatomic feature of these lesions is that, even with large suprasellar extensions, the diaphragma is always intact and the cysts remain entirely subdiaphragmatic. In rare instances, a pinhole communication with the suprasellar subarachnoid space is noted; however, as evidenced by their failure to admit contrast agents delivered cisternally, most intrasellar arachnoid cysts lack free communication with the subarachnoid space. Radiologically, these lesions are associated with sellar enlargement, appearing as homogeneous cysts whose fluid contents exhibit density and signal characteristics identical to CSF. Intrasellar arachnoid cysts are distinguished from the intrasellar herniation of the chiasmatic cistern that occurs with the empty sella syndrome and other cystic pathologies that occur in the region (craniopharyngioma, Rathke's cleft cyst, cystic pituitary adenoma, epidermoids, dermoids). Symptomatic intrasellar arachnoid cysts, as well as those in which imaging studies fail to exclude a cystic neoplasm, are best managed by transsphenoidal exploration, excision of the cyst wall for diagnosis, and marsupialization of the cyst cavity. Symptomatic improvement is the rule, headache, visual function, and hyperprolactinemia improving virtually always; however, anterior pituitary deficits, particularly if long-standing, are generally less responsive to surgical decompression.
Suprasellar arachnoid cysts are predominantly lesions of the pediatric population, with the majority of patients presenting within the first two decades of life (32,136). Arising within the chiasmatic cistern, suprasellar arachnoid cysts may become sizeable, causing compression of the hypothalamus and third ventricle, stretching of the optic nerve, and distortion of the pituitary stalk. Lateral and posterior extensions into the medial temporal lobe and interpeduncular cistern, respectively, may also occur. Hydrocephalus, on the basis of third ventricular compression or aqueductal distortion, is often the dominant presenting feature, resulting in headache, macrocephaly, and retardation; precocious puberty or hypopituitarism reflects hypothalamic and/or stalk compression. Visual field deficits and loss of acuity are often present. A peculiar and intermittent pattern of head movements dubbed the "bobble-head doll" syndrome is a rare but classic presentation of arachnoid cysts in this location.
The management of suprasellar arachnoid cysts is surgical; however, the optimal form of intervention remains a matter of debate. Marsupialization of the cyst wall, with or without some form of shunting procedure, is the principal therapeutic option; ventriculoperitoneal shunting alone rarely produces a satisfactory or durable response (32,136). Access to the cyst wall for purposes of marsupialization can be accomplished by a variety of approaches, including subfrontal, transcortical-transventricular, transsylvian, or transcallosal routes. Once a wide excision of the cyst wall has been achieved, some authors advocate the routine placement of a cystoperitoneal or ventriculoperitoneal shunt, because marsupialization alone may fail to improve hydrocephalus, despite seemingly adequate decompression of the cyst.
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