Arachnoid cysts are commonly seen supratentorial, being fossa media the most frequent location. Intrasellar distribution is extremely rare, accounting for 0.5% in autopsy studies3. There are few operated cases reported in literature.
The pathophysiology is still unknown, although several theories have been advocated such as being produced by a defective diaphragma sellae through which basal arachnoid membrane herniates. This defect may close as a result of meningitis –as it happened in our case-, hemorrhage, or inflammatory event creating a non-communicating cyst4.
IAC clinically resemble a nonfunctional pituitary adenoma, thus endocrine symptoms are observed to be less common5.
IAC are characterized on the MRI images by cystic intrasellar lesion with no contrast enhancement and calcification with a typical CSF-like signal which is hypointense on T1 and hyperintense on T2-weighted sequences. Suprasellar extension is seen regularly, adopting a balloon regular shape molding but no invading cavernous sinus5,6.
The differential diagnosis should be done with other benign cysts and tumors, being sometimes complicated preoperatively because symptoms and images may mimic each other. Benign intrasellar cysts can be divided into arachnoid cysts, Rathke’s cleft cysts, epidermoid cysts and pars intermedia cysts. These cysts have no free communication with the surrounding subarachnoid space. The appearance of rim like enhancement of the cyst wall is typical of Rathke’s cleft cysts and craniopharyngiomas, moreover, these entities may show calcifications. Solid, heterogeneous contrast-enhancing parts within the cyst and elevated protein concentration in the fluid are classically characteristics of craniopharyngioma, showing slightly higher intensity than CSF and isointense with CSF on T1 and T2-weighted sequences, respectively5,7.
Surgical treatment options of IAC are indicated when appears pituitary dysfunction, visual pathway affectation and symptoms of intracranial hypertension. Generally, the elective surgical technique is endoscopic endonasal transsphenoidal approach. Surgical morbidity for those cysts seems to be higher than that for pituitary adenomas; being CSF fistulae the most frequent complication.
Despite being a very exceptional entity, intrasellar arachnoid cyst should be taken in consideration in the differential diagnosis of sellar cystic lesions. Surgical treatment may be necessary in cases of hormonal alteration, visual affectation or symptomatic intracranial hypertension.