====== Posterior fossa arachnoid cyst ======

Posterior fossa [[intracranial arachnoid cyst]] or [[infratentorial]] [[arachnoid cyst]] are classified into supracerebellar arachnoid cyst, infracerebellar, hemispheric, clivus, 

===== Cerebellopontine angle arachnoid cyst =====

[[Cerebellopontine angle arachnoid cyst]].

===== Cisterna magna arachnoid cyst =====

[[Cisterna magna arachnoid cyst]].

===== Fourth ventricle arachnoid cyst =====

[[Fourth ventricle arachnoid cyst]].

===== Clinical features =====
[[Supratentorial arachnoid cyst]]s are usually [[asymptomatic]] and may be discovered by chance at [[autopsy]]; however, [[infratentorial arachnoid cyst]]s, which correspond to liquid forms enclosed by an [[arachnoid sheet]] but whose [[pathogenesis]] is unknown, might cause symptoms.


===== Differential diagnosis =====
[[Posterior fossa neuroglial cyst]].

===== Treatment =====
[[Endoscopic Neurosurgery]] is currently recommended as the first choice to treat [[posterior fossa arachnoid cyst]]s. It has proven to be effective, providing improved [[outcome]], and safe, having a low [[complication]] rate. Compared to [[craniotomy]] and [[shunt]] placement, it has lower surgical [[morbidity]], minimizing or avoiding risks of [[subdural fluid collection]]s, [[shunt infection]], [[malfunction]], [[overdrainage]], and dependence. Usually, rigid scopes maneuvered through a suboccipital approach are used. When symptomatic obstructive hydrocephalus develops, CSF diversion is the first aim of surgery.

In these patients, a flexible [[endoscope]] introduced through a frontal burr hole allows not only immediate and efficient management of [[hydrocephalus]] with [[endoscopic third ventriculostomy]], but in selected cases also direct cyst inspection and fenestration. Navigation of an enlarged cerebral aqueduct is actually safe when performed by experienced neurosurgeons.

This minimally invasive technique gives the possibility of performing both endoscopic third-ventriculostomy and cyst fenestration, which alone may not be enough to efficiently treat hydrocephalus
((Feletti A, Alicandri-Ciufelli M, Pavesi G. Transaqueductal trans-Magendie
fenestration of arachnoid cyst in the posterior fossa. Acta Neurochir (Wien).
2016 Apr;158(4):655-62. doi: 10.1007/s00701-016-2734-3. Epub 2016 Feb 17. PubMed 
PMID: 26883551.)).

====Case reports====
Verghese et al. report an [[Posterior fossa arachnoid cyst]] that caused [[Painful tic convulsif]] (PTC) in a 50-year-old woman. Her radiological evaluation revealed a median, well-circumscribed, cystic lesion of the posterior fossa suggestive of arachnoid cyst, pushing the cerebellum and brainstem anteriorly. Midline suboccipital craniotomy and marsupialization of cyst was performed with complete recovery of symptoms. This is the first report of a retrocerebellar arachnoid cyst causing PTC
((Verghese J, Mahore A, Goel A. Arachnoid cyst associated with painful tic
convulsif. J Clin Neurosci. 2012 May;19(5):763-4. doi:
10.1016/j.jocn.2011.07.039. Epub 2012 Feb 8. PubMed PMID: 22321360.
)).
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{{::posterior_fossa_neuroglial_cyst11361mrisagital.jpg?400|}}

A lesion of 41 x 46 x 52 mm centered in the left [[cerebellar hemisphere]], with thin walls, homogeneous [[cyst]]ic interior signal, absence of a solid [[mass]], [[nodule]] or [[enhancement]] after [[contrast]] administration, no [[restricted diffusion]] or signal alterations in the adjacent [[parenchyma]]. The lesion exerts [[mass effect]] on the [[IV ventricle]] with [[ventriculomegaly]] and a [[periventricular]] signal alteration suggesting hydrocephalus and [[transependymal edema]]. There is descent of the [[cerebellar tonsil]]s below the [[foramen magnum]] about 15 mm, decrease of the [[subarachnoid space]] and ectasia of the nerve sheath due to [[intracranial hypertension]]. Collapsed [[cisterna ambiens]] due to [[transtentorial herniation]]. 
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In [[prone position]]. Head fixed to [[Mayfield]].
Left paramedian incision with craniotomy from under the transverse sinus to [[Foramen Magnum]].
Linear [[durotomy]] respecting cyst walls. Extraction of 2cc of [[CSF]] prior to opening.
A capsule fragment is removed from the cyst for AP analysis. Opening of the cyst whose content is similar to CSF ​​(clear-transparent). Inspection of the interior of the cyst with an endoscope without visualizing suspicious tumor nodules. The cyst is communicated with the [[cisterna magna]] with decompression of the cerebellum and cerebellar tonsil that appears descending below the foramen magnum.
[[Tachosil]] reinforced the [[Water-tight dural closure]]. Bone flap replacement with trephine plate.