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KEY POINTS
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Key Points

  • Arachnoid cysts are rare central nervous system malformations that represent 1% of neonatal intracranial masses.

  • Arachnoid cysts are diagnosed prenatally as a simple, echolucent area within the fetal head and in which no communication with the ventricular system is seen.

  • The main differential diagnosis for a posterior fossa arachnoid cyst is between mega cisterna magna and Dandy–Walker malformation.

  • Most arachnoid cysts remain stable antenatally, but some may cause hydrocephalus by their mass effect as the pregnancy progresses.

  • Management of pregnancy is generally not altered with an arachnoid cyst, unless significant hydrocephalus is present. Careful head imaging in the neonatal period is required to confirm the diagnosis and exclude associated abnormalities.

  • Prognosis depends on the presence of other malformations, parenchymal hemorrhage, rate of cyst growth, and progession of ventriculomegaly.

  • Pediatric management is usually either expectant, for asymptomatic cysts, or open or endoscopic fenestration or obliteration of the cyst if symptomatic.

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CONDITION
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Arachnoid cysts represent collections of cerebrospinal fluid enclosed within layers of pia arachnoid, that do not communicate with the intracranial ventricular system (Osborn and Preece, 2006). They are lined by arachnoidal cells and collagen. Arachnoid cysts are unilocular, round, oval, or crescentlike in shape. They initially communicate with the subarachnoid space and have the potential to grow due to this continued communication. Fluid accumulates as a result of a ball-valve mechanism (Diakoumakis et al., 1986). Choroid plexus-like tissue can be present within the cyst wall. This ectopic tissue secretes cerebrospinal fluid, resulting in progressive distention of the cyst (Diakoumakis et al., 1986). These fluid-filled masses must be distinguished on prenatal sonographic evaluation from the fourth ventricle and vallecula. The cerebellar vermis, hemispheres, and brainstem are usually normal in this condition except when the cyst compresses these structures (Altman et al., 1992).

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Two types of arachnoid cysts exist: the congenital type, which is considered to be the result of maldevelopment of the leptomeninges, and the acquired type, which is the result of hemorrhage, trauma, or infection (Meizner et al., 1988).

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Two-thirds of prenatally detected cases are supratentorial in location, whereas one-third are located within the posterior fossa (Estroff et al., 1995). In contrast, most postnatally detected cases are located within the posterior fossa (Hogge et al., 1995). Five percent of supratentorial interhemispheric arachnoid cysts are associated with agenesis of the corpus callosum (Lena et al., 1995).

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Structural features of the arachnoid cyst wall that distinguish it from the normal arachnoid membrane include

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  1. splitting of the arachnoid membrane at the margin of the cyst;

  2. a very thick layer of collagen present in the cyst wall;

  3. absence of the traversing trabecular processes within the cyst;

  4. presence of hyperplastic arachnoid cells located within the cyst wall that presumably participate in collagen synthesis (Rengachary and Watanabe, 1981).

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