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).

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).

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).

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

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).

Electron micrographic studies of arachnoid cysts reveal a striking and nearly invariable association of the arachnoid cyst with the normal subarachnoid cistern. Some investigators have demonstrated that in their early stages, arachnoid cysts communicate with the subarachnoid space, revealing the exclusively intra-arachnoid nature of the cyst (Rengachary and Watanabe, 1981). Other investigators, however, postulate that the retrocerebellar arachnoid cyst may represent a persistent diverticulum of the fourth ventricle that fails to involute, the so-called Blake pouch (Altman et al., 1992). The precise cause for the development of arachnoid cyst is unknown. The factors responsible for the formation of the normal subarachnoid cistern, however, also produce splitting of the arachnoid membrane in that location. This results in a diverticulum or enclosed cystic space. Thus, arachnoid cysts are thought to be a developmental anomaly of the subarachnoid cistern (Rengachary and Watanabe, 1981).

The incidence of arachnoid cysts is unknown. They are rare in prenatal life. Arachnoid cysts represent 1% of all space-occupying masses in childhood (Estroff et al., 1995) and are an incidental finding in 0.5% of autopsy studies (Rafferty et al., 1998). Arachnoid cysts are more common in males than in females (Kollias et al., 1993; Lena et al., 1995). The left side of the brain is more commonly affected than the right.

Sonographic diagnosis of arachnoid cyst relies on the finding of a sonolucent mass with smooth and thin walls within the brain (Figure 8-1). This cyst does not communicate with the lateral ventricles, but may be associated with hydrocephalus (Figure 8-2) due to a mass effect that obstructs the flow of cerebrospinal fluid. In this condition, the cerebellar vermis is normal in size. The cyst is usually completely echolucent, with the same signal intensity as CSF (Figure 8-2). Occasionally, hemorrhage into an arachnoid cyst may obscure its simple appearance (Osborn and Preece, 2006).

In 1986, Diakoumakis et al. described a fetal suprasellar arachnoid cyst that was detected on a routine antenatal sonographic examination at 32 weeks of gestation, and resulted in development of hydrocephalus. Postnatal cranial computed tomographic (CT) examination revealed a characteristic “head of the bunny” appearance with a midline cyst and two dilated lateral ventricles (Diakoumakis et al., 1986). In another case report, a fetus was described at 22 weeks of gestation with an isolated extraventricular supratentorial arachnoid cyst, which appeared as a rounded, fluid-filled cyst in the right parieto-occipital lobe of the brain (Meizner et al., 1988). There was splaying of the right lateral ventricle due to a pressure effect, and the cyst enlarged progressively over the duration of pregnancy (Meizner et al., 1988).

Most cases of arachnoid cysts are detected at >20 weeks of gestation (Hogge et al., 1995; Rafferty et al., 1998). Only rarely are extracranial abnormalities detected. In one case report, a 1-by 1-cm posterior fossa cyst was detected at 18 weeks of gestation. The pregnancy was aborted because of associated chromosomal abnormalities (Hogge et al., 1995).

Estroff et al. (1995) described two cases of arachnoid cysts that, on the basis of prenatal sonographic findings, were suspected antenatally to be a Dandy–Walker malformation. These authors noted that sonographic distinction between the retrocerebellar arachnoid cysts and the Dandy–Walker cyst may be difficult. Both malformations may be associated with hydrocephalus. In arachnoid cysts, the underlying cerebellar hemispheres and vermis are normal, although they are displaced and compressed anteriorly. Typically, no extracranial abnormalities are present in arachnoid cysts (Estroff et al., 1995). At least 27 cases of fetal arachnoid cysts have been described in the literature (reviewed in Chen, 2007). Of these, 9 had prenatal MR imaging in addition to sonography. In a few cases, the diagnosis was made in the second trimester, but the majority were third trimester diagnoses. In one report, using transvaginal sonography, a posterior fossa arachnoid cyst was diagnosed at 13 weeks of gestation (Bretelle et al., 2002).

The differential diagnosis for arachnoid cysts is given in Table 8-1, and antenatal magnetic resonance imaging (MRI) may be needed for accurate evaluation (Golash et al., 2001). Mega cisterna magna can be confused with arachnoid cyst but, there is no mass effect on the cerebellar hemispheres nor hydrocephalus in the setting of mega cisterna magna (Estroff et al., 1995). Arachnoid cysts in the midline that lie wholly posterior to the cerebellum may be difficult to distinguish from the mega cisterna magna (Altman et al., 1992).