Retrocerebellar space enlargement: Mega Cisterna Magna and Arachnoid Cyst

The cisterna magna (CM, aka cerebellomedullary cistern) is the largest opening in the extracerebral space, positioned between the arachnoid and the pia mater meninges. The CM is located between the postero-inferior margin of the cerebellum and the dorsal rim of the medulla oblongata, up to the posterior rim of the foramen magnum (1). The cerebrospinal fluid drains from the 4th ventricle into the CM through the foramina of Lushka and Magendie and its normal dimensions in the axial transcerebellar plane ranges between 3 mm and 8 mm. Enlargement of the cisterna magna, known as mega cisterna magna (MCM) has been defined as a CM >10 mm, in the absence of an open communication with the fourth ventricle and with a normal cerebellum and vermis (biometric and morphologic) (1). The dilatation of the CM is symmetric and does not compress the cerebellar structures or exert a mass effect. Retrocerebellar arachnoid cysts (RCAC) are cystic structures enclosed between the pia and arachnoid layers of the meninges, filled with anechoic fluid (1). With this anomaly no communication with the fourth ventricle or the subarachnoid space is observed and their placement may be asymmetric (1). A mass effect on the cerebellum and vermis is often observed.   

No specific ICD code for MCM is provided.
Q04.8 - Other specified congenital anomaly of the brain
Q04.6 Congenital Cerebral Cysts (arachnoid) 
 

HP:0002280 Enlarged cisterna magna 
HP:0100702 Arachnoid Cyst 

MCM
The exact incidence of MCM in the general population is unknown, with some authors reporting it up to 0.4%, without gender predilection (2). It is often found as an incidental asymptomatic finding during an exam performed for an unrelated indication. The fetal incidence is probably higher as some cases have spontaneous resolution and third trimester cases are often not diagnosed.  

RCAC
A recent publication reported an overall incidence of fetal arachnoid cysts as 0.2% (3). The rate of RCAC differs among authors, being only 3% from all arachnoid cysts in the fetus (3) up to 40% (4). There is also higher frequency of arachnoid retrocerebellar cysts reported in males by some authors, but the reason for the sex difference remains unclear. 
 

The cisterna magna embryologically originates from the permeabilization of the Blake´s pouch, which allows CSF to flow from the 4th ventricle to the subarachnoid space. It is completely formed at the end of the 7th gestational week (5). Anatomically, it occupies the vallecula cerebelli (a depression between the inferior surface of the vermis and the medial aspect of the tonsils) and extends inferiorly to the first cervical space and anteriorly to the foramen of Magendie. Posteriorly, it is limited by the arachnoid membrane, which extends from the upper cervical cord to the posteroinferior surface of the cerebellum at the level of the vermian pyramid. This arachnoid membrane separates the cisterna magna from the supra-vermian cistern. The posterior part of the cisterna magna is divided by the falx cerebelli. The arachnoid space is created by the expansion of the extracellular space of loose primitive mesenchyme, which surrounds the neural tube early in fetal life (6). The outer layer of cells form the arachnoid web and the inner layer the pia mater. The intervening space is filled by fine trabecule (7). Abnormal splitting of this layers and entrapment of cerebrospinal fluid causes retrocerebellar arachnoid cysts to develop.

Most cases of MCM are isolated and considered to appear due to a partial permeabilization of the Blake’s pouch and a mild dilatation of the CM without elevation and rotation of the vermis. It is a developmental finding without a clear explanation of the etiology. Nevertheless, MCM is often part of genetic syndromes and can appear as a no-isolated sonographic finding in the fetus. It can be part of classic aneuploidies as trisomy 13,18 and monosomy X0 or 20 other syndromes (chromosomal and monogenic) as listed by the OMIM database (8).  

Postnatally, arachnoid cysts can originate from trauma, infection and hemorrhage and usually communicate with the arachnoid space (9). In the fetus the mechanisms are not well defined, although there are different theories to explain the pathogenesis of arachnoid cysts in different locations in the brain. Retrocerebellar arachnoid cysts, could arise as a result of developmental variation of the meninx primitive that surrounds the neural tube during the differentiation of the mesenchyme, or, alternatively, from abnormal development of the inferior membranous area with persistence of the Blake pouch and the formation of a closed ependymo-arachnoid cyst that does not communicate with the ventricular system or the subarachnoid space (10). 
 

MCM has been associated with infarction, inflammation and infection, particularly cytomegalovirus, as well as with genetic syndromes, particularly trisomy 18. It is critical to exclude ventriculomegaly and cerebellar hypoplasia when diagnosing mega cisterna magna. However, when indeed isolated, MCM is a benign finding without clinical significance (11). 
In RCAC, hydrocephalus has been reported in some cases and may be communicating or non- communicating (11). RCAC may cause compression of the cerebellum due to mass effect. They rarely present with other anomalies besides ventriculomegaly but may present as part of genetic syndromes such as 15q13.3 deletion syndrome or absence of tibia, polydactyly arachnoid cyst syndrome. Malformations of cortical development in the peri-cystic regions were also reported, mainly polymicrogyria (4,6). 

When antenatally detected a thorough fetal anatomical survey, including neurosonography is indicated. Genetic evaluation is advised in non-solated cases. MRI should be offered according to the specific case and the level of fetal neurosonography available (4).  
 

Depends on the cause. In isolated cases it is considered sporadic. 

Enlarged retrocerebellar spaces are a common indication for prenatal evaluation and counselling. These entities are best considered in three major categories: “open” fourth ventricle, enlargement of the cisterna magna (mega cisterna magna), and retrocerebellar arachnoid cyst (1). An open fourth ventricle after 18 gestational weeks suggests a differential diagnosis including Blake’s pouch cyst, Dandy Walker Malformation and Vermian hypoplasia, which are not the scope of this chapter. MCM is suspected in the presence of a cisterna magna >10mm in the axial transcerebellar plane, with symmetric dilatation, normal cerebellar biometry, no mass effects over the cerebellar surface and noted in the absence of an open 4th ventricle. 
RCAC are well-delineated cystic structures with anechoic content, displaying smooth walls and are nearly always unilocular. They do not communicate with the surrounding subarachnoid space or with the fourth ventricle (9,11); Frequently, but not always, RCAC compress the cerebellum and / or vermis, altering configuration of the surface in contact with the cyst. Rarely the cyst leads to rotation and elevation of the vermis simulating a Blake’s Pouch Cyst. Sometimes the compression is severe impeding the normal drainage of CSF and leading to hydrocephalus or displacement of the cerebellum and brainstem significantly. In contrast to MCM, the RCAC are much more dynamic and they may either grow, shrink or disappear altogether throughout pregnancy.

Many publications still group the anomalies with open 4th ventricle (Blakes Pouch Cyst, Dandy Walker Malformation and Vermian hypoplasia) in the differential diagnosis of MCM although morphologically they should not, as once an open 4th ventricle is identified, the diagnosis of MCM is excluded. The main differential diagnoses are MCM, RCAR, other rare posterior fossa cysts such as epidermoid cyst and pilocytic astrocytoma (the most common pediatric brain tumour) (11).

In MCM prognosis depends on the presence of additional findings or a genetic disorder, and will be related to the underlying cause in non-isolated cases. A recent meta analysis including 144 fetuses with MCM described 30% of associated CNS and non-CNS anomalies in fetuses without chromosomal anomalies. Whole Exome Sequencing data was not available for these patients, however. In isolated cases, cognitive functioning is within normal limits, according to postnatal references. A small case series found that these patients score lower on specific parameters such as memory, executive functioning and verbal fluency (2). Other studies also described psychiatric associations with isolated MCM, including bipolar disorder, schizophrenia and catatonia (12,13). 

In isolated arachnoid cyst, the prognosis is reportedly good, although further studies are needed to enable better counselling. The prognosis of a fetus with arachnoid cyst depends on the brain integrity rather than on the volume or location of the cyst, although cysts bigger than 2 cm have a higher tendency to grow and require neurosurgical interventions. The presence of normal brain morphology, slow growth of the cyst and absence of ventriculomegaly are favourable variables (14). Apart from hydrocephalus, macrocephaly, seizures, headache and motor deficit or neurodevelopmental delay in childhood were reported in association with RCAC (15). 

An ultrasound finding suggestive of MCM requires full morphological fetal assessment, including follow-up in the third trimester. In up to 15% of cases the condition may be falsely assumed to be isolatated and therefore invasive genetic diagnosis is prudent.  MCM itself requires no treatment or drainage. Associated findings will receive treatment accordingly.
Most RCAC need no neurosurgical intervention. Decompression surgery may be required to prevent long term sequelae (6). The best available treatment for arachnoid cysts remains controversial; surgery has been found to be beneficial in symptomatic cases. Endoscopic fenestration is considered the first-line surgical option and it may be followed by shunting, if necessary (16). Shunts may be preferred in very young children where there is associated hydrocephalus or macrocephaly. 

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