Congenital basis of posterior fossa anomalies

Abstract

The classification of posterior fossa congenital anomalies has been a controversial topic. Advances in genetics and imaging have allowed a better understanding of the embryologic development of these abnormalities. A new classification schema correlates the embryologic, morphologic, and genetic bases of these anomalies in order to better distinguish and describe them. Although they provide a better understanding of the clinical aspects and genetics of these disorders, it is crucial for the radiologist to be able to diagnose the congenital posterior fossa anomalies based on their morphology, since neuroimaging is usually the initial step when these disorders are suspected. We divide the most common posterior fossa congenital anomalies into two groups: 1) hindbrain malformations, including diseases with cerebellar or vermian agenesis, aplasia or hypoplasia and cystic posterior fossa anomalies; and 2) cranial vault malformations. In addition, we will review the embryologic development of the posterior fossa and, from the perspective of embryonic development, will describe the imaging appearance of congenital posterior fossa anomalies. Knowledge of the developmental bases of these malformations facilitates detection of the morphological changes identified on imaging, allowing accurate differentiation and diagnosis of congenital posterior fossa anomalies.

Keywords: Posterior fossa anomalies; congenital posterior fossa abnormalities; development posterior fossa; embryology posterior fossa; imaging posterior fossa.

Figure 1.

(a, b and c) Representation of the differentiation of the ventral encephalic vesicles. The cephalic end of the neural tube demonstrates two constrictions that result in three vesicles. The most caudal vesicle (the rhombencephalon) will develop into the hindbrain. The rhombencephalon is divided into two vesicles (the metencephalon and myelencephalon). (d, e and f) The rhombic flexure causes the dorsal aspect of the neural tube to open, exposing the floor of the 4^th ventricle with its characteristic rhomboidal shape, hence the name rhombencephalon.

Figure 2.

Posterior (a–c) and cross-sectional (d–f) representation of the brainstem showing the sequence of growth of the cerebellum from the rhombic lips (green). These develop along the lateral edges of the rhombencephalon and are the site of development of cerebellar differentiation.

Figure 3.

Cerebellar agenesis. (a) Sagittal T1WI, (b) axial T2WI and (c) coronal T2WI demonstrate absence of the cerebellum and significant hypoplasia of the pons (between arrows). Notice the downward herniation of the occipital lobes (arrowheads).

Figure 4.

Rhombencephalosynapsis. (a) Axial T2, (b) sagittal T1 and (c) coronal FLAIR sequences show absence of the vermis with fusion of the cerebellar hemispheres (asterisk). The sagittal view shows the absence of a cerebellar vermis with a gray–white matter configuration similar to a cerebellar hemisphere. An interhemispheric cyst (arrow head) was present as an associated finding.

Figure 5.

Cerebellar hypoplasia. (a) Sagittal T1WI and (b) axial T2WI show hypoplasia of the left cerebellar hemisphere (arrows), resulting in an enlarged surrounding subarachnoid space. Normal-sized fissures in relation to the folia are seen. A small cerebellar cleft is present in the left cerebellar hemisphere.

Figure 6.

Vermian-cerebellar hypoplasia. (a) Sagittal T1WI and (b) coronal T2WI show a prominent cystic space in the posterior fossa (asterisk) with a small cerebellum (arrows) and pons. The midbrain is elongated. Note associated agenesis of the corpus callosum along with ventricular dilatation.

Figure 7.

Lhermitte–Duclos disease (dysplastic gangliocytoma). (a) Coronal FLAIR and (b) axial T2WI show increased signal of the left cerebellar hemisphere in a “corduroy pattern” with slight enlargement of the left cerebellar hemisphere (arrows). The 4^th ventricle is small, appearing to be compressed by the lesion.

Figure 8.

Walker–Warburg syndrome. (a) Sagittal T1WI, (b) axial T2WI and (c) coronal T2WI show vermian hypoplasia with a prominent cerebrospinal fluid space (asterisk) elevating the torcula which communicates with the 4^th ventricle. Migration anomalies with small white matter cysts in the cerebellum are demonstrated (black arrowheads). Brain type II lissencephaly is also present (white arrowheads). Note the “z”-shaped brainstem with concavity of the floor of the 4^th ventricle, typical of Walker–Warburg syndrome.

Figure 9.

Joubert syndrome. (a) Sagittal T1WI without contrast and (b) axial T2WI show vermian hypoplasia (arrowhead) and thickened superior cerebellar peduncles giving the “molar tooth” appearance (arrows).

Figure 10.

Dandy–Walker malformation. (a) Axial CT and sagittal T2WI images show a posterior fossa cyst (arrows) that communicates with the 4^th ventricle and displaces the cerebellar hemispheres laterally. Hydrocephalus is present (asterisk). The vermis is small and tilted superiorly. The pons is also decreased in size.

Figure 11.

Arachnoid cyst. (a) Sagittal T1WI shows a large cyst surrounding the cerebellum which displaces the corpus callosum superiorly (arrow). (b) Axial T2WI shows anterior displacement and mass effect against the cerebellum by the cyst (asterisk). The cerebellar falx is not seen at the midline (arrowhead).

Figure 12.

Blake's pouch cyst. (a) Sagittal T1 with contrast and (b) axial T2WI show elevation of a normal vermis by a cystic lesion that extends from the 4^th ventricle to the foramen magnum (asterisk); the 4^th ventricle is enlarged. The choroid plexus is visualized along the superior aspect of the cyst (arrowheads).

Figure 13.

Megacisterna magna. (a) Sagittal T1WI and (b) axial T2WI show an abnormally widened cisterna magna in the posterior fossa (arrowheads). There is no compression or mass effect against the cerebellum. Note that the cerebellar falx is seen at the midline (arrows). The vermis and the 4^th ventricle are normal in size.

Figure 14.

Chiari I malformation: (a) Axial T2WI shows crowding at the level of the foramen magnum with herniation of the cerebellar tonsils (arrow). (b)Sagittal T1WI shows caudal herniation of the “peg shaped” cerebellar tonsils (arrow). (c) Sagittal T2WI of a different patient shows associated syringomyelia (arrowheads).

Figure 15.

Chiari II malformation. (a)Sagittal T1WI shows a small posterior fossa with a low torcular insertion (arrow) and tectal beaking (arrowhead). (b) Axial T2WI fails to demonstrate the cerebellum between the occipital lobes. (c) T2WI shows spinal dysraphysm.

Figure 16.

Chiari III malformation. (a)Sagittal T2WI, (b) axial T2WI and (c) coronal T2WI demonstrate an occipital encephalocele (arrows), an elongated appearance of the brainstem and descent of the cerebellar tonsils (arrowheads).

Figure 17.

A classification based on development alone is not possible. We have divided the most common posterior fossa anomalies based on their imaging morphology into two groups: 1) hindbrain malformations, including diseases with cerebellar or vermian agenesis, aplasia or hypoplasia and cystic posterior fossa anomalies; and 2) cranial vault malformations. *not a true cystic lesion.