Analysis and classification of cerebellar malformations

Abstract

Background and purpose: Because of improved visualization of posterior fossa structures with MR imaging, cerebellar malformations are recognized with increasing frequency. Herein we attempt to describe and propose a rational classification of cerebellar malformations.

Methods: MR images obtained in 70 patients with cerebellar malformations were retrospectively reviewed. The cerebellar malformations were initially divided into those with hypoplasia and those with dysplasia. They were then divided into focal and diffuse malformations. Finally, they were separated according to other features, such as brain stem involvement and cerebral involvement.

Results: All patients with diffuse cerebellar dysplasia (muscular dystrophy [n = 10], cytomegalovirus [n = 6], lissencephaly [n = 3],) had abnormalities of the cerebrum. Patients with focal cerebellar dysplasia of the Joubert (n = 12) and rhombencephalosynapsis (n = 8) types had variable cerebral dysplasia. Patients with nonsyndromic focal cerebellar dysplasia (isolated focal cerebellar cortical dysplasia [n = 2], cerebellar heterotopia with cerebellar cortical dysplasia [n = 1], idiopathic diffuse cerebellar dysplasia [n = 1], Lhermitte-Duclos syndrome [n = 1]) and those with cerebellar hypoplasia (isolated cerebellar hypoplasia [n = 6], pontocerebellar hypoplasia type 1 [n = 1]) had normal cerebra. Patients with features of Dandy-Walker malformation (n = 19) had both hypoplasia and dysplasia of the cerebellum. No notable difference was found between the cerebella of patients with large fourth ventricle cysts (Dandy-Walker malformations) and those without large fourth ventricle cysts (isolated cerebellar hypoplasia). Therefore, the Dandy-Walker malformation seems to be heterogeneous.

Conclusion: Use of this classification system helps in the segregation and understanding of the relationship among cerebellar malformations. Although it will undoubtedly require revisions, this classification is a first step in combining imaging with molecular biology to facilitate understanding of cerebellar development and maldevelopment.

Fig 1.

Dandy-Walker malformation with cerebellar dysplasia in a 4-month-old infant. A, Coronal fast spin-echo (3000/102 [TR/TE]) image shows that the vermis is hypoplastic and the posterior fossa large, with the cerebellar hemispheres widely separated. The cerebellar hemispheres have an abnormal folial pattern, compatible with dysplasia. The cerebral hemispheres are abnormal with marked thinned white matter, enlarged ventricles, and shallow sulci. B, Axial fast spin-echo (3500/112) image shows, in addition to the large posterior fossa fluid collection and dysplastic cerebellar hemispheres, an abnormality of the pons, which is small and has a ventral cleft.

Fig 2.

Dandy-Walker malformation with cerebellar dysplasia in a 6-day-old neonate. A, Sagittal spin-echo (550/16) image shows large posterior fossa CSF collection and dysplastic appearing cerebellar vermis. B, Axial spin-echo (3000/120) image shows abnormal folial pattern of cerebellar hemispheres.

Fig 3.

Cerebellar hypoplasia in a 33-year-old man. Sagittal spin-echo (500/11) image shows a profoundly small cerebellum in a fluid-filled, but normal-sized, posterior fossa. The pons and medulla are abnormally small.

Fig 4.

Unilateral cerebellar hypoplasia in 29-year-old woman. Axial spin-echo (2500/30) image shows a small right cerebellar hemisphere. The left hemisphere and vermis appear normal.

Fig 5.

Lissencephaly and cerebellar hypoplasia in a 4-day-old neonate. A, Sagittal spin-echo (550/11) image shows complete cerebral lissencephaly and a very small cerebellum. The brain stem is abnormally thin. The colliculi are fused. B, Axial spin-echo (550/16) image shows the very small cerebellar hemispheres and the small pons with a central cleft, presumably due to absence of the crossing ventral pontine axons.

Fig 6.

Cerebellar dysplasia in a 10-month-old patient with congential muscular dystrophy. A, Axial spin-echo (3000/120) image shows abnormal folial pattern and small cysts in the cerebellar hemispheres. B, Coronal gradient-echo (35/7) (theta of 45 degrees) image shows that the cysts are primarily in the superior aspects of the cerebellar hemispheres. Note the cobblestone cortex of the cerebrum.

Fig 7.

Diffuse cerebellar dysplasia in a neonate with Walker-Warburg syndrome. A, Axial spin-echo (3000/120) image shows small cerebellar hemispheres with dysplastic folial pattern. B, Coronal spin-echo (600/16) image shows marked ventriculomegaly and cobblestone cortex.

Fig 8.

Cerebellar dysplasia in a neonate with congenital cytomegalovirus infection. Coronal spin-echo (600/16) image shows markedly small cerebellar hemispheres with almost no foliation. The hyperintensity of the right hemisphere was the result of calcification. Note the lissencephaly of the cerebrum with large calcifications.

Fig 9.

Cerebellar cortical dysplasia with subcortical heterotopia in a 3-month-old patient. A, Axial spin-echo (3000/120) image shows abnormal cerebellar cortical folial pattern and the presence of nodules of gray matter intensity (arrows) in the cerebellar white matter. B, Coronal spin-echo (600/16) image shows that the nodule in the cerebellar white matter (arrows) remains isointense to gray matter on this T1-weighted image. The abnormal cerebellar folial pattern is again seen. In addition, the abnormality of the left cerebral hemisphere, which appears to represent polymicrogyria, is seen on this image.

Fig 10.

Rhombencephalosynapsis in a 6-year-old boy. A, Sagittal spin-echo (550/15) image shows abnormal cerebellar vermis. B, Axial spin-echo (2500/80) image shows continuity of the cerebral hemispheres across the midline without a midline cerebellar vermis. The cerebral hemispheres are abnormal, with reduced white matter and inward folding of the cortex as a result of ventriculoperitoneal shunting.

Fig 11.

Molar tooth malformation in a 16-month-old patient with Joubert syndrome. A, Sagittal spin-echo (550/11) image shows a small, dysplastic cerebellar vermis. The folial pattern is abnormal. Note the narrow isthmus (junction of the mesencephalon and pons) (arrow). B, Axial spin-echo (550/15) image shows the very small vermis (small arrow), the broad, horizontal superior cerebellar peduncles, and the narrow isthmus (large arrow). C, Coronal spin-echo (600/11) image shows a midline cleft separating the two dysplastic areas of vermis (arrows).

Fig 12.

Focal cerebellar dysplasia in a 20-month-old female patient. Coronal view fast spin-echo (3500/112) image shows a focal region of abnormal foliation (arrows) in the inferomedial portion of the left cerebellar hemisphere.

Fig 13.

Drawings illustrate normal cerebellar development. During the 5th gestational week, cellular proliferation within the alar plates in conjunction with formation of the pontine flexure forms the rhombic lips. The neuroepithelial zones, in the roof of the fourth ventricle and the rhombic lips, are the locations of the germinal matrices where the cells of the cerebellum and many brain stem nuclei will form. Between 9 and 13 postconceptional weeks, the Purkinje cells of the cerebellar cortex and the neurons of the deep cerebellar nuclei migrate radially outward from this germinal matrix. In contrast, the neurons of the granular layer of the cerebellar cortex migrate tangentially from the germinal zone of the rhombic lips, over the cerebellar surface to form a transient external (Ext.) granular layer, which acts as a secondary germinal matrix. The external granular layer forms between the 10th and 11th postconceptional weeks and persists until approximately 15 months postnatal. The cells in the external granular layer proliferate, and the granule cell neuroblasts begin to migrate inward between clusters of homophilic Purkinje cells with the presumed aid of radial glial (Bergman) fibers, forming the internal (Int.) granular layer (reprinted with permission from Lippincott Williams & Wilkins [27]).