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  • Updated 10.16.2023
  • Released 03.22.2006
  • Expires For CME 10.16.2026




In this article, the author defines a congenital malformation of agenesis of the vermis with fusion of, rather than subarachnoid space between, the cerebellar hemispheres. The dentate nuclei also are fused. This dysgenesis may be isolated or associated with other cerebral malformations, including holoprosencephaly and Chiari malformations, aqueductal stenosis, agenesis of the corpus callosum, and chromosomopathies. Clinical, imaging, and neuropathological features are defined, though the clinical picture is variable. The diagnosis may be established prenatally by fetal neuroimaging. It does not cause obstructive hydrocephalus unless associated with Chiari malformations, aqueductal stenosis, or X-linked hydrocephalus; no treatment is available or required unless obstructive hydrocephalus is a complication. In rare cases, the diagnosis may be delayed until later childhood or adult life.

Key points

• This malformation is characterized by congenital absence of the vermis with fusion of the medial sides of the cerebellar hemispheres and of the dentate nuclei, unlike subarachnoid space in the cerebellar midline in Joubert syndrome and Dandy-Walker malformation.

• The malformation may be isolated or associated with other malformations, particularly dorsal midline defects such as noncleavage of the tectal plate of the midbrain, septo-optic dysplasia, holoprosencephaly, aqueductal stenosis, Chiari malformations, and occasionally, agenesis of the corpus callosum.

• The syndrome is described in various chromosomopathies and other genetic defects but also occurs without an evident genetic etiology.

• Isolated rhombencephalosynapsis rarely causes obstructive hydrocephalus or requires treatment but is frequently associated with aqueductal stenosis, including X-linked hydrocephalus and, occasionally, Chiari malformations.

• The diagnosis can be established prenatally by fetal ultrasound or fetal MRI and postnatally by MRI.

Historical note and terminology

Absence of the vermis with fusion of the cerebellar hemispheres was first described in 1914 by Obersteiner from an autopsy of a 28-year-old man who committed suicide (78). The term “rhombencephalosynapsis” was coined in 1959 by Gross and Gross and Hoff (37; 37). The association of some cases with septo-optic-pituitary dysplasia also was first demonstrated by Gross and Hoff and has been confirmed neuropathologically by several other authors (37; 73; 94; 101; 04).

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