Harvey B Sarnat MD FRCPC MS (

Dr. Sarnat of the University of Calgary has no relevant financial relationships to disclose.

Originally released March 22, 2006; last updated September 26, 2020; expires September 26, 2023


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, 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.

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 as 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 a variety of chromosomopathies and other genetic defects but also occurs without an evident genetic etiology.


• Isolated rhombencephalosynapsis does not cause obstructive hydrocephalus or require 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.

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 (Obersteiner 1914). The term “rhombencephalosynapsis” was coined in 1959 by Gross and Gross and Hoff (Gross 1959; Gross and Hoff 1959). 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 (Gross and Hoff 1959; Michaud et al 1982; Roessmann et al 1987; Sarnat 1992; Adle-Biassette et al 2013).

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