Vermian agenesis without posterior fossa cyst.

We report 11 cases of vermian partial agenesis without posterior fossa cyst or hemispheric abnormalities. Characteristic MR signs were: absence of the posterior lobe, hypoplasia of the anterior lobe, a narrow sagittal cleft separating the hemispheres ("buttocks sign") and fourth ventricle deformity. The main clinical signs were complex oculomotor dysfunction and developmental delay. None of the patients had respiratory symptoms. Consideration is given to the relationship between Joubert syndrome and this entity as well as to embryological data.