8 month old child underwent cranial MRI for delayed developmental milestone, hypotonia, episodes of abnormal breathing and abnormal eye movement.
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MRI study revealed vermian agenesis which resulted in two cerebellar hemispheres in contact at the midline. Axial T1 and T2 images showed dysplasia of the superior cerebellar peduncles that resulted in a molar tooth configuration. The more caudal T2-weighted transverse MR image showed the fourth ventricle shaped like opened bat wing. Furthermore, severe hypoplasia involving the inferior vermis resulted in an open communication between the fourth ventricle and the extracerebellar subarachnoid space but without evidence of a posterior fossa cyst. Size of the posterior fossa appears normal.
Joubert syndrome is an autosomal recessive disorder with characteristic pathologic findings of cerebellar vermian hypoplasia with a midline cleft. In 1969, a French neurologist, Marie Joubert, reported a series of cases of children with mental retardation, episodes of abnormally deep and rapid breathing, abnormal eye movements, and ataxia, which were associated with agenesis of the cerebellar vermis . The name Joubert syndrome was given several years later when an additional set children with similar findings was reported. The primary imaging features of Joubert syndrome are absence of the vermis, thickening and reorientation of the superior cerebellar peduncles, and fourth ventricle deformity. The isthmus( part of the brainstem between pons and inferior colliculus) , is seen as elongation and thinning of ponto-mesencephalic junction (due its dysgenesis ).There is also thickening of superior cerebellar peduncles (due to absence of decussating superior cerebellar peduncular axons) and deep interpeduncular cistern (due to hypoplasia of vermis ). The combination of these three finding result in the characteristic “molar tooth sign” in patients with Joubert syndrome. The absence of a normal vermis creates a midline cleft between the two normal-appearing cerebellar hemispheres, which results in a characteristic “bat wing” appearance of the fourth ventricle transverse images.