Joubert syndrome is an autosomal recessive disorder characterized by cerebellar vermian hypoplasia with a midline cleft and lack of decussation of the superior cerebellar peduncles, the latter causing an enlargement of the peduncles. Other decussation defects of the central pontine and corticospinal tracts as well as dysplasia of the olivary nuclei may also be present. Associated renal, retinal, and hepatic abnormalities are often seen.
The molar tooth sign refers to the characteristic appearance of the midbrain seen on axial imaging (CT or MR) in patients with Joubert syndrome. Thickened superior cerebellar peduncles surrounding an elongated fourth ventricle give this appearance. A vermian cleft may also be recognized.
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