Molar tooth sign in fetal brain magnetic resonance imaging leading to the prenatal diagnosis of Joubert syndrome and related disorders.

Joubert syndrome is a rare autosomal recessive disorder characterized by ataxia, developmental delay, and oculomotor and respiratory abnormalities in relation to cerebellar vermian and midbrain dysgenesis. The midbrain dysgenesis is responsible for the molar tooth sign on axial magnetic resonance imaging (MRI). This classic hallmark of Joubert syndrome has been identified in other disorders sharing overlapping clinical and radiologic features with Joubert syndrome. Recent identification of two different genes points to genetic heterogeneity in this group of disorders, now entitled Joubert syndrome and related disorders, making a genetic prenatal diagnosis not readily available. In addition, fetal ultrasonography lacks sensitivity in regard to posterior fossa malformation. Fetal MRI is now acknowledged as the method of choice to delineate posterior fossa malformation in a fetus. The identification of a molar tooth sign has, however, rarely been documented by a fetal brain MRI. We report a case of Joubert syndrome diagnosed prenatally using fetal MRI. We also discuss the etiology of Joubert syndrome in view of the recent genetic advances and murine models of cerebellar dysgenesis.