The molar tooth sign refers to the appearance of the midbrain in an axial section in which the elongated superior cerebellar peduncles give the midbrain an appearance reminiscent of a molar or wisdom tooth.
It was initially described in Joubert syndrome and related disorders (JSRD) 2 but is now recognized to occur in a number of other conditions, e.g. nephronophthisis, hepatic fibrosis, Cogan's syndrome, and pontine tegmental cap dysplasia 4.
Molar tooth appearance results from a lack of normal decussation of superior cerebellar peduncular fiber tracts which in turn leads to enlargement of the peduncles, which also follow a more horizontal course 3.
The absence of crossing fibers also leads to a reduction in the anteroposterior diameter of the midbrain and deepening of the interpeduncular cistern 3.