Specific Imaging Findings

The molar tooth sign on axial brain MRI or CT images is pathognomonic of Joubert syndrome (JS). This sign results from the combination of marked hypoplasia or aplasia of the cerebellar vermis, a narrow isthmus (the area of the midbrain caudal to the inferior colliculi and immediately above the pons), a triangular shaped or “umbrella-shaped” mid-fourth ventricle and a “batwing” appearance of the superior fourth ventricle. The key finding are thickened, elongated and horizontally oriented superior cerebellar peduncles (SCP) with a very deep interpeduncular fossa. In JS patients older than 30 months the absence of SCP decussation can be identified on a mid-sagittal T1WI as a lack of the rounded low signal-intensity area at the level of the pontomesencephalic junction, representing the SCP decussation. Diffusion-tensor MR imaging and fiber tractography have been employed to demonstrate the abnormal morphology and aberrant course of the SCP and corticospinal tract. Associated abnormalities may include migration disorders (such as heterotopia), callosal dysgenesis, ventriculomegaly, and cephaloceles.

Pertinent Clinical Information

JS is a rare, autosomal recessive disorder with a distinctive hindbrain malformation. The molar tooth sign was first described in JS; however, it is present in numerous other rare syndromes that are partially overlapping with JS and characterized by abnormalities outside of the CNS in addition to the brainstem and cerebellar findings [e.g. Joubert–polymicrogyria syndrome, Dekaban–Arima syndrome, COACH (Cerebellar vermis hypoplasia, Oligophrenia, Ataxia, Coloboma, and Hepatic fibrosis) syndrome, Varadi–Papp syndrome, Senior–Löchen syndrome].