Anterior Mesencephalic Cap Dysplasia: Novel Brain Stem Malformative Features Associated with Joubert Syndrome.

F Arrigoni,R Romaniello,E.M Valente,C Parazzini,A De Luca,D Peruzzo,R Borgatti,F Triulzi

doi:10.3174/ajnr.a5360

Abstract

In Joubert syndrome, the "molar tooth" sign can be associated with several additional supra- and infratentorial malformations. Here we report on 3 subjects (2 siblings, 8-14 years of age) with Joubert syndrome, showing an abnormal thick bulging of the anterior profile of the mesencephalon causing a complete obliteration of the interpeduncular fossa. DTI revealed that the abnormal tissue consisted of an ectopic white matter tract with a laterolateral transverse orientation. Tractographic reconstructions support the hypothesis of impaired axonal guidance mechanisms responsible for the malformation. The 2 siblings were compound heterozygous for 2 missense variants in the TMEM67 gene, while no mutations in a panel of 120 ciliary genes were detected in the third patient. The name "anterior mesencephalic cap dysplasia," referring to the peculiar aspect of the mesencephalon on sagittal MR imaging, is proposed for this new malformative feature.

Highlights

We report on 3 patients from 2 different families (2 brothers with genetically defined Joubert syndrome (JS) and 1 unrelated girl), presenting an additional complex malformation of the brain stem characterized by an abnormal thick bulging of the anterior profile of the mesencephalon
The presence of an anterior mesencephalic bulging due to an ectopic transverse white matter bundle represents a new malformative pattern of the midbrain for which we propose the name “anterior mesencephalic cap dysplasia” (AMCD)
Careful interpretation of these findings is needed because the algorithm we used for tractography has limitations in resolving crossing or sharply angulated fibers,[18] which are better handled by more sophisticated approaches, such as high angular resolution diffusion imaging (HARDI)

Summary

Introduction

We report on 3 patients from 2 different families (2 brothers with genetically defined JS and 1 unrelated girl), presenting an additional complex malformation of the brain stem characterized by an abnormal thick bulging of the anterior profile of the mesencephalon. ● Color-encoded DTI maps and tractography reconstructions revealed that the anterior mesencephalic bulging corresponded to a transversely orientated white matter tract in the interpeduncular cistern (Fig 2).

Results

Conclusion