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Molar Tooth Malformations (Joubert)

• Joubert syndrome and related disorders (JSRD)
○ Hindbrain anomaly characterized by dysmorphic vermis, lack of decussation of superior cerebellar peduncle, central pontine tracts, corticospinal tracts
○ JSRD considered as a ciliopathy
• Also sometimes called molar tooth malformation (MTM)

Diagnostic criteria  
• Molar tooth appearance of midbrain on axial images
• Midline vermian clefting
• Thick, horizontal (perpendicular to brainstem), superior cerebellar peduncles
• Midline depression of 4th ventricular floor at isthmus (thin isthmus on midline)
• High-definition T2 (CISS/FIESTA) provides exquisite structural analysis
• HASTE allows clear identification of molar tooth sign in fetuses
• DTI is useful complementary technique

• > 20 causative genes have been discovered so far
○ All encode for proteins expressed in primary cilium/its apparatus
• Absence of decussation of superior cerebellar peduncles
• Near total absence of pyramidal decussation

• Ataxia, developmental delay, oculomotor and respiratory abnormalities

Best imaging tool
○ MR

Protocol advice
○ Use high definition; posterior fossa structures are small
○ DTI is useful complementary technique

• MTM whenever scanning infants/children with severe hypotonia and ocular anomalies

Image Interpretation Pearls
• If vermis not recognized or looks small and dysmorphic on sagittal image
• Sometimes vermis looks nearly normal
○ Look for vermian cleft and molar tooth sign on axials

Imaging gallery
 Axial graphic depicts Joubert malformation. Thickened superior cerebellar peduncles  around the elongated 4th ventricle form the classic molar tooth seen in this anomaly. Note the cleft cerebellar vermis .

Sagittal T1WI in a patient with classic Joubert syndrome shows a small, grossly malformed cerebellar vermis  with upwardly convex superior 4th ventricle ſt and a rounded fastigial point .

Axial T1 MP-RAGE image in the previous patient shows a classic molar tooth sign with a foreshortened midbrain and a narrow isthmus . Thick, elongated superior cerebellar peduncles surround an elongated 4th ventricle. Note the vermis clefting .

Axial T2WI in the same case shows the molar tooth sign even more clearly.

Sagittal T2WI MR (HASTE) of a 27-week fetus shows a dysmorphic 4th ventricle with a hardly recognizable vermis  (abnormal shape, no normal fissures identified). The posterior fossa cisterns are markedly enlarged. Supratentorial structures are normal.

 Axial T2WI MR (HASTE) in the previous  patient demonstrates an anteriorly pointed 4th ventricle  and thick superior cerebellar peduncles , giving an appearance like a molar tooth.

Sagittal T1WI MR shows classic molar tooth malformation (MTM). Midbrain is elongated and narrow at isthmus. Vermis st is small and dysplastic (abnormal foliation). Fourth ventricle is high, at pons midbrain junction.

 Axial T1WI MR shows molar tooth sign. Midbrain is narrow in midline due to absent decussation of superior cerebellar peduncles (SCPs). SCPs ſt are large and horizontal, forming the "roots" of the molar tooth.

Sagittal T1WI MR shows typical features of Joubert with narrow midbrain isthmus , small dysplastic vermis , and a hypothalamic hamartoma st that is characteristic of the Varadi-Papp syndrome.

Sagittal T1WI MR shows subtle MTM with slightly high 4th ventricle and small vermis. The cerebellar hemisphere is seen to come into the midline below the abnormal vermis.

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