What causes the molar tooth sign on brain MRI?
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Answer:
The molar tooth sign is seen on axial MRI at the level of the midbrain-hindbrain junction and comprises three components: hypoplasia or aplasia of the cerebellar vermis, an abnormally deep interpeduncular fossa, and elongated, thickened, horizontally orientated superior cerebellar peduncles. Together these reproduce the crown and two roots of a molar tooth. It is the mandatory diagnostic criterion for Joubert syndrome and related disorders, a group of ciliopathies presenting with hypotonia, ataxia, developmental delay, oculomotor apraxia and episodic hyperpnoea, with variable retinal, renal, hepatic and skeletal involvement. It is accompanied on more caudal sections by the bat wing or umbrella-shaped fourth ventricle, produced by the midline cleft between the unfused cerebellar hemispheres. Because the sign requires a correctly angled axial section at the right level, it is missed when the imaging plane is oblique, and it should be actively sought in any child with congenital ataxia or oculomotor apraxia.
Why is it called so?
The thick horizontal peduncles and deep interpeduncular fossa together look like the crown and roots of a molar tooth in cross-section.
Pathophysiology
Joubert syndrome and related disorders are ciliopathies: mutations in genes encoding primary cilium and basal body proteins disrupt sonic hedgehog and other signalling pathways that govern midline patterning of the mid- and hindbrain. The superior cerebellar peduncular fibres fail to decussate in the midbrain tegmentum, so instead of crossing and being embedded within the tegmentum they run straight and horizontally, becoming thickened and elongated. Absence of the decussating fibre mass narrows the isthmus and deepens the interpeduncular fossa. The same midline patterning failure prevents the cerebellar vermis from forming, leaving the hemispheres unapposed and the fourth ventricle deformed.
Alternative names: Molar tooth malformation
Other associated named signs: Bat wing fourth ventricle, umbrella sign, the hot cross bun sign as a contrasting brainstem pattern, and the Dandy-Walker malformation as a posterior fossa differential
References
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