A 6-month-old male infant was referred to our institute for evaluation of abnormal eye movements and delayed development. The infant had no breathing abnormality in the neonatal period. He was born after a full-term uncomplicated pregnancy and was the first birth of a non-consanguineous marriage. Neurological examination revealed microcephaly (occipitofrontal circumference 39 cm), pendular nystagmus, tremulous movements of hands on attempt to approach objects, hypotonia and motor mental developmental delay. Fundus and the rest of the systemic examination was normal.

MRI of the brain revealed a deep and wide interpeduncular cistern with dysplastic and thick superior cerebellar peduncles giving the ‘molar tooth’ appearance in the axial images (figure 1A). The fourth ventricle was wide with a deformed roof and an anteriorly convex floor appearing as a bat-wing (figure 1B). The cerebellar hemispheres appeared to come together with the midline being separated by a thin sagittal cleft due to dysgenesis of the vermis known as ‘buttock sign’ (figure 1B). T1-weighted sagittal image showed dilated fourth ventricle with rounding of the roof; thick superior cerebellar peduncles oriented almost at right angles to the posterior surface of the brain stem (figure 1C,D). The foramen of Magendie was wide with an enlarged cisterna magna (figure 1C). There was hypoplasia of the corpus callosum and cortical atrophy along with prominence of the extra-axial cerebrospinal space predominantly in the region of the frontal lobes (figure 1C). Abdominal organs were evaluated as normal with ultrasonographic examination. All these findings were in consonance with Joubert syndrome type 1.

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Figure 1

A: Section of axial T1-weighted MRI showing molar tooth sign. B: Section of axial T1-weighted MRI showing bat-wing appearance/umbrella sign with buttock appearance of cerebellar hemispheres. C: T1-weighted midline sagittal MRI showing a dilated fourth ventricle with rounding of the roof with enlarged cisterna magna and cortical atrophy along with prominence of the extra-axial cerebrospinal fluid space predominantly in the region of the frontal lobes. D: T1-weighted sagittal MRI depicting thick superior cerebellar peduncle oriented almost at right angles to the posterior surface of the brain stem.

Joubert syndrome, an autosomal recessive disorder, is clinically characterised by a constellation of episodic hyperpnoea and apnoea in the neonatal period, ocular abnormalities (oculomotor apraxia and ocular coloboma), truncal ataxia, hypotonia and mental retardation with neuropathological abnormalities of cerebellum and brainstem.1 Based on the presence of retinal dystrophy, Joubert syndrome is classified into two types. Retinal and renal dysplastic changes have been described in type II, which has a worse prognosis.2 Considering the absence of retinal changes, the index case belongs to Joubert syndrome type 1.

The distinctive feature in MRI is molar tooth sign, which has been reported in about 85% patients with Joubert syndrome.3 Moreover, other syndromes with posterior-fossa malformations, including Dekaban-Arima, Senior-Löken and cerebellar vermis hypoplasia/aplasia, oligophrenia, ataxia, coloboma and hepatic fibrosis (COACH), frequently lead to diagnostic dilemmas. Presence of other signs and features on MRI suggestive of Joubert syndrome akin to the index case is noteworthy and further support the radiological diagnosis and aid further management.3 Furthermore, Joubert syndrome may present with variable clinical features especially in early infancy. Therefore, a high index of suspicion and a search for classical neuroimaging findings are essential in the appropriate diagnosis, management and genetic counselling of Joubert syndrome.