Isolated Joubert syndrome

Isolated Joubert syndrome (also known as classic Joubert syndrome or pure Joubert syndrome) is a rare autosomal recessive neurodevelopmental disorder characterized by a distinctive cerebellar and brainstem malformation known as the "molar tooth sign" (MTS) on axial brain MRI. This hallmark radiological finding results from cerebellar vermis hypoplasia or aplasia, deepened interpeduncular fossa, and thickened, elongated superior cerebellar peduncles. The condition primarily affects the central nervous system, particularly the cerebellum and brainstem, which are critical for coordination, balance, and regulation of breathing. Unlike Joubert syndrome with associated features (such as retinal, renal, or hepatic involvement), isolated Joubert syndrome is confined to neurological manifestations without significant multi-organ involvement. Key clinical features typically present in the neonatal or infantile period and include hypotonia (low muscle tone), episodic abnormal breathing patterns such as alternating tachypnea (rapid breathing) and apnea, abnormal eye movements (particularly oculomotor apraxia and nystagmus), and cerebellar ataxia. Developmental delays, including motor and cognitive impairment of variable severity, are common. Some individuals may have intellectual disability ranging from mild to severe, while others may have near-normal cognitive function. Episodic breathing abnormalities tend to improve with age and often resolve in childhood. Isolated Joubert syndrome is caused by pathogenic variants in genes encoding proteins involved in the structure and function of primary cilia, making it a ciliopathy. Multiple genes have been implicated, including but not limited to INPP5E, TMEM216, AHI1, CC2D2A, TMEM67, RPGRIP1L, CPLANE1 (C5orf42), and others. There is currently no cure or disease-specific treatment. Management is supportive and multidisciplinary, involving physical therapy, occupational therapy, speech therapy, and developmental support. Monitoring of breathing abnormalities is important in infancy. Genetic counseling is recommended for affected families, and periodic surveillance for potential organ involvement (kidneys, eyes, liver) is advisable even in cases initially classified as isolated, as additional features may emerge over time.

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