Lai, Sammie

Background Cerebellar tonsillar ectopia may arise from altered cerebrospinal fluid (CSF) dynamics and can mimic Chiari I malformation. Differentiating between pressure-induced tonsillar descent and congenital malformations is essential. We present an interesting case that visualizes variable tonsillar displacement levels as an indicator of CSF dynamics. Case A 3-year-old boy presented with headache and sixth nerve palsy and was diagnosed with idiopathic intracranial hypertension. Initial magnetic resonance imaging (MRI) revealed a 5 mm descent of the cerebellar tonsils. During hospitalization, the patient underwent mastoidectomy for an incidental T2 hyperintense lesion. Subsequent MRI scans demonstrated the resolution of tonsillar displacement. At age 5, the patient experienced worsening headache. Brain MRI revealed a 13 mm descent of the cerebellar tonsils below the foramen magnum and pachymeningeal enhancement. Further evaluation revealed transtemporal CSF leakage through the previous mastoidectomy site into the soft neck tissues. Conclusion This case exemplifies cerebellar tonsillar displacement associated with both intracranial hypertension (push) and hypotension (pull), illustrating opposite causal directions of CSF dynamics in the same patient. Understanding the phenomenon as a result of dynamic CSF balance rather than unidirectional force or fixed pathophysiology can enhance monitoring of patients.