Tonsillar herniation
Cerebellar tonsils “cone” through the foramen magnum, compressing medulla → respiratory arrest. Usually rapidly fatal.
This occurs with either supra- or infratentorial masses or with elevated ICP. May be precipitated by LP. In many cases, there may simply be pressure on the brainstem without actual herniation. There are also cases with significant cerebellar herniation through the foramen magnum with the patient remaining alert.
It has been well documented that, along with tonsillar herniation, Chiari Malformation Type I (CMI) is associated with smaller posterior cranial fossa (PCF) and altered cerebrospinal fluid (CSF) flow and tissue motion in the craniocervical junction.
CM-I in children is not a radiologically static entity but rather is a dynamic one. Radiological changes were seen throughout the 7 years of follow-up. A reduction in tonsillar herniation was substantially more common than an increase. Radiological changes did not correlate with neurological examination finding changes, symptom development, or the need for future surgery. Follow-up imaging of asymptomatic children with CM-I did not alter treatment for any patient. It would be reasonable to follow these children with clinical examinations but without regular surveillance MRI 1).
As subdural empyema SDE along the tentorial surface may remain undetected in axial sections of CT scans, a lumbar puncture may precipitate fatal tonsillar herniation