Aetiology of herniation of the hindbrain in craniosynostosis. An investigation incorporating intracranial pressure monitoring and magnetic resonance imaging.

The occurrence and extent of herniation of the hindbrain has been evaluated in a population of children with craniosynostosis by means of magnetic resonance imaging of the craniocervical junction. The role of intracranial pressure (ICP), posterior fossa size and hydrocephalus in the development of this deformity has also been assessed. Magnetic resonance imaging (Siemens Magnetom 1.5T) was reviewed in 27 cases of craniosynostosis in whom there had been no previous cranial vault surgery. The position of the cerebellar tonsils in relation to the plane of the foramen magnum was measured and an index of the size of the posterior fossa relative to the rest of the cranial vault was also calculated for each case. The presence of hydrocephalus (requiring a cerebrospinal fluid diversion procedure) was documented. In 22 of these cases overnight, subdural ICP monitoring using the Camino fibre optic device had also been performed. Herniation of the hindbrain below the plane of the foramen magnum occurred in 10 of 27 cases (37%). The level of ICP showed a significant correlation with the extent of hindbrain herniation (p < 0.001) as did small posterior fossa size (p = 0.0035). Hydrocephalus was present in 4 patients, all of whom had hindbrain herniation. The extent of hindbrain herniation did not correlate with age (p = 0.48). We propose that herniation of the hindbrain in craniosynostosis is a consequence of brain deformation occurring in response to the physical forces imposed by a combination of the anatomical deformity at the skull base and intracranial hypertension rather than a primary malformation of brain development as commonly supposed.