Tonsillar ectopia and headaches.

Tonsillar ectopia, encompassing slight descent of the cerebellar tonsils and Chiari I malformations, are disorders observed routinely in older children and adults and are believed to be an acquired form of the Chiari malformations. This entity is different from the other Chiari malformations in that hydrocephalus plays no role in its evolution. More likely it is a disorder of para-axial mesoderm, characterized by posterior fossa hypoplasia and content overcrowding, and not an embryologic anomaly of neuroectoderm. The prevalence of tonsillar ectopia to my knowledge has not been studied. The incidence of Chiari I malformations ranges from less than 1%to 3% at various tertiary centers. The occipital and exertional headache associated with this malformation can be observed in subjects who have new-onset tonsillar ectopia resulting from repeated lumbar puncture,idiopathic intracranial hypotension, lumboperitoneal shunting, or spontaneous development [34-36]. This new-onset headache can remit with return to normal tonsil positioning. These findings support the proposal that pressure dissociation gradients and perhaps traction of pain-sensitive tissues play a role in generation of the headache observed in this malformation. In accordance with this idea, the extent of tonsillar descent is irrelevant alone,and the degree of posterior fossa hypoplasia and decrement of CSF flow velocity are better defining factors for determining the clinical significance of tonsillar ectopia.