Chronic impaction of the cerebellar tonsils in the Chiari I malformation can lead to fibrosis of tonsillar tips as illustrated in a previous post. Another pathological effect of chronic impaction on the tips of the tonsils is cystic degeneration. This finding occurred in only 3 of 440 children undergoing posterior fossa decompression for CM-I reported by Stevenson et al. The incidence in adults is unknown, but has been rare in my experience.
The case above shows a moderate cystic degeneration (a blister) at the tip of the left tonsil. (This adult patient is in the prone position with the head to the left and spine to the right.)