The cerebellar tonsils as visualized during posterior fossa decompression can range from a relatively normal appearance (other than herniation through the foramen magnum) to marked deformity with blister formation at their tips.
In this image, the dura is opened in a triangular fashion with the brain to the left and the spinal cord to the right. The tonsils are elongated and their superficial vessels are stretched (lower arrow). Callous formation is seen at the tips of both tonsils, the upper arrow marking the callous running along tip of right tonsil. The elongation and callous formation occurs due to chronic impaction and repetitive pulsations of the cerebellar tonsils with each cardiac systole. Valsalva maneuvers such as coughing and sneezing also likely aggravate the process.