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Figure 1. Sagittal view shows caudal protrusion of peg-shaped cerebellar tonsil (yellow line) measuring 8mm beneath the foramen magnum (red line). The clivus is shortened (dashed green line) or accurately the basiocciput is shortened. Morphometric study by Nishikawa et al demonstrated basiossiput to be significantly smaller/shortened in adult Chiari I patient.1 On the other hand there is no significant difference in the posterior fossa brain volume or axial length of the hindbrain when compared to normal subjects.1 Interestingly it was the strikingly shortened basiocciput that caught my eye when I first look through the sagittal images for this study.
Figure 2. Coronal and axial views again demonstrates the caudal protrusion of the cerebellar tonsils. Subtle tonsil asymmetry is present with the right cerebellar tonsil displaced to a greater extent compared to the left.
Figure 3. Axial view shows noticeably small posterior fossa evident by the absence of the cisterna magna.
Figure 4. In addition to shortened clivus mentioned in figure 1 other associated bone anomalies in this patient include notching of the posterior arch of C1 and a prominent internal occipital protuberance that extends deeply between the cerebellar hemisphere.