I read with interest the paper by Fischbein et al in which prominent motion of the tonsils was seen in a patient with a Chiari I malformation on a phase-contrast velocity MR image (1). This observation confirms our report in which similar findings were made in seven of eight patients with the malformation (2). We attributed this prominent motion to an increase in the downward systolic velocity of the tonsils in the Chiari I malformation. It has been postulated that this increase in the tonsillar velocities may contribute to the development of syringomyelia by causing an increased systolic pressure wave forcing CSF into the cord parenchyma during systole (3). Another cause for the increased flow of CSF into the cord parenchyma may be an increase in the duration of CSF systole at the C2-C3 level in patients with the Chiari I malformation and syringomyelia, as reported by our group (4). This alone could lead to an increase in the bulk flow of CSF into the spinal cord. After surgical removal of bony structures at the craniocervical junction and decompression of the tonsils, the syrinx cavity collapsed and the duration of systole decreased (4). The increase in systolic pressure, forcing CSF into the spinal cord parenchyma during systole postulated by Fischbein et al (1) and others (3), while intuitively correct, needs to be proved by direct CSF pressure measurements of the cervical subarachnoid space in these patients.
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