Oldfield's theory (piston/pressure wave)

What it is

Oldfield's theory is a widely cited modern explanation for syrinx formation in Chiari malformation. It proposes that the cerebellar tonsils, moving downward with each heartbeat in the obstructed foramen magnum, act as a piston, driving pressure waves through the spinal subarachnoid space that force fluid into the spinal cord.

Understanding Oldfield's theory (piston/pressure wave)

Based on intraoperative observations, this theory describes how, with each systolic pulse, the low-lying tonsils move downward like a piston in the crowded foramen magnum, generating a pressure wave in the spinal CSF. This wave is thought to drive fluid into the spinal cord, contributing to syrinx formation and expansion. Oldfield's theory is essentially the detailed mechanism behind the "piston effect" and has been influential in explaining why relieving the obstruction (by decompression and creating more space at the foramen magnum) often causes a syrinx to shrink. Understanding Oldfield's theory clarifies a leading, observation-based modern model of syrinx formation and its direct connection to the rationale for surgery.

Why it matters

Understanding Oldfield's theory helps families see a leading modern explanation for syrinx formation — the tonsils acting as a piston — and why it directly supports the idea that decompression can shrink a syrinx.