Contrary to popular belief, a Chiari Malformation does not indicate a large brain or a small skull. Most doctors ascribe to the belief that a Chiari Malformation is nothing but a congenitally underdeveloped posterior cranial fossa (posterior fossa hypoplasia) in fact, the surgical treatment that they recommend is technically called a Posterior Fossa Decompression (not a Chiari Decompression, as patients tend to call it).
A small posterior fossa does not cause a problem unless it pushes the cerebellar tonsils down into the foramen magnum (the hole at the base of the skull), causing a blockage of cerebrospinal fluid. Yet, the size of that herniation is THE ONLY THING that doctors routinely measure when they diagnose a Chiari Malformation. They diagnose according to a criteria that only considers the size and appearance of the cerebellar tonsils (without measuring the posterior cranial fossa at all).
✅ Are the cerebellar tonsils herniated by at least 3-5mm?
✅ Are the cerebellar tonsils pointy enough?
If the answer to both of these questions is affirmative, most surgeons will diagnose it as a congenital Chiari Malformation (caused by the small posterior fossa THAT THEY NEVER MEASURED) and recommend a posterior fossa decompression based solely on their assumptions.
While a small posterior fossa can cause the cerebellar tonsils to prolapse, there are many conditions that can also cause the tonsils to herniate. When these pathological (causal/attributing) factors go untreated and are allowed to continue pushing/pulling the cerebellar tonsils downward, the brain fails to become buoyant and the patient is left with an even larger hole for the brain to slump into, once again restricting the flow of cerebrospinal fluid. While the patient may initially get a measure of relief from the decompression allowing the CSF to flow, once the brain drops into the hole, the symptoms resume, and doctors are less likely to admit a FAILED DECOMPRESSION.
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