Core Terms & Classification
Acquired Chiari malformation
Acquired Chiari malformation is a less common form that develops after birth due to another condition affecting pressure or space around the brain.
In short: Acquired Chiari malformation
- It develops after birth from another cause.
- Causes include masses, cysts, hydrocephalus, or CSF overdrainage.
- It can sometimes be reversed by treating the cause.
- It contrasts with the usual congenital form.
What it is
Acquired Chiari malformation is a less common form that develops after birth as a result of another condition that changes the pressure or available space around the brain, causing the cerebellar tonsils to be displaced downward. It contrasts with the usual congenital form.
Understanding Acquired Chiari malformation
Various conditions can lead to acquired tonsillar descent: a mass or tumor, an arachnoid cyst, hydrocephalus pushing structures down, or, conversely, low CSF pressure (intracranial hypotension) — including from overdrainage by a lumboperitoneal shunt — pulling the tonsils down. Because acquired Chiari results from an identifiable cause, addressing that cause (for example, treating the mass, the hydrocephalus, or the source of low pressure) can sometimes reverse or improve the tonsillar descent. Recognizing an acquired cause is important because the management differs from congenital Chiari — the underlying problem is treated rather than (or in addition to) decompression. Understanding acquired Chiari helps explain why identifying a cause matters.
Why it matters
Understanding acquired Chiari helps families see that, in some cases, tonsillar descent results from another treatable condition, and that addressing that cause — rather than decompression alone — may be the appropriate approach.
Common questions
What is acquired Chiari malformation?
A form developing after birth due to another condition.
What can cause it?
Masses, cysts, hydrocephalus, or CSF overdrainage (low pressure).
Can it be reversed?
Sometimes — by treating the underlying cause.
Bottom line
Acquired Chiari malformation develops after birth from another condition affecting pressure or space, and may improve by treating that cause.
Browse all 265 Chiari malformation definitions
Read the Chiari malformation patient guides
Not sure which page you need? Put words to it with the symptom navigator