Acquired causes (overview)

What it is

Acquired causes are conditions that develop after birth and lead to downward displacement of the cerebellar tonsils, producing a Chiari-like picture. They are distinct from the usual congenital form (which arises from a developmentally small posterior fossa).

Understanding Acquired causes (overview)

Several conditions can cause acquired (secondary) tonsillar descent: a mass or tumor pushing structures downward, an arachnoid cyst, hydrocephalus increasing pressure above the foramen magnum, or — importantly — low CSF pressure (intracranial hypotension, as from a CSF leak or shunt overdrainage) pulling the tonsils down. The key point is that these causes are identifiable and often treatable, and addressing the underlying cause can improve or reverse the tonsillar descent. This contrasts with congenital Chiari, where decompression addresses the small bony compartment. Recognizing an acquired cause is crucial because the right treatment is to address the cause, not necessarily to perform decompression. Understanding acquired causes clarifies the category of conditions that can mimic Chiari and why identifying them matters for treatment.

Why it matters

Understanding acquired causes helps families see that tonsillar descent can result from various treatable conditions developing after birth, and why identifying and treating the cause — rather than assuming congenital Chiari — is important.