Secondary Chiari

What it is

Secondary Chiari refers to downward tonsillar descent that is caused by another, identifiable condition — rather than arising from a developmentally small posterior fossa (primary Chiari). It overlaps closely with the concept of acquired Chiari malformation.

Understanding Secondary Chiari

In secondary Chiari, the tonsils are displaced downward because of some other process affecting pressure or space — for example, a mass pushing structures down, hydrocephalus increasing pressure above, or low CSF pressure (intracranial hypotension, including from a spinal fluid leak or overdrainage) pulling the tonsils down. Because there is an underlying cause, identifying and treating it can sometimes improve or resolve the tonsillar descent, and the approach differs from primary Chiari (where decompression addresses the crowded bony compartment). Recognizing secondary Chiari is important so that the actual cause is addressed rather than assuming a developmental malformation. Understanding secondary Chiari reinforces why determining the cause of tonsillar descent matters.

Why it matters

Understanding secondary Chiari helps families see that tonsillar descent sometimes reflects another treatable condition, and that finding and addressing that cause — rather than assuming a developmental malformation — can be the key to management.