Posterior fossa volume (small posterior fossa)

What it is

A small posterior fossa refers to a reduced-size bony compartment at the back of the skull — too small to comfortably hold the cerebellum and brainstem. A small or underdeveloped posterior fossa is one of the leading factors in the development of Chiari malformation type I.

Understanding Posterior fossa volume (small posterior fossa)

Research suggests that many people with Chiari I have a posterior fossa that is smaller than average, often due to underdevelopment of the bones of the skull base. With too little room, the cerebellum is crowded, and the cerebellar tonsils are pushed downward through the foramen magnum. This "overcrowding" model — a normal-sized brain in an undersized compartment — is a leading explanation for type I (as opposed to the brain itself being too large). It also helps explain associations with conditions affecting bone and connective tissue development. Posterior fossa decompression surgery effectively enlarges this compartment. Understanding posterior fossa volume clarifies why Chiari I develops and the rationale for decompression.

Why it matters

Understanding posterior fossa volume helps families see that Chiari I often results from a small bony compartment crowding a normal-sized cerebellum, which explains the tonsillar descent and the logic of enlarging the space surgically.