How can Arnold Chiari malformation be described?

• A. Enlargement of the cerebellum
• B. Displacement of cerebellar and brain stem tissue into the foramen magnum
• C. Compression of spinal cord structures only
• D. Thinning of the brain cortex

Answer: (B)

Arnold Chiari malformation is primarily characterized by the displacement of cerebellar and brainstem tissue into the foramen magnum, which is the large opening at the base of the skull. This condition typically occurs when the skull is misshapen or smaller than normal, leading to the cerebellum and parts of the brainstem being pushed downward into the spinal canal. This displacement can result in a variety of neurological symptoms due to the obstruction of cerebrospinal fluid flow and pressure on the brain and spinal cord.

The other choices do not accurately represent the nature of Arnold Chiari malformation. For instance, the enlargement of the cerebellum does not occur in this condition; rather, the structures are displaced or compressed. Similarly, while there may be effects on spinal cord structures, the defining characteristic of this malformation involves the displacement rather than sole compression. Thinning of the brain cortex is not a typical feature associated with this malformation either, as it specifically pertains to the positional alteration of cerebellar and brainstem tissues rather than changes in cortical thickness. Thus, option B is the most suitable description of Arnold Chiari malformation.