What is a characteristic feature of Chiari II malformation?

• A. Herniation of cerebellar tonsils
• B. Enlargement of the lateral ventricles
• C. Absence of the cerebellar vermis
• D. Severe muscle weakness

Answer: (A)

Chiari II malformation, also known as Arnold-Chiari malformation, is primarily characterized by the herniation of cerebellar tonsils through the foramen magnum into the spinal canal. This displacement can lead to a variety of neurological symptoms and is often associated with other spinal conditions such as myelomeningocele. The herniated tonsils can result in obstruction of cerebrospinal fluid (CSF) flow, potentially leading to various complications, including hydrocephalus.

The presence of cerebellar tonsil herniation is crucial in differentiating Chiari II from other types of Chiari malformations, such as Chiari I, which also involves tonsillar herniation but typically occurs without associated structural defects. The herniation in Chiari II is considered more significant due to its association with neural tube defects.

In this context, other options provided relate to different aspects or conditions. Enlargement of the lateral ventricles is commonly associated with obstructive hydrocephalus due to impaired CSF flow but is not a defining feature of Chiari II. Similarly, the absence of the cerebellar vermis is more characteristic of Dandy-Walker malformation rather than Chiari II. Severe muscle weakness may occur