Cerebellar tonsillar herniation

Cerebellar tonsillar herniation is primarily caused by abnormal development of the midline structures of the posterior cranial fossa during the embryonic period, leading to the herniation of the cerebellar tonsils, cerebellar vermis, medulla oblongata, and the lower part of the fourth ventricle into the foramen magnum. Cerebellar tonsillar herniation is typically classified into four types: Type I: This is the most common type clinically, characterized by cerebellar tonsillar herniation below the foramen magnum (>5mm), which may be accompanied by hydrocephalus and syringomyelia. Type II: Based on Type I, this type also involves downward displacement and deformation of the brainstem, cerebellar vermis, and fourth ventricle, often accompanied by myelomeningocele and hydrocephalus. Type III: This is very rare. Based on Type II, it also involves occipital or cervical encephalocele and an enlarged foramen magnum. The encephalocele sac contains the medulla oblongata, cerebellum, fourth ventricle, and various pathological neural tissue components. Type IV: This is extremely rare, characterized by hypoplasia or absence of the cerebellar tonsils and tentorium cerebelli.