ACS refers to a spectrum of anatomical hindbrain (rhombencephalon) anomalies varying from herniation of posterior fossa contents through the foramen magnum into the spinal canal all the way to cerebellar aplasia. It may be associated with additional intra- and extracranial findings like hydrocephalus, syringomyelia, spinal dysraphism (eg, [myelo-] meningocele, diastematomyelia), and dysfunction of the respiratory and/or cardiac center.
Cerebromedullary Malformation Syndrome. Chiari Malformation. Chiari Deformity. (In ACS Type I [see below], the cerebellar tonsils are not mal-, but deformed, hence the preferred name is Chiari Deformity.)
In 1883, John Cleland, a British poet and anatomist, first described the abnormalities of spina bifida, encephalocele, and anencephalus. The Austrian pathologist Hans Chiari (1851-1916) in 1891 and the German pathologist Julius Arnold (1835-1915) in 1894 further detailed this anomaly in children with hydrocephalus. The designation Arnold-Chiari Syndrome (ACS) was coined in 1907 by Arnold’s students (without honoring Cleland’s work).
It is based on the radiological and/or pathological morphology of the anatomical anomalies.
ACS Type I: The main finding is displacement of the often elongated cerebellar tonsils through the foramen magnum into the cervical spinal canal of more than or equal to 5 mm beyond the opisthion-basion line (“line of McRae”). The medulla oblongata may be slightly displaced, kinked, or compressed. The vermis cerebelli and the fourth ventricle are usually not or only minimally involved. The degree of hydrocephalus, hydromyelia (distension of the central canal), and cervical syringomyelia (distension of the paracentral cavities) is variable. While myelomeningocele (MMC) is not a feature of this type of ACS, other cranial and vertebral abnormalities occur in approximately one fourth of patients and may include craniosynostosis, platybasia, atlanto-occipital fusion, abnormal positioning of the odontoid process, ☞Klippel-Feil Syndrome, cervical ribs, fused thoracic ribs, and kyphoscoliosis. This type of ACS is also called the adult type because many of the patients were not (and often still are not) diagnosed before the second or third decade of life in the era before CT and MRI.
ACS Type II: Radiologically characterized by a relatively small posterior fossa, which results in caudal displacement of the cerebellar vermis, medulla oblongata, and the fourth ventricle into the spinal canal and crowding of the cervical cord roots into an upward course (upward herniation). Superior displacement of the cerebellum through the tentorial incisura may occur, which is also known as towering cerebellum. Kinking of the medulla at the cervicomedullary junction occurs in more than 50% of cases. The most characteristic anatomical finding in ACS Type II though is a lumbar MMC. This type of ACS is frequently (in up to 90%) associated with other anomalies of the central nervous system, such as aqueductal stenosis with significant occlusive hydrocephalus (often presenting after repair of MMC), holoprosencephaly, dys- or agenesis of the corpus callosum, syringo- or hydromyelia, ...