Arnold-Chiari Malformations are rare diseases that denote to a spectrum of congenital malformation of the brain. It is often called Chiari Malformation or tonsillar herniation. In this disorder, congenital hindbrain abnormalities are present that affects the structural relationships between the cerebellum, brainstem, the upper cervical cord and the bony cranial bone.
The disorder is named after Hans Chiari who was an Austrian pathologist. Hans Chiari classified the Chiari malformations through types I to III in 1891. However, Cleland was the first person to describe the first case of Chiari malformation in 1833. Julius Arnold, Chiari’s colleague, made additional information and contributions to the definition of Chiari II malformation. Because of this, student of Dr. Arnold later named type II malformation as Ranold-Chiari malformation in his honor. The type IV malformation was later added by other investigators.
- 1. Chiari type I malformation – this is the most common form of Chiari malformation. Aside from being common it is the least severe type and is often diagnosed in adulthood. In this type, generally, the individual is asymptomatic during childhood.
- Chiari type II malformation – this type is less common but is more severe than type I. it is usually accompanied by a lumbar myelomenigocele which results to either a partial or complete paralysis below the spinal defect. It becomes symptomatic in infancy or early childhood.
- Chiari Type III malformation – this is an exceedingly rare and generally incompatible with life. It causes severe neurological defects and is associated with an occipital encephalocele.
- Chiari Type IV malformation – like type III, this is a very rare disease and is generally incompatible with life. It is characterized by a lack of cerebellar development.
Chiari Type I Malformation
- Genetic basis –There are suggestions that occurrence of type I malformation is linked to chromosomes 9 and 15.
- Disorder of para-axial mesoderm – results in the formation of a small posterior fossa. In return, the cerebellum development within the small compartment would lead to overcrowding of the posterior fossa, herniation of the cerbellar tonsils and the impaction of the foramen magnum.
Chiari Type II Maformation
- CSF loss theory – because chiari II malformation is associated with meylomeningocele, fluids escape through the open area in the myelomeningocele that leads to the inadequate stimulus for condensation at the base of the skull. Because of this, inadequate and disordered growth of the fossa occurs that results to the upward herniation of the vermis and the downward herniation of the brainstem.
image from choa.org