Arnold-Chiari syndrome is a relatively rare malformation of the cerebellum. It occurs mainly in infants and is characterised typically by an association with spina bifida, and an abnormal arrangement of the posterior, or rear, part of the brain (cerebellum and bulb), which engages the upper portion of the cervical spine.
Arnold-Chiari malformation is also associated with hydrocephalus, which is the presence of cerebrospinal fluid in the cavities in excess of the brain, and brain damage. There are several forms of Chiari syndrome, with varying degrees of severity. For some, there may be three or four types of defects, which primarily relate to children.
Type I Chiari malformation is constituted by a misshapen cerebellum, which is lowered through the foramen magnum (a very large opening in the occipital bone of the cranium, that connects the spinal cord to the brain). The diagnosis of Chiari Type I is based on MRI and the extent of the flow of cerebrospinal fluid. In children who show no symptoms it is not always useful to intervene surgically, but the child is placed under strict surveillance.
Type II Arnold-Chiari malformation is also associated with other nerve structures (cerebellar tonsils, brain), but not in all cases. Sometimes syringomyelia is also associated with Type I. This chronic disorder is identified characterized by the presence of a cyst in the spinal cord that contains a yellow fluid, and leads to progressive destruction of nerve fibers, resulting in skin sensitivity to heat, cold and pain. Platybasia may also be evident, illustrated by a flat and expanded skull base. This type of malformation is found more frequently in adolescents and young adults.
Type II Chiari malformation is typically associated with myelomeningocele and hydrocephalus. The severest and most common form of myelomeningocele is also known as spina bifida. Hydrocephalus is an excessive accumulation of cerebrospinal fluid within the ventricles of the brain, due to poor circulation or poor absorption of cerebrospinal fluid.
Type III Arnold-Chiari syndrome is illustrated by a combination of a malformation of the area between the spinal cord and medulla cerebellum, and other defects such as myelomeningocele or encephalocele. Encephalocele is a protrusion of part of the brain, along with its meninges, outside the skull. It is sometimes congenital, but most commonly it follows a concussion. Hydrocephalus is typically seen in these cases.
Type IV Arnold-Chiari syndrome is characterised by a lack of cerebellar development, along with hypoplasia. The cerebellum is underdeveloped or incomplete, and sections of the skull and spinal cord may be visible.
The 19th century Austrian pathologist Hans Chiari first described anomalies of the hind brain, known as the Chiari malformations I, II and III. Chiari IV was identified later by other scientists. Severity ratings are scaled from I-IV, with IV being severest. Types III and IV are the rarest.
Chiari malformation symptoms most commonly present as:
In children, Type I malformation rarely has symptoms. If symptoms are present, typically they are: