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What are the different Chiari malformations?
CM0
Chiari malformation type 0 (CM0) is a rare type of Chiari malformation that occurs when there is little to no cerebellar tonsil descent and crowding at the base of the skull
Abnormalities associated with CM0:
- A small posterior fossa
- An obex situated lower than normal
- An increased AP diameter of the foramen magnum
- An increased AP distance of the spinomedullary junction to the foramen magnum
- An increased angle between the floor of the fourth ventricle and the clivus
CM I
The most common type is type 1, which occurs when the cerebellar tonsils push into the foramen magnum, the opening between the skull and spinal cord. This can happen when the skull is too small, or the brain grows faster than the skull. Symptoms can appear in any age group, but are often first noticed in adolescence or adulthood.
CM 1.5
The most common type is type 1, which occurs when the cerebellar tonsils push into the foramen magnum, the opening between the skull and spinal cord. This can happen when the skull is too small, or the brain grows faster than the skull. Symptoms can appear in any age group, but are often first noticed in adolescence or adulthood.
Type II
CM Type II involves both the cerebellum and brain stem tissue pushing into the foramen magnum. The nerve tissue that connects the two halves of the cerebellum may be missing or only partially formed. Type II is usually accompanied by a myelomeningocele—a form of spina bifida that occurs when the spinal canal and backbone do not close before birth (see below). A myelomeningocele usually results in partial or complete paralysis of the body below the spinal opening. It is the CM type most associated with hydrocephalus. The term Arnold-Chiari malformation is specific to CM Type II.
Type III - IV
A critical condition where the cerebellum and brainstem herniate through an opening in the back of the skull.
Type III:
CM Type III is a critical condition characterized by some of the cerebellum and the brain stem sticking out, or herniating, through an opening in the back of the skull. Membranes surrounding the brain or spinal cord may also herniate. Symptoms of this very rare form of CM appear in infancy and can cause debilitating and life-threatening complications. Babies with CM Type III can have additional severe neurological defects such as seizures and mental and physical delays.
Type IV:
A very rare condition, CM Type IV involves an incomplete or underdeveloped cerebellum (a condition known as cerebellar hypoplasia). While the cerebellum is in its normal position, parts of it are missing, and portions of the skull and spinal cord may be visible.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4900115/