Classic Arnold-Chiari malformations are inheritied deformities in the nervous system that develop from ectodermal tissues in the embryo. Ectodermal tissues turn into nerves and skin. Chiari 1 malformations, on the other hand, appear to be due to design flaws in the posterior fossa of the skull, such as one that is too small. Smaller posterior fossas may explain why females have a higher incidence of Chiari malformations. In contrast to classic Chiari malformations, the skull develops from mesodermal tissues, which also develop into muscles and connective tissues. What's more, the latest research shows that adults can acquire Chiari 1 malformation due to degeneration and trauma of the spine. The cause in many cases may be due the tethering of the brain and cord.
Humans are susceptible to Chiari malformations due to the unique design of the skull, spine and circulatory system of the brain as a result of upright posture. Among other things, Chiari malformations may play a far greater role than we currently realize in the cause of neurodegenerative diseases such as Alzheimer's, Parkinson's and multiple sclerosis, as well as others.
For further information on cord tethering and Chiari 1 malformations click the link below to a page I published on the foramem magnum: