The association between Chiari malformation, type I (CM-I) and connective tissue disorders (CTDs) has only recently been discovered. A group of neurosurgeons noticed that some patients with CM-I who had a failed surgery had other suspicious symptoms.
Further physical examinations and genetic testing showed a second inherited condition (one of several different types of CTD). The combination of these two problems resulted in hypermobility of the upper cervical spine. Hypermobility refers to excess motion as one vertebra slides over the one below it.
Patients with CTD often have joint dislocations, loose and fragile skin, and poor wound healing. Muscle and tendon rupture can also occur. Easy bruising, hernias, and changes in blood vessels have also been reported.
CM-I results in a herniation or downward slip of the lower brain through the foramen magnum. The foramen magnum is the natural opening in the skull through which the spinal cord descends.
When CM-I is present along with CTD, there is a greater degree of herniation. Instability of the upper cervical spine also results in cranial settling. This means the cranium (skull) slips down over the vertebrae of the upper cervical spine.
Slightly more than 10 per cent of patients with CM-I also have a CTD. The chance of having both these problems at the same time is estimated to be around one to five for every 10 million people. In other words, it's very rare. When it does occur, there is a strong positive family history of CM-I, CTD, or both.