Many researchers believe that the Chiari malformation results from underdevelopment of the lower part of the skull. The Posterior Fossa is too small and does not provide the room needed for the Cerebellum and the lower brain stem. Thus, the lower part of the cerebellum, and sometimes the lower part of the brain stem, develops below the foramen magnum.

Why most people don’t show symptoms in infancy is not well understood.  What is known is that the brain moves with every heartbeat. Special MRI studies, called CINE MRI, have shown that the brain expands quickly with each heartbeat as blood is pumped into the brain. The briefly expanding brain, restricted by the fixed skull, moves downward toward the foramen magnum, which is already crowded in patients with the Chiari I malformation. This continuous downward pulsation of the brain can result in malfunction of the neurological tissue in the area. Just think about it - the heart beats 72 times each minute – 60 minutes each hour – 24 hours per day – for a grand total of 103,680 brain pulses per day! Even worse, when a person with the Chiari I malformation coughs, sneezes or strains, there is additional downward pressure which causes the Tonsils to impact into the foramen and cause an increase in the headache.

For most patients the Chiari malformation does not run in their family. However, in some cases, it can pass down from the mother or father to son or daughter.  Further research in this area may lead to a better understanding of the causes of the Chiari malformation.

Blockage at the foramen magnum means that the spinal fluid flow into the spinal canal will not be normal. In some patients with Chiari, this abnormal flow will cause fluid to build up inside the spinal cord, the condition called Syringomyelia (the cavity itself is often referred to as a Syrinx). The number of patients that develop a syrinx from the Chiari I malformation varies from 20% to 70% depending on the patients studied and the type of test used to determine its presence. Although the leading cause of syringomyelia is Chiari malformation, it can also be caused by trauma, tumor or infection. In some cases, the cause is idiopathic (unknown).

This illustration shows a large syrinx in the upper part of the spinal cord. It extends from just below the C1 level to C4.